Merge remote-tracking branches 'asoc/fix/amd', 'asoc/fix/arizona', 'asoc/fix/dpcm', 'asoc/fix/dwc', 'asoc/fix/fsl' and 'asoc/fix/fsl-ssi' into asoc-linus
diff --git a/.mailmap b/.mailmap
index b1e9a97..7e6c533 100644
--- a/.mailmap
+++ b/.mailmap
@@ -21,6 +21,7 @@
 Andrew Morton <akpm@linux-foundation.org>
 Andrew Vasquez <andrew.vasquez@qlogic.com>
 Andy Adamson <andros@citi.umich.edu>
+Antonio Ospite <ao2@ao2.it> <ao2@amarulasolutions.com>
 Archit Taneja <archit@ti.com>
 Arnaud Patard <arnaud.patard@rtp-net.org>
 Arnd Bergmann <arnd@arndb.de>
diff --git a/CREDITS b/CREDITS
index 8207cc6..a3887b5 100644
--- a/CREDITS
+++ b/CREDITS
@@ -534,6 +534,7 @@
 E: neil@brown.name
 P: 4096R/566281B9 1BC6 29EB D390 D870 7B5F  497A 39EC 9EDD 5662 81B9
 D: NFSD Maintainer 2000-2007
+D: MD Maintainer 2001-2016
 
 N: Zach Brown
 E: zab@zabbo.net
@@ -1507,6 +1508,14 @@
 S: Griffith, ACT 2603 
 S: Australia
 
+N: Andreas Herrmann
+E: herrmann.der.user@gmail.com
+E: herrmann.der.user@googlemail.com
+D: Key developer of x86/AMD64
+D: Author of AMD family 15h processor power monitoring driver
+D: Maintainer of AMD Athlon 64 and Opteron processor frequency driver
+S: Germany
+
 N: Sebastian Hetze
 E: she@lunetix.de
 D: German Linux Documentation,
@@ -1847,6 +1856,16 @@
 S: 1403 ND  BUSSUM
 S: The Netherlands
 
+N: Martin Kepplinger
+E: martink@posteo.de
+E: martin.kepplinger@theobroma-systems.com
+W: http://www.martinkepplinger.com
+D: mma8452 accelerators iio driver
+D: Kernel cleanups
+S: Garnisonstraße 26
+S: 4020 Linz
+S: Austria
+
 N: Karl Keyte
 E: karl@koft.com
 D: Disk usage statistics and modifications to line printer driver
diff --git a/Documentation/ABI/testing/configfs-iio b/Documentation/ABI/testing/configfs-iio
new file mode 100644
index 0000000..2483756
--- /dev/null
+++ b/Documentation/ABI/testing/configfs-iio
@@ -0,0 +1,21 @@
+What:		/config/iio
+Date:		October 2015
+KernelVersion:	4.4
+Contact:	linux-iio@vger.kernel.org
+Description:
+		This represents Industrial IO configuration entry point
+		directory. It contains sub-groups corresponding to IIO
+		objects.
+
+What:		/config/iio/triggers
+Date:		October 2015
+KernelVersion:	4.4
+Description:
+		Industrial IO software triggers directory.
+
+What:		/config/iio/triggers/hrtimers
+Date:		October 2015
+KernelVersion:	4.4
+Description:
+		High resolution timers directory. Creating a directory here
+		will result in creating a hrtimer trigger in the IIO subsystem.
diff --git a/Documentation/ABI/testing/configfs-rdma_cm b/Documentation/ABI/testing/configfs-rdma_cm
new file mode 100644
index 0000000..5c389aa
--- /dev/null
+++ b/Documentation/ABI/testing/configfs-rdma_cm
@@ -0,0 +1,22 @@
+What: 		/config/rdma_cm
+Date: 		November 29, 2015
+KernelVersion:  4.4.0
+Description: 	Interface is used to configure RDMA-cable HCAs in respect to
+		RDMA-CM attributes.
+
+		Attributes are visible only when configfs is mounted. To mount
+		configfs in /config directory use:
+		# mount -t configfs none /config/
+
+		In order to set parameters related to a specific HCA, a directory
+		for this HCA has to be created:
+		mkdir -p /config/rdma_cm/<hca>
+
+
+What: 		/config/rdma_cm/<hca>/ports/<port-num>/default_roce_mode
+Date: 		November 29, 2015
+KernelVersion:  4.4.0
+Description: 	RDMA-CM based connections from HCA <hca> at port <port-num>
+		will be initiated with this RoCE type as default.
+		The possible RoCE types are either "IB/RoCE v1" or "RoCE v2".
+		This parameter has RW access.
diff --git a/Documentation/ABI/testing/configfs-usb-gadget-sourcesink b/Documentation/ABI/testing/configfs-usb-gadget-sourcesink
index bc7ff73..f56335a 100644
--- a/Documentation/ABI/testing/configfs-usb-gadget-sourcesink
+++ b/Documentation/ABI/testing/configfs-usb-gadget-sourcesink
@@ -10,3 +10,5 @@
 		isoc_mult	- 0..2 (hs/ss only)
 		isoc_maxburst	- 0..15 (ss only)
 		buflen		- buffer length
+		bulk_qlen	- depth of queue for bulk
+		iso_qlen	- depth of queue for iso
diff --git a/Documentation/ABI/testing/configfs-usb-gadget-tcm b/Documentation/ABI/testing/configfs-usb-gadget-tcm
new file mode 100644
index 0000000..a29ed2d
--- /dev/null
+++ b/Documentation/ABI/testing/configfs-usb-gadget-tcm
@@ -0,0 +1,6 @@
+What:		/config/usb-gadget/gadget/functions/tcm.name
+Date:		Dec 2015
+KernelVersion:	4.5
+Description:
+		There are no attributes because all the configuration
+		is performed in the "target" subsystem of configfs.
diff --git a/Documentation/ABI/testing/sysfs-bus-iio-ina2xx-adc b/Documentation/ABI/testing/sysfs-bus-iio-ina2xx-adc
new file mode 100644
index 0000000..8916f7e
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-bus-iio-ina2xx-adc
@@ -0,0 +1,24 @@
+What:		/sys/bus/iio/devices/iio:deviceX/in_allow_async_readout
+Date:		December 2015
+KernelVersion:	4.4
+Contact:	linux-iio@vger.kernel.org
+Description:
+		By default (value '0'), the capture thread checks for the Conversion
+		Ready Flag to being set prior to committing a new value to the sample
+		buffer. This synchronizes the in-chip conversion rate with the
+		in-driver readout rate at the cost of an additional register read.
+
+		Writing '1' will remove the polling for the Conversion Ready Flags to
+		save the additional i2c transaction, which will improve the bandwidth
+		available for reading data. However, samples can be occasionally skipped
+		or repeated, depending on the beat between the capture and conversion
+		rates.
+
+What:		/sys/bus/iio/devices/iio:deviceX/in_shunt_resistor
+Date:		December 2015
+KernelVersion:	4.4
+Contact:	linux-iio@vger.kernel.org
+Description:
+		The value of the shunt resistor may be known only at runtime fom an
+		eeprom content read by a client application. This attribute allows to
+		set its value in ohms.
diff --git a/Documentation/ABI/testing/sysfs-bus-usb b/Documentation/ABI/testing/sysfs-bus-usb
index 3a4abfc..0bd731c 100644
--- a/Documentation/ABI/testing/sysfs-bus-usb
+++ b/Documentation/ABI/testing/sysfs-bus-usb
@@ -134,19 +134,21 @@
 		enabled for the device. Developer can write y/Y/1 or n/N/0 to
 		the file to enable/disable the feature.
 
-What:		/sys/bus/usb/devices/.../power/usb3_hardware_lpm
-Date:		June 2015
+What:		/sys/bus/usb/devices/.../power/usb3_hardware_lpm_u1
+		/sys/bus/usb/devices/.../power/usb3_hardware_lpm_u2
+Date:		November 2015
 Contact:	Kevin Strasser <kevin.strasser@linux.intel.com>
+		Lu Baolu <baolu.lu@linux.intel.com>
 Description:
 		If CONFIG_PM is set and a USB 3.0 lpm-capable device is plugged
 		in to a xHCI host which supports link PM, it will check if U1
 		and U2 exit latencies have been set in the BOS descriptor; if
-		the check is is passed and the host supports USB3 hardware LPM,
+		the check is passed and the host supports USB3 hardware LPM,
 		USB3 hardware LPM will be enabled for the device and the USB
-		device directory will contain a file named
-		power/usb3_hardware_lpm. The file holds a string value (enable
-		or disable) indicating whether or not USB3 hardware LPM is
-		enabled for the device.
+		device directory will contain two files named
+		power/usb3_hardware_lpm_u1 and power/usb3_hardware_lpm_u2. These
+		files hold a string value (enable or disable) indicating whether
+		or not USB3 hardware LPM U1 or U2 is enabled for the device.
 
 What:		/sys/bus/usb/devices/.../removable
 Date:		February 2012
@@ -187,6 +189,17 @@
 		The file will read "hotplug", "wired" and "not used" if the
 		information is available, and "unknown" otherwise.
 
+What:		/sys/bus/usb/devices/.../(hub interface)/portX/usb3_lpm_permit
+Date:		November 2015
+Contact:	Lu Baolu <baolu.lu@linux.intel.com>
+Description:
+		Some USB3.0 devices are not friendly to USB3 LPM.  usb3_lpm_permit
+		attribute allows enabling/disabling usb3 lpm of a port. It takes
+		effect both before and after a usb device is enumerated. Supported
+		values are "0" if both u1 and u2 are NOT permitted, "u1" if only u1
+		is permitted, "u2" if only u2 is permitted, "u1_u2" if both u1 and
+		u2 are permitted.
+
 What:		/sys/bus/usb/devices/.../power/usb2_lpm_l1_timeout
 Date:		May 2013
 Contact:	Mathias Nyman <mathias.nyman@linux.intel.com>
diff --git a/Documentation/ABI/testing/sysfs-class-infiniband b/Documentation/ABI/testing/sysfs-class-infiniband
new file mode 100644
index 0000000..a86abe6
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-class-infiniband
@@ -0,0 +1,16 @@
+What:		/sys/class/infiniband/<hca>/ports/<port-number>/gid_attrs/ndevs/<gid-index>
+Date:		November 29, 2015
+KernelVersion:	4.4.0
+Contact:	linux-rdma@vger.kernel.org
+Description: 	The net-device's name associated with the GID resides
+		at index <gid-index>.
+
+What:		/sys/class/infiniband/<hca>/ports/<port-number>/gid_attrs/types/<gid-index>
+Date:		November 29, 2015
+KernelVersion:	4.4.0
+Contact:	linux-rdma@vger.kernel.org
+Description: 	The RoCE type of the associated GID resides at index <gid-index>.
+		This could either be "IB/RoCE v1" for IB and RoCE v1 based GODs
+		or "RoCE v2" for RoCE v2 based GIDs.
+
+
diff --git a/Documentation/ABI/testing/sysfs-class-net-cdc_ncm b/Documentation/ABI/testing/sysfs-class-net-cdc_ncm
index 5cedf72d..f7be0e8 100644
--- a/Documentation/ABI/testing/sysfs-class-net-cdc_ncm
+++ b/Documentation/ABI/testing/sysfs-class-net-cdc_ncm
@@ -19,6 +19,25 @@
 		Set to 0 to pad all frames. Set greater than tx_max to
 		disable all padding.
 
+What:		/sys/class/net/<iface>/cdc_ncm/ndp_to_end
+Date:		Dec 2015
+KernelVersion:	4.5
+Contact:	Bjørn Mork <bjorn@mork.no>
+Description:
+		Boolean attribute showing the status of the "NDP to
+		end" quirk.  Defaults to 'N', except for devices
+		already known to need it enabled.
+
+		The "NDP to end" quirk makes the driver place the NDP
+		(the packet index table) after the payload.  The NCM
+		specification does not mandate this, but some devices
+		are known to be more restrictive. Write 'Y' to this
+		attribute for temporary testing of a suspect device
+		failing to work with the default driver settings.
+
+		A device entry should be added to the driver if this
+		quirk is found to be required.
+
 What:		/sys/class/net/<iface>/cdc_ncm/rx_max
 Date:		May 2014
 KernelVersion:	3.16
diff --git a/Documentation/ABI/testing/sysfs-class-net-mesh b/Documentation/ABI/testing/sysfs-class-net-mesh
index c464062..c2b956d 100644
--- a/Documentation/ABI/testing/sysfs-class-net-mesh
+++ b/Documentation/ABI/testing/sysfs-class-net-mesh
@@ -8,7 +8,7 @@
 
 What:           /sys/class/net/<mesh_iface>/mesh/<vlan_subdir>/ap_isolation
 Date:           May 2011
-Contact:        Antonio Quartulli <antonio@meshcoding.com>
+Contact:        Antonio Quartulli <a@unstable.cc>
 Description:
                 Indicates whether the data traffic going from a
                 wireless client to another wireless client will be
@@ -70,7 +70,7 @@
 
 What:		/sys/class/net/<mesh_iface>/mesh/isolation_mark
 Date:		Nov 2013
-Contact:	Antonio Quartulli <antonio@meshcoding.com>
+Contact:	Antonio Quartulli <a@unstable.cc>
 Description:
 		Defines the isolation mark (and its bitmask) which
 		is used to classify clients as "isolated" by the
diff --git a/Documentation/ABI/testing/sysfs-class-net-qmi b/Documentation/ABI/testing/sysfs-class-net-qmi
new file mode 100644
index 0000000..fa5a00b
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-class-net-qmi
@@ -0,0 +1,23 @@
+What:		/sys/class/net/<iface>/qmi/raw_ip
+Date:		Dec 2015
+KernelVersion:	4.4
+Contact:	Bjørn Mork <bjorn@mork.no>
+Description:
+		Boolean.  Default: 'N'
+
+		Set this to 'Y' to change the network device link
+		framing from '802.3' to 'raw-ip'.
+
+		The netdev will change to reflect the link framing
+		mode.  The netdev is an ordinary ethernet device in
+		'802.3' mode, and the driver expects to exchange
+		frames with an ethernet header over the USB link. The
+		netdev is a headerless p-t-p device in 'raw-ip' mode,
+		and the driver expects to echange IPv4 or IPv6 packets
+		without any L2 header over the USB link.
+
+		Userspace is in full control of firmware configuration
+		through the delegation of the QMI protocol. Userspace
+		is responsible for coordination of driver and firmware
+		link framing mode, changing this setting to 'Y' if the
+		firmware is configured for 'raw-ip' mode.
diff --git a/Documentation/ABI/testing/sysfs-class-watchdog b/Documentation/ABI/testing/sysfs-class-watchdog
new file mode 100644
index 0000000..736046b
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-class-watchdog
@@ -0,0 +1,51 @@
+What:		/sys/class/watchdog/watchdogn/bootstatus
+Date:		August 2015
+Contact:	Wim Van Sebroeck <wim@iguana.be>
+Description:
+		It is a read only file. It contains status of the watchdog
+		device at boot. It is equivalent to WDIOC_GETBOOTSTATUS of
+		ioctl interface.
+
+What:		/sys/class/watchdog/watchdogn/identity
+Date:		August 2015
+Contact:	Wim Van Sebroeck <wim@iguana.be>
+Description:
+		It is a read only file. It contains identity string of
+		watchdog device.
+
+What:		/sys/class/watchdog/watchdogn/nowayout
+Date:		August 2015
+Contact:	Wim Van Sebroeck <wim@iguana.be>
+Description:
+		It is a read only file. While reading, it gives '1' if that
+		device supports nowayout feature else, it gives '0'.
+
+What:		/sys/class/watchdog/watchdogn/state
+Date:		August 2015
+Contact:	Wim Van Sebroeck <wim@iguana.be>
+Description:
+		It is a read only file. It gives active/inactive status of
+		watchdog device.
+
+What:		/sys/class/watchdog/watchdogn/status
+Date:		August 2015
+Contact:	Wim Van Sebroeck <wim@iguana.be>
+Description:
+		It is a read only file. It contains watchdog device's
+		internal status bits. It is equivalent to WDIOC_GETSTATUS
+		of ioctl interface.
+
+What:		/sys/class/watchdog/watchdogn/timeleft
+Date:		August 2015
+Contact:	Wim Van Sebroeck <wim@iguana.be>
+Description:
+		It is a read only file. It contains value of time left for
+		reset generation. It is equivalent to WDIOC_GETTIMELEFT of
+		ioctl interface.
+
+What:		/sys/class/watchdog/watchdogn/timeout
+Date:		August 2015
+Contact:	Wim Van Sebroeck <wim@iguana.be>
+Description:
+		It is a read only file. It is read to know about current
+		value of timeout programmed.
diff --git a/Documentation/ABI/testing/sysfs-fs-f2fs b/Documentation/ABI/testing/sysfs-fs-f2fs
index 0345f2d..e5200f3 100644
--- a/Documentation/ABI/testing/sysfs-fs-f2fs
+++ b/Documentation/ABI/testing/sysfs-fs-f2fs
@@ -87,6 +87,12 @@
 Description:
 		 Controls the checkpoint timing.
 
+What:		/sys/fs/f2fs/<disk>/idle_interval
+Date:		January 2016
+Contact:	"Jaegeuk Kim" <jaegeuk@kernel.org>
+Description:
+		 Controls the idle timing.
+
 What:		/sys/fs/f2fs/<disk>/ra_nid_pages
 Date:		October 2015
 Contact:	"Chao Yu" <chao2.yu@samsung.com>
diff --git a/Documentation/ABI/testing/sysfs-kernel-livepatch b/Documentation/ABI/testing/sysfs-kernel-livepatch
index 5bf42a8..da87f43 100644
--- a/Documentation/ABI/testing/sysfs-kernel-livepatch
+++ b/Documentation/ABI/testing/sysfs-kernel-livepatch
@@ -33,7 +33,7 @@
 		The object directory contains subdirectories for each function
 		that is patched within the object.
 
-What:		/sys/kernel/livepatch/<patch>/<object>/<function>
+What:		/sys/kernel/livepatch/<patch>/<object>/<function,sympos>
 Date:		Nov 2014
 KernelVersion:	3.19.0
 Contact:	live-patching@vger.kernel.org
@@ -41,4 +41,8 @@
 		The function directory contains attributes regarding the
 		properties and state of the patched function.
 
+		The directory name contains the patched function name and a
+		sympos number corresponding to the nth occurrence of the symbol
+		name in kallsyms for the patched object.
+
 		There are currently no such attributes.
diff --git a/Documentation/ABI/testing/sysfs-ptp b/Documentation/ABI/testing/sysfs-ptp
index 44806a6..a17f817 100644
--- a/Documentation/ABI/testing/sysfs-ptp
+++ b/Documentation/ABI/testing/sysfs-ptp
@@ -74,7 +74,7 @@
 		assignment may be changed by two writing numbers into
 		the file.
 
-What:		/sys/class/ptp/ptpN/pps_avaiable
+What:		/sys/class/ptp/ptpN/pps_available
 Date:		September 2010
 Contact:	Richard Cochran <richardcochran@gmail.com>
 Description:
diff --git a/Documentation/CodingStyle b/Documentation/CodingStyle
index c06f817..db65377 100644
--- a/Documentation/CodingStyle
+++ b/Documentation/CodingStyle
@@ -430,7 +430,7 @@
 		return result;
 	}
 
-A common type of bug to be aware of it "one err bugs" which look like this:
+A common type of bug to be aware of is "one err bugs" which look like this:
 
 	err:
 		kfree(foo->bar);
diff --git a/Documentation/DMA-API-HOWTO.txt b/Documentation/DMA-API-HOWTO.txt
index d69b3fc..781024e 100644
--- a/Documentation/DMA-API-HOWTO.txt
+++ b/Documentation/DMA-API-HOWTO.txt
@@ -951,16 +951,6 @@
    alignment constraints (e.g. the alignment constraints about 64-bit
    objects).
 
-3) Supporting multiple types of IOMMUs
-
-   If your architecture needs to support multiple types of IOMMUs, you
-   can use include/linux/asm-generic/dma-mapping-common.h. It's a
-   library to support the DMA API with multiple types of IOMMUs. Lots
-   of architectures (x86, powerpc, sh, alpha, ia64, microblaze and
-   sparc) use it. Choose one to see how it can be used. If you need to
-   support multiple types of IOMMUs in a single system, the example of
-   x86 or powerpc helps.
-
 			   Closing
 
 This document, and the API itself, would not be in its current
diff --git a/Documentation/DMA-API.txt b/Documentation/DMA-API.txt
index 1e98a7e..45ef3f2 100644
--- a/Documentation/DMA-API.txt
+++ b/Documentation/DMA-API.txt
@@ -236,7 +236,7 @@
 
 DMA_TO_DEVICE synchronisation must be done after the last modification
 of the memory region by the software and before it is handed off to
-the driver.  Once this primitive is used, memory covered by this
+the device.  Once this primitive is used, memory covered by this
 primitive should be treated as read-only by the device.  If the device
 may write to it at any point, it should be DMA_BIDIRECTIONAL (see
 below).
diff --git a/Documentation/DocBook/Makefile b/Documentation/DocBook/Makefile
index 91f6d89..d70f9b6 100644
--- a/Documentation/DocBook/Makefile
+++ b/Documentation/DocBook/Makefile
@@ -50,8 +50,7 @@
 
 HTML := $(sort $(patsubst %.xml, %.html, $(BOOKS)))
 htmldocs: $(HTML)
-	$(call build_main_index)
-	$(call build_images)
+	$(call cmd,build_main_index)
 	$(call install_media_images)
 
 MAN := $(patsubst %.xml, %.9, $(BOOKS))
@@ -139,7 +138,8 @@
 
 index = index.html
 main_idx = $(obj)/$(index)
-build_main_index = rm -rf $(main_idx); \
+quiet_cmd_build_main_index = HTML    $(main_idx)
+      cmd_build_main_index = rm -rf $(main_idx); \
 		   echo '<h1>Linux Kernel HTML Documentation</h1>' >> $(main_idx) && \
 		   echo '<h2>Kernel Version: $(KERNELVERSION)</h2>' >> $(main_idx) && \
 		   cat $(HTML) >> $(main_idx)
@@ -227,6 +227,10 @@
 	@echo  '  mandocs         - man pages'
 	@echo  '  installmandocs  - install man pages generated by mandocs'
 	@echo  '  cleandocs       - clean all generated DocBook files'
+	@echo
+	@echo  'make DOCBOOKS="s1.xml s2.xml" [target] Generate only docs s1.xml s2.xml'
+	@echo  '  valid values for DOCBOOKS are: $(DOCBOOKS)'
+
 
 ###
 # Temporary files left by various tools
diff --git a/Documentation/DocBook/device-drivers.tmpl b/Documentation/DocBook/device-drivers.tmpl
index 42a2d85..cdd8b24 100644
--- a/Documentation/DocBook/device-drivers.tmpl
+++ b/Documentation/DocBook/device-drivers.tmpl
@@ -238,83 +238,32 @@
 !Iinclude/media/videobuf2-memops.h
      </sect1>
      <sect1><title>Digital TV (DVB) devices</title>
-!Idrivers/media/dvb-core/dvb_ca_en50221.h
-!Idrivers/media/dvb-core/dvb_frontend.h
+	<sect1><title>Digital TV Common functions</title>
 !Idrivers/media/dvb-core/dvb_math.h
 !Idrivers/media/dvb-core/dvb_ringbuffer.h
 !Idrivers/media/dvb-core/dvbdev.h
-	<sect1><title>Digital TV Demux API</title>
-	    <para>The kernel demux API defines a driver-internal interface for
-	    registering low-level, hardware specific driver to a hardware
-	    independent demux layer. It is only of interest for Digital TV
-	    device driver writers. The header file for this API is named
-	    <constant>demux.h</constant> and located in
-	    <constant>drivers/media/dvb-core</constant>.</para>
-
-	<para>The demux API should be implemented for each demux in the
-	system. It is used to select the TS source of a demux and to manage
-	the demux resources. When the demux client allocates a resource via
-	the demux API, it receives a pointer to the API of that
-	resource.</para>
-	<para>Each demux receives its TS input from a DVB front-end or from
-	memory, as set via this demux API. In a system with more than one
-	front-end, the API can be used to select one of the DVB front-ends
-	as a TS source for a demux, unless this is fixed in the HW platform.
-	The demux API only controls front-ends regarding to their connections
-	with demuxes; the APIs used to set the other front-end parameters,
-	such as tuning, are not defined in this document.</para>
-	<para>The functions that implement the abstract interface demux should
-	be defined static or module private and registered to the Demux
-	core for external access. It is not necessary to implement every
-	function in the struct <constant>dmx_demux</constant>. For example,
-	a demux interface might support Section filtering, but not PES
-	filtering. The API client is expected to check the value of any
-	function pointer before calling the function: the value of NULL means
-	that the &#8220;function is not available&#8221;.</para>
-	<para>Whenever the functions of the demux API modify shared data,
-	the possibilities of lost update and race condition problems should
-	be addressed, e.g. by protecting parts of code with mutexes.</para>
-	<para>Note that functions called from a bottom half context must not
-	sleep. Even a simple memory allocation without using GFP_ATOMIC can
-	result in a kernel thread being put to sleep if swapping is needed.
-	For example, the Linux kernel calls the functions of a network device
-	interface from a bottom half context. Thus, if a demux API function
-	is called from network device code, the function must not sleep.
-	</para>
-    </sect1>
-
-    <section id="demux_callback_api">
-	<title>Demux Callback API</title>
-	<para>This kernel-space API comprises the callback functions that
-	deliver filtered data to the demux client. Unlike the other DVB
-	kABIs, these functions are provided by the client and called from
-	the demux code.</para>
-	<para>The function pointers of this abstract interface are not
-	packed into a structure as in the other demux APIs, because the
-	callback functions are registered and used independent of each
-	other. As an example, it is possible for the API client to provide
-	several callback functions for receiving TS packets and no
-	callbacks for PES packets or sections.</para>
-	<para>The functions that implement the callback API need not be
-	re-entrant: when a demux driver calls one of these functions,
-	the driver is not allowed to call the function again before
-	the original call returns. If a callback is triggered by a
-	hardware interrupt, it is recommended to use the Linux
-	&#8220;bottom half&#8221; mechanism or start a tasklet instead of
-	making the callback function call directly from a hardware
-	interrupt.</para>
-	<para>This mechanism is implemented by
-	<link linkend='API-dmx-ts-cb'>dmx_ts_cb()</link> and
-	<link linkend='API-dmx-section-cb'>dmx_section_cb()</link>.</para>
-    </section>
-
+	</sect1>
+	<sect1><title>Digital TV Frontend kABI</title>
+!Pdrivers/media/dvb-core/dvb_frontend.h Digital TV Frontend
+!Idrivers/media/dvb-core/dvb_frontend.h
+	</sect1>
+	<sect1><title>Digital TV Demux kABI</title>
+!Pdrivers/media/dvb-core/demux.h Digital TV Demux
+	<sect1><title>Demux Callback API</title>
+!Pdrivers/media/dvb-core/demux.h Demux Callback
+	</sect1>
 !Idrivers/media/dvb-core/demux.h
-    </sect1>
+	</sect1>
+	<sect1><title>Digital TV Conditional Access kABI</title>
+!Idrivers/media/dvb-core/dvb_ca_en50221.h
+	</sect1>
+     </sect1>
     <sect1><title>Remote Controller devices</title>
 !Iinclude/media/rc-core.h
 !Iinclude/media/lirc_dev.h
     </sect1>
     <sect1><title>Media Controller devices</title>
+!Pinclude/media/media-device.h Media Controller
 !Iinclude/media/media-device.h
 !Iinclude/media/media-devnode.h
 !Iinclude/media/media-entity.h
diff --git a/Documentation/DocBook/gpu.tmpl b/Documentation/DocBook/gpu.tmpl
index 201dcd3..a866933 100644
--- a/Documentation/DocBook/gpu.tmpl
+++ b/Documentation/DocBook/gpu.tmpl
@@ -124,6 +124,43 @@
     <para>
       [Insert diagram of typical DRM stack here]
     </para>
+  <sect1>
+    <title>Style Guidelines</title>
+    <para>
+      For consistency this documentation uses American English. Abbreviations
+      are written as all-uppercase, for example: DRM, KMS, IOCTL, CRTC, and so
+      on. To aid in reading, documentations make full use of the markup
+      characters kerneldoc provides: @parameter for function parameters, @member
+      for structure members, &amp;structure to reference structures and
+      function() for functions. These all get automatically hyperlinked if
+      kerneldoc for the referenced objects exists. When referencing entries in
+      function vtables please use -&gt;vfunc(). Note that kerneldoc does
+      not support referencing struct members directly, so please add a reference
+      to the vtable struct somewhere in the same paragraph or at least section.
+    </para>
+    <para>
+      Except in special situations (to separate locked from unlocked variants)
+      locking requirements for functions aren't documented in the kerneldoc.
+      Instead locking should be check at runtime using e.g.
+      <code>WARN_ON(!mutex_is_locked(...));</code>. Since it's much easier to
+      ignore documentation than runtime noise this provides more value. And on
+      top of that runtime checks do need to be updated when the locking rules
+      change, increasing the chances that they're correct. Within the
+      documentation the locking rules should be explained in the relevant
+      structures: Either in the comment for the lock explaining what it
+      protects, or data fields need a note about which lock protects them, or
+      both.
+    </para>
+    <para>
+      Functions which have a non-<code>void</code> return value should have a
+      section called "Returns" explaining the expected return values in
+      different cases and their meanings. Currently there's no consensus whether
+      that section name should be all upper-case or not, and whether it should
+      end in a colon or not. Go with the file-local style. Other common section
+      names are "Notes" with information for dangerous or tricky corner cases,
+      and "FIXME" where the interface could be cleaned up.
+    </para>
+  </sect1>
   </chapter>
 
   <!-- Internals -->
@@ -615,18 +652,6 @@
           <function>drm_gem_object_init</function>. Storage for private GEM
           objects must be managed by drivers.
         </para>
-        <para>
-          Drivers that do not need to extend GEM objects with private information
-          can call the <function>drm_gem_object_alloc</function> function to
-          allocate and initialize a struct <structname>drm_gem_object</structname>
-          instance. The GEM core will call the optional driver
-          <methodname>gem_init_object</methodname> operation after initializing
-          the GEM object with <function>drm_gem_object_init</function>.
-          <synopsis>int (*gem_init_object) (struct drm_gem_object *obj);</synopsis>
-        </para>
-        <para>
-          No alloc-and-init function exists for private GEM objects.
-        </para>
       </sect3>
       <sect3>
         <title>GEM Objects Lifetime</title>
@@ -635,10 +660,10 @@
           acquired and release by <function>calling drm_gem_object_reference</function>
           and <function>drm_gem_object_unreference</function> respectively. The
           caller must hold the <structname>drm_device</structname>
-          <structfield>struct_mutex</structfield> lock. As a convenience, GEM
-          provides the <function>drm_gem_object_reference_unlocked</function> and
-          <function>drm_gem_object_unreference_unlocked</function> functions that
-          can be called without holding the lock.
+	  <structfield>struct_mutex</structfield> lock when calling
+	  <function>drm_gem_object_reference</function>. As a convenience, GEM
+	  provides <function>drm_gem_object_unreference_unlocked</function>
+	  functions that can be called without holding the lock.
         </para>
         <para>
           When the last reference to a GEM object is released the GEM core calls
@@ -649,15 +674,9 @@
         </para>
         <para>
           <synopsis>void (*gem_free_object) (struct drm_gem_object *obj);</synopsis>
-          Drivers are responsible for freeing all GEM object resources, including
-          the resources created by the GEM core. If an mmap offset has been
-          created for the object (in which case
-          <structname>drm_gem_object</structname>::<structfield>map_list</structfield>::<structfield>map</structfield>
-          is not NULL) it must be freed by a call to
-          <function>drm_gem_free_mmap_offset</function>. The shmfs backing store
-          must be released by calling <function>drm_gem_object_release</function>
-          (that function can safely be called if no shmfs backing store has been
-          created).
+          Drivers are responsible for freeing all GEM object resources. This includes
+          the resources created by the GEM core, which need to be released with
+          <function>drm_gem_object_release</function>.
         </para>
       </sect3>
       <sect3>
@@ -740,17 +759,10 @@
           DRM identifies the GEM object to be mapped by a fake offset passed
           through the mmap offset argument. Prior to being mapped, a GEM object
           must thus be associated with a fake offset. To do so, drivers must call
-          <function>drm_gem_create_mmap_offset</function> on the object. The
-          function allocates a fake offset range from a pool and stores the
-          offset divided by PAGE_SIZE in
-          <literal>obj-&gt;map_list.hash.key</literal>. Care must be taken not to
-          call <function>drm_gem_create_mmap_offset</function> if a fake offset
-          has already been allocated for the object. This can be tested by
-          <literal>obj-&gt;map_list.map</literal> being non-NULL.
+          <function>drm_gem_create_mmap_offset</function> on the object.
         </para>
         <para>
           Once allocated, the fake offset value
-          (<literal>obj-&gt;map_list.hash.key &lt;&lt; PAGE_SHIFT</literal>)
           must be passed to the application in a driver-specific way and can then
           be used as the mmap offset argument.
         </para>
@@ -836,10 +848,11 @@
           abstracted from the client in libdrm.
         </para>
       </sect3>
-      <sect3>
-        <title>GEM Function Reference</title>
+    </sect2>
+    <sect2>
+      <title>GEM Function Reference</title>
 !Edrivers/gpu/drm/drm_gem.c
-      </sect3>
+!Iinclude/drm/drm_gem.h
     </sect2>
     <sect2>
       <title>VMA Offset Manager</title>
@@ -970,12 +983,10 @@
     <sect2>
       <title>Atomic Mode Setting Function Reference</title>
 !Edrivers/gpu/drm/drm_atomic.c
+!Idrivers/gpu/drm/drm_atomic.c
     </sect2>
     <sect2>
-      <title>Frame Buffer Creation</title>
-      <synopsis>struct drm_framebuffer *(*fb_create)(struct drm_device *dev,
-				     struct drm_file *file_priv,
-				     struct drm_mode_fb_cmd2 *mode_cmd);</synopsis>
+      <title>Frame Buffer Abstraction</title>
       <para>
         Frame buffers are abstract memory objects that provide a source of
         pixels to scanout to a CRTC. Applications explicitly request the
@@ -994,73 +1005,6 @@
 	and so expects TTM handles in the create ioctl and not GEM handles.
       </para>
       <para>
-        Drivers must first validate the requested frame buffer parameters passed
-        through the mode_cmd argument. In particular this is where invalid
-        sizes, pixel formats or pitches can be caught.
-      </para>
-      <para>
-        If the parameters are deemed valid, drivers then create, initialize and
-        return an instance of struct <structname>drm_framebuffer</structname>.
-        If desired the instance can be embedded in a larger driver-specific
-	structure. Drivers must fill its <structfield>width</structfield>,
-	<structfield>height</structfield>, <structfield>pitches</structfield>,
-        <structfield>offsets</structfield>, <structfield>depth</structfield>,
-        <structfield>bits_per_pixel</structfield> and
-        <structfield>pixel_format</structfield> fields from the values passed
-        through the <parameter>drm_mode_fb_cmd2</parameter> argument. They
-        should call the <function>drm_helper_mode_fill_fb_struct</function>
-        helper function to do so.
-      </para>
-
-      <para>
-	The initialization of the new framebuffer instance is finalized with a
-	call to <function>drm_framebuffer_init</function> which takes a pointer
-	to DRM frame buffer operations (struct
-	<structname>drm_framebuffer_funcs</structname>). Note that this function
-	publishes the framebuffer and so from this point on it can be accessed
-	concurrently from other threads. Hence it must be the last step in the
-	driver's framebuffer initialization sequence. Frame buffer operations
-	are
-        <itemizedlist>
-          <listitem>
-            <synopsis>int (*create_handle)(struct drm_framebuffer *fb,
-		     struct drm_file *file_priv, unsigned int *handle);</synopsis>
-            <para>
-              Create a handle to the frame buffer underlying memory object. If
-              the frame buffer uses a multi-plane format, the handle will
-              reference the memory object associated with the first plane.
-            </para>
-            <para>
-              Drivers call <function>drm_gem_handle_create</function> to create
-              the handle.
-            </para>
-          </listitem>
-          <listitem>
-            <synopsis>void (*destroy)(struct drm_framebuffer *framebuffer);</synopsis>
-            <para>
-              Destroy the frame buffer object and frees all associated
-              resources. Drivers must call
-              <function>drm_framebuffer_cleanup</function> to free resources
-              allocated by the DRM core for the frame buffer object, and must
-              make sure to unreference all memory objects associated with the
-              frame buffer. Handles created by the
-              <methodname>create_handle</methodname> operation are released by
-              the DRM core.
-            </para>
-          </listitem>
-          <listitem>
-            <synopsis>int (*dirty)(struct drm_framebuffer *framebuffer,
-	     struct drm_file *file_priv, unsigned flags, unsigned color,
-	     struct drm_clip_rect *clips, unsigned num_clips);</synopsis>
-            <para>
-              This optional operation notifies the driver that a region of the
-              frame buffer has changed in response to a DRM_IOCTL_MODE_DIRTYFB
-              ioctl call.
-            </para>
-          </listitem>
-        </itemizedlist>
-      </para>
-      <para>
 	The lifetime of a drm framebuffer is controlled with a reference count,
 	drivers can grab additional references with
 	<function>drm_framebuffer_reference</function>and drop them
@@ -1197,137 +1141,6 @@
           pointer to CRTC functions.
         </para>
       </sect3>
-      <sect3 id="drm-kms-crtcops">
-        <title>CRTC Operations</title>
-        <sect4>
-          <title>Set Configuration</title>
-          <synopsis>int (*set_config)(struct drm_mode_set *set);</synopsis>
-          <para>
-            Apply a new CRTC configuration to the device. The configuration
-            specifies a CRTC, a frame buffer to scan out from, a (x,y) position in
-            the frame buffer, a display mode and an array of connectors to drive
-            with the CRTC if possible.
-          </para>
-          <para>
-            If the frame buffer specified in the configuration is NULL, the driver
-            must detach all encoders connected to the CRTC and all connectors
-            attached to those encoders and disable them.
-          </para>
-          <para>
-            This operation is called with the mode config lock held.
-          </para>
-          <note><para>
-	    Note that the drm core has no notion of restoring the mode setting
-	    state after resume, since all resume handling is in the full
-	    responsibility of the driver. The common mode setting helper library
-	    though provides a helper which can be used for this:
-	    <function>drm_helper_resume_force_mode</function>.
-          </para></note>
-        </sect4>
-        <sect4>
-          <title>Page Flipping</title>
-          <synopsis>int (*page_flip)(struct drm_crtc *crtc, struct drm_framebuffer *fb,
-                   struct drm_pending_vblank_event *event);</synopsis>
-          <para>
-            Schedule a page flip to the given frame buffer for the CRTC. This
-            operation is called with the mode config mutex held.
-          </para>
-          <para>
-            Page flipping is a synchronization mechanism that replaces the frame
-            buffer being scanned out by the CRTC with a new frame buffer during
-            vertical blanking, avoiding tearing. When an application requests a page
-            flip the DRM core verifies that the new frame buffer is large enough to
-            be scanned out by  the CRTC in the currently configured mode and then
-            calls the CRTC <methodname>page_flip</methodname> operation with a
-            pointer to the new frame buffer.
-          </para>
-          <para>
-            The <methodname>page_flip</methodname> operation schedules a page flip.
-            Once any pending rendering targeting the new frame buffer has
-            completed, the CRTC will be reprogrammed to display that frame buffer
-            after the next vertical refresh. The operation must return immediately
-            without waiting for rendering or page flip to complete and must block
-            any new rendering to the frame buffer until the page flip completes.
-          </para>
-          <para>
-            If a page flip can be successfully scheduled the driver must set the
-            <code>drm_crtc-&gt;fb</code> field to the new framebuffer pointed to
-            by <code>fb</code>. This is important so that the reference counting
-            on framebuffers stays balanced.
-          </para>
-          <para>
-            If a page flip is already pending, the
-            <methodname>page_flip</methodname> operation must return
-            -<errorname>EBUSY</errorname>.
-          </para>
-          <para>
-            To synchronize page flip to vertical blanking the driver will likely
-            need to enable vertical blanking interrupts. It should call
-            <function>drm_vblank_get</function> for that purpose, and call
-            <function>drm_vblank_put</function> after the page flip completes.
-          </para>
-          <para>
-            If the application has requested to be notified when page flip completes
-            the <methodname>page_flip</methodname> operation will be called with a
-            non-NULL <parameter>event</parameter> argument pointing to a
-            <structname>drm_pending_vblank_event</structname> instance. Upon page
-            flip completion the driver must call <methodname>drm_send_vblank_event</methodname>
-            to fill in the event and send to wake up any waiting processes.
-            This can be performed with
-            <programlisting><![CDATA[
-            spin_lock_irqsave(&dev->event_lock, flags);
-            ...
-            drm_send_vblank_event(dev, pipe, event);
-            spin_unlock_irqrestore(&dev->event_lock, flags);
-            ]]></programlisting>
-          </para>
-          <note><para>
-            FIXME: Could drivers that don't need to wait for rendering to complete
-            just add the event to <literal>dev-&gt;vblank_event_list</literal> and
-            let the DRM core handle everything, as for "normal" vertical blanking
-            events?
-          </para></note>
-          <para>
-            While waiting for the page flip to complete, the
-            <literal>event-&gt;base.link</literal> list head can be used freely by
-            the driver to store the pending event in a driver-specific list.
-          </para>
-          <para>
-            If the file handle is closed before the event is signaled, drivers must
-            take care to destroy the event in their
-            <methodname>preclose</methodname> operation (and, if needed, call
-            <function>drm_vblank_put</function>).
-          </para>
-        </sect4>
-        <sect4>
-          <title>Miscellaneous</title>
-          <itemizedlist>
-            <listitem>
-              <synopsis>void (*set_property)(struct drm_crtc *crtc,
-                     struct drm_property *property, uint64_t value);</synopsis>
-              <para>
-                Set the value of the given CRTC property to
-                <parameter>value</parameter>. See <xref linkend="drm-kms-properties"/>
-                for more information about properties.
-              </para>
-            </listitem>
-            <listitem>
-              <synopsis>void (*gamma_set)(struct drm_crtc *crtc, u16 *r, u16 *g, u16 *b,
-                        uint32_t start, uint32_t size);</synopsis>
-              <para>
-                Apply a gamma table to the device. The operation is optional.
-              </para>
-            </listitem>
-            <listitem>
-              <synopsis>void (*destroy)(struct drm_crtc *crtc);</synopsis>
-              <para>
-                Destroy the CRTC when not needed anymore. See
-                <xref linkend="drm-kms-init"/>.
-              </para>
-            </listitem>
-          </itemizedlist>
-        </sect4>
-      </sect3>
     </sect2>
     <sect2>
       <title>Planes (struct <structname>drm_plane</structname>)</title>
@@ -1344,7 +1157,7 @@
         <listitem>
         DRM_PLANE_TYPE_PRIMARY represents a "main" plane for a CRTC.  Primary
         planes are the planes operated upon by CRTC modesetting and flipping
-        operations described in <xref linkend="drm-kms-crtcops"/>.
+	operations described in the page_flip hook in <structname>drm_crtc_funcs</structname>.
         </listitem>
         <listitem>
         DRM_PLANE_TYPE_CURSOR represents a "cursor" plane for a CRTC.  Cursor
@@ -1381,52 +1194,6 @@
           primary plane with standard capabilities.
         </para>
       </sect3>
-      <sect3>
-        <title>Plane Operations</title>
-        <itemizedlist>
-          <listitem>
-            <synopsis>int (*update_plane)(struct drm_plane *plane, struct drm_crtc *crtc,
-                        struct drm_framebuffer *fb, int crtc_x, int crtc_y,
-                        unsigned int crtc_w, unsigned int crtc_h,
-                        uint32_t src_x, uint32_t src_y,
-                        uint32_t src_w, uint32_t src_h);</synopsis>
-            <para>
-              Enable and configure the plane to use the given CRTC and frame buffer.
-            </para>
-            <para>
-              The source rectangle in frame buffer memory coordinates is given by
-              the <parameter>src_x</parameter>, <parameter>src_y</parameter>,
-              <parameter>src_w</parameter> and <parameter>src_h</parameter>
-              parameters (as 16.16 fixed point values). Devices that don't support
-              subpixel plane coordinates can ignore the fractional part.
-            </para>
-            <para>
-              The destination rectangle in CRTC coordinates is given by the
-              <parameter>crtc_x</parameter>, <parameter>crtc_y</parameter>,
-              <parameter>crtc_w</parameter> and <parameter>crtc_h</parameter>
-              parameters (as integer values). Devices scale the source rectangle to
-              the destination rectangle. If scaling is not supported, and the source
-              rectangle size doesn't match the destination rectangle size, the
-              driver must return a -<errorname>EINVAL</errorname> error.
-            </para>
-          </listitem>
-          <listitem>
-            <synopsis>int (*disable_plane)(struct drm_plane *plane);</synopsis>
-            <para>
-              Disable the plane. The DRM core calls this method in response to a
-              DRM_IOCTL_MODE_SETPLANE ioctl call with the frame buffer ID set to 0.
-              Disabled planes must not be processed by the CRTC.
-            </para>
-          </listitem>
-          <listitem>
-            <synopsis>void (*destroy)(struct drm_plane *plane);</synopsis>
-            <para>
-              Destroy the plane when not needed anymore. See
-              <xref linkend="drm-kms-init"/>.
-            </para>
-          </listitem>
-        </itemizedlist>
-      </sect3>
     </sect2>
     <sect2>
       <title>Encoders (struct <structname>drm_encoder</structname>)</title>
@@ -1483,27 +1250,6 @@
           encoders they want to use to a CRTC.
         </para>
       </sect3>
-      <sect3>
-        <title>Encoder Operations</title>
-        <itemizedlist>
-          <listitem>
-            <synopsis>void (*destroy)(struct drm_encoder *encoder);</synopsis>
-            <para>
-              Called to destroy the encoder when not needed anymore. See
-              <xref linkend="drm-kms-init"/>.
-            </para>
-          </listitem>
-          <listitem>
-            <synopsis>void (*set_property)(struct drm_plane *plane,
-                     struct drm_property *property, uint64_t value);</synopsis>
-            <para>
-              Set the value of the given plane property to
-              <parameter>value</parameter>. See <xref linkend="drm-kms-properties"/>
-              for more information about properties.
-            </para>
-          </listitem>
-        </itemizedlist>
-      </sect3>
     </sect2>
     <sect2>
       <title>Connectors (struct <structname>drm_connector</structname>)</title>
@@ -1707,27 +1453,6 @@
             connector_status_unknown.
           </para>
         </sect4>
-        <sect4>
-          <title>Miscellaneous</title>
-          <itemizedlist>
-            <listitem>
-              <synopsis>void (*set_property)(struct drm_connector *connector,
-                     struct drm_property *property, uint64_t value);</synopsis>
-              <para>
-                Set the value of the given connector property to
-                <parameter>value</parameter>. See <xref linkend="drm-kms-properties"/>
-                for more information about properties.
-              </para>
-            </listitem>
-            <listitem>
-              <synopsis>void (*destroy)(struct drm_connector *connector);</synopsis>
-              <para>
-                Destroy the connector when not needed anymore. See
-                <xref linkend="drm-kms-init"/>.
-              </para>
-            </listitem>
-          </itemizedlist>
-        </sect4>
       </sect3>
     </sect2>
     <sect2>
@@ -1854,462 +1579,6 @@
       entities.
     </para>
     <sect2>
-      <title>Helper Functions</title>
-      <itemizedlist>
-        <listitem>
-          <synopsis>int drm_crtc_helper_set_config(struct drm_mode_set *set);</synopsis>
-          <para>
-            The <function>drm_crtc_helper_set_config</function> helper function
-            is a CRTC <methodname>set_config</methodname> implementation. It
-            first tries to locate the best encoder for each connector by calling
-            the connector <methodname>best_encoder</methodname> helper
-            operation.
-          </para>
-          <para>
-            After locating the appropriate encoders, the helper function will
-            call the <methodname>mode_fixup</methodname> encoder and CRTC helper
-            operations to adjust the requested mode, or reject it completely in
-            which case an error will be returned to the application. If the new
-            configuration after mode adjustment is identical to the current
-            configuration the helper function will return without performing any
-            other operation.
-          </para>
-          <para>
-            If the adjusted mode is identical to the current mode but changes to
-            the frame buffer need to be applied, the
-            <function>drm_crtc_helper_set_config</function> function will call
-            the CRTC <methodname>mode_set_base</methodname> helper operation. If
-            the adjusted mode differs from the current mode, or if the
-            <methodname>mode_set_base</methodname> helper operation is not
-            provided, the helper function performs a full mode set sequence by
-            calling the <methodname>prepare</methodname>,
-            <methodname>mode_set</methodname> and
-            <methodname>commit</methodname> CRTC and encoder helper operations,
-            in that order.
-          </para>
-        </listitem>
-        <listitem>
-          <synopsis>void drm_helper_connector_dpms(struct drm_connector *connector, int mode);</synopsis>
-          <para>
-            The <function>drm_helper_connector_dpms</function> helper function
-            is a connector <methodname>dpms</methodname> implementation that
-            tracks power state of connectors. To use the function, drivers must
-            provide <methodname>dpms</methodname> helper operations for CRTCs
-            and encoders to apply the DPMS state to the device.
-          </para>
-          <para>
-            The mid-layer doesn't track the power state of CRTCs and encoders.
-            The <methodname>dpms</methodname> helper operations can thus be
-            called with a mode identical to the currently active mode.
-          </para>
-        </listitem>
-        <listitem>
-          <synopsis>int drm_helper_probe_single_connector_modes(struct drm_connector *connector,
-                                            uint32_t maxX, uint32_t maxY);</synopsis>
-          <para>
-            The <function>drm_helper_probe_single_connector_modes</function> helper
-            function is a connector <methodname>fill_modes</methodname>
-            implementation that updates the connection status for the connector
-            and then retrieves a list of modes by calling the connector
-            <methodname>get_modes</methodname> helper operation.
-          </para>
-         <para>
-            If the helper operation returns no mode, and if the connector status
-            is connector_status_connected, standard VESA DMT modes up to
-            1024x768 are automatically added to the modes list by a call to
-            <function>drm_add_modes_noedid</function>.
-          </para>
-          <para>
-            The function then filters out modes larger than
-            <parameter>max_width</parameter> and <parameter>max_height</parameter>
-            if specified. It finally calls the optional connector
-            <methodname>mode_valid</methodname> helper operation for each mode in
-            the probed list to check whether the mode is valid for the connector.
-          </para>
-        </listitem>
-      </itemizedlist>
-    </sect2>
-    <sect2>
-      <title>CRTC Helper Operations</title>
-      <itemizedlist>
-        <listitem id="drm-helper-crtc-mode-fixup">
-          <synopsis>bool (*mode_fixup)(struct drm_crtc *crtc,
-                       const struct drm_display_mode *mode,
-                       struct drm_display_mode *adjusted_mode);</synopsis>
-          <para>
-            Let CRTCs adjust the requested mode or reject it completely. This
-            operation returns true if the mode is accepted (possibly after being
-            adjusted) or false if it is rejected.
-          </para>
-          <para>
-            The <methodname>mode_fixup</methodname> operation should reject the
-            mode if it can't reasonably use it. The definition of "reasonable"
-            is currently fuzzy in this context. One possible behaviour would be
-            to set the adjusted mode to the panel timings when a fixed-mode
-            panel is used with hardware capable of scaling. Another behaviour
-            would be to accept any input mode and adjust it to the closest mode
-            supported by the hardware (FIXME: This needs to be clarified).
-          </para>
-        </listitem>
-        <listitem>
-          <synopsis>int (*mode_set_base)(struct drm_crtc *crtc, int x, int y,
-                     struct drm_framebuffer *old_fb)</synopsis>
-          <para>
-            Move the CRTC on the current frame buffer (stored in
-            <literal>crtc-&gt;fb</literal>) to position (x,y). Any of the frame
-            buffer, x position or y position may have been modified.
-          </para>
-          <para>
-            This helper operation is optional. If not provided, the
-            <function>drm_crtc_helper_set_config</function> function will fall
-            back to the <methodname>mode_set</methodname> helper operation.
-          </para>
-          <note><para>
-            FIXME: Why are x and y passed as arguments, as they can be accessed
-            through <literal>crtc-&gt;x</literal> and
-            <literal>crtc-&gt;y</literal>?
-          </para></note>
-        </listitem>
-        <listitem>
-          <synopsis>void (*prepare)(struct drm_crtc *crtc);</synopsis>
-          <para>
-            Prepare the CRTC for mode setting. This operation is called after
-            validating the requested mode. Drivers use it to perform
-            device-specific operations required before setting the new mode.
-          </para>
-        </listitem>
-        <listitem>
-          <synopsis>int (*mode_set)(struct drm_crtc *crtc, struct drm_display_mode *mode,
-                struct drm_display_mode *adjusted_mode, int x, int y,
-                struct drm_framebuffer *old_fb);</synopsis>
-          <para>
-            Set a new mode, position and frame buffer. Depending on the device
-            requirements, the mode can be stored internally by the driver and
-            applied in the <methodname>commit</methodname> operation, or
-            programmed to the hardware immediately.
-          </para>
-          <para>
-            The <methodname>mode_set</methodname> operation returns 0 on success
-	    or a negative error code if an error occurs.
-          </para>
-        </listitem>
-        <listitem>
-          <synopsis>void (*commit)(struct drm_crtc *crtc);</synopsis>
-          <para>
-            Commit a mode. This operation is called after setting the new mode.
-            Upon return the device must use the new mode and be fully
-            operational.
-          </para>
-        </listitem>
-      </itemizedlist>
-    </sect2>
-    <sect2>
-      <title>Encoder Helper Operations</title>
-      <itemizedlist>
-        <listitem>
-          <synopsis>bool (*mode_fixup)(struct drm_encoder *encoder,
-                       const struct drm_display_mode *mode,
-                       struct drm_display_mode *adjusted_mode);</synopsis>
-          <para>
-            Let encoders adjust the requested mode or reject it completely. This
-            operation returns true if the mode is accepted (possibly after being
-            adjusted) or false if it is rejected. See the
-            <link linkend="drm-helper-crtc-mode-fixup">mode_fixup CRTC helper
-            operation</link> for an explanation of the allowed adjustments.
-          </para>
-        </listitem>
-        <listitem>
-          <synopsis>void (*prepare)(struct drm_encoder *encoder);</synopsis>
-          <para>
-            Prepare the encoder for mode setting. This operation is called after
-            validating the requested mode. Drivers use it to perform
-            device-specific operations required before setting the new mode.
-          </para>
-        </listitem>
-        <listitem>
-          <synopsis>void (*mode_set)(struct drm_encoder *encoder,
-                 struct drm_display_mode *mode,
-                 struct drm_display_mode *adjusted_mode);</synopsis>
-          <para>
-            Set a new mode. Depending on the device requirements, the mode can
-            be stored internally by the driver and applied in the
-            <methodname>commit</methodname> operation, or programmed to the
-            hardware immediately.
-          </para>
-        </listitem>
-        <listitem>
-          <synopsis>void (*commit)(struct drm_encoder *encoder);</synopsis>
-          <para>
-            Commit a mode. This operation is called after setting the new mode.
-            Upon return the device must use the new mode and be fully
-            operational.
-          </para>
-        </listitem>
-      </itemizedlist>
-    </sect2>
-    <sect2>
-      <title>Connector Helper Operations</title>
-      <itemizedlist>
-        <listitem>
-          <synopsis>struct drm_encoder *(*best_encoder)(struct drm_connector *connector);</synopsis>
-          <para>
-            Return a pointer to the best encoder for the connecter. Device that
-            map connectors to encoders 1:1 simply return the pointer to the
-            associated encoder. This operation is mandatory.
-          </para>
-        </listitem>
-        <listitem>
-          <synopsis>int (*get_modes)(struct drm_connector *connector);</synopsis>
-          <para>
-            Fill the connector's <structfield>probed_modes</structfield> list
-            by parsing EDID data with <function>drm_add_edid_modes</function>,
-            adding standard VESA DMT modes with <function>drm_add_modes_noedid</function>,
-            or calling <function>drm_mode_probed_add</function> directly for every
-            supported mode and return the number of modes it has detected. This
-            operation is mandatory.
-          </para>
-          <para>
-            Note that the caller function will automatically add standard VESA
-            DMT modes up to 1024x768 if the <methodname>get_modes</methodname>
-            helper operation returns no mode and if the connector status is
-            connector_status_connected. There is no need to call
-            <function>drm_add_edid_modes</function> manually in that case.
-          </para>
-          <para>
-            When adding modes manually the driver creates each mode with a call to
-            <function>drm_mode_create</function> and must fill the following fields.
-            <itemizedlist>
-              <listitem>
-                <synopsis>__u32 type;</synopsis>
-                <para>
-                  Mode type bitmask, a combination of
-                  <variablelist>
-                    <varlistentry>
-                      <term>DRM_MODE_TYPE_BUILTIN</term>
-                      <listitem><para>not used?</para></listitem>
-                    </varlistentry>
-                    <varlistentry>
-                      <term>DRM_MODE_TYPE_CLOCK_C</term>
-                      <listitem><para>not used?</para></listitem>
-                    </varlistentry>
-                    <varlistentry>
-                      <term>DRM_MODE_TYPE_CRTC_C</term>
-                      <listitem><para>not used?</para></listitem>
-                    </varlistentry>
-                    <varlistentry>
-                      <term>
-        DRM_MODE_TYPE_PREFERRED - The preferred mode for the connector
-                      </term>
-                      <listitem>
-                        <para>not used?</para>
-                      </listitem>
-                    </varlistentry>
-                    <varlistentry>
-                      <term>DRM_MODE_TYPE_DEFAULT</term>
-                      <listitem><para>not used?</para></listitem>
-                    </varlistentry>
-                    <varlistentry>
-                      <term>DRM_MODE_TYPE_USERDEF</term>
-                      <listitem><para>not used?</para></listitem>
-                    </varlistentry>
-                    <varlistentry>
-                      <term>DRM_MODE_TYPE_DRIVER</term>
-                      <listitem>
-                        <para>
-                          The mode has been created by the driver (as opposed to
-                          to user-created modes).
-                        </para>
-                      </listitem>
-                    </varlistentry>
-                  </variablelist>
-                  Drivers must set the DRM_MODE_TYPE_DRIVER bit for all modes they
-                  create, and set the DRM_MODE_TYPE_PREFERRED bit for the preferred
-                  mode.
-                </para>
-              </listitem>
-              <listitem>
-                <synopsis>__u32 clock;</synopsis>
-                <para>Pixel clock frequency in kHz unit</para>
-              </listitem>
-              <listitem>
-                <synopsis>__u16 hdisplay, hsync_start, hsync_end, htotal;
-    __u16 vdisplay, vsync_start, vsync_end, vtotal;</synopsis>
-                <para>Horizontal and vertical timing information</para>
-                <screen><![CDATA[
-             Active                 Front           Sync           Back
-             Region                 Porch                          Porch
-    <-----------------------><----------------><-------------><-------------->
-
-      //////////////////////|
-     ////////////////////// |
-    //////////////////////  |..................               ................
-                                               _______________
-
-    <----- [hv]display ----->
-    <------------- [hv]sync_start ------------>
-    <--------------------- [hv]sync_end --------------------->
-    <-------------------------------- [hv]total ----------------------------->
-]]></screen>
-              </listitem>
-              <listitem>
-                <synopsis>__u16 hskew;
-    __u16 vscan;</synopsis>
-                <para>Unknown</para>
-              </listitem>
-              <listitem>
-                <synopsis>__u32 flags;</synopsis>
-                <para>
-                  Mode flags, a combination of
-                  <variablelist>
-                    <varlistentry>
-                      <term>DRM_MODE_FLAG_PHSYNC</term>
-                      <listitem><para>
-                        Horizontal sync is active high
-                      </para></listitem>
-                    </varlistentry>
-                    <varlistentry>
-                      <term>DRM_MODE_FLAG_NHSYNC</term>
-                      <listitem><para>
-                        Horizontal sync is active low
-                      </para></listitem>
-                    </varlistentry>
-                    <varlistentry>
-                      <term>DRM_MODE_FLAG_PVSYNC</term>
-                      <listitem><para>
-                        Vertical sync is active high
-                      </para></listitem>
-                    </varlistentry>
-                    <varlistentry>
-                      <term>DRM_MODE_FLAG_NVSYNC</term>
-                      <listitem><para>
-                        Vertical sync is active low
-                      </para></listitem>
-                    </varlistentry>
-                    <varlistentry>
-                      <term>DRM_MODE_FLAG_INTERLACE</term>
-                      <listitem><para>
-                        Mode is interlaced
-                      </para></listitem>
-                    </varlistentry>
-                    <varlistentry>
-                      <term>DRM_MODE_FLAG_DBLSCAN</term>
-                      <listitem><para>
-                        Mode uses doublescan
-                      </para></listitem>
-                    </varlistentry>
-                    <varlistentry>
-                      <term>DRM_MODE_FLAG_CSYNC</term>
-                      <listitem><para>
-                        Mode uses composite sync
-                      </para></listitem>
-                    </varlistentry>
-                    <varlistentry>
-                      <term>DRM_MODE_FLAG_PCSYNC</term>
-                      <listitem><para>
-                        Composite sync is active high
-                      </para></listitem>
-                    </varlistentry>
-                    <varlistentry>
-                      <term>DRM_MODE_FLAG_NCSYNC</term>
-                      <listitem><para>
-                        Composite sync is active low
-                      </para></listitem>
-                    </varlistentry>
-                    <varlistentry>
-                      <term>DRM_MODE_FLAG_HSKEW</term>
-                      <listitem><para>
-                        hskew provided (not used?)
-                      </para></listitem>
-                    </varlistentry>
-                    <varlistentry>
-                      <term>DRM_MODE_FLAG_BCAST</term>
-                      <listitem><para>
-                        not used?
-                      </para></listitem>
-                    </varlistentry>
-                    <varlistentry>
-                      <term>DRM_MODE_FLAG_PIXMUX</term>
-                      <listitem><para>
-                        not used?
-                      </para></listitem>
-                    </varlistentry>
-                    <varlistentry>
-                      <term>DRM_MODE_FLAG_DBLCLK</term>
-                      <listitem><para>
-                        not used?
-                      </para></listitem>
-                    </varlistentry>
-                    <varlistentry>
-                      <term>DRM_MODE_FLAG_CLKDIV2</term>
-                      <listitem><para>
-                        ?
-                      </para></listitem>
-                    </varlistentry>
-                  </variablelist>
-                </para>
-                <para>
-                  Note that modes marked with the INTERLACE or DBLSCAN flags will be
-                  filtered out by
-                  <function>drm_helper_probe_single_connector_modes</function> if
-                  the connector's <structfield>interlace_allowed</structfield> or
-                  <structfield>doublescan_allowed</structfield> field is set to 0.
-                </para>
-              </listitem>
-              <listitem>
-                <synopsis>char name[DRM_DISPLAY_MODE_LEN];</synopsis>
-                <para>
-                  Mode name. The driver must call
-                  <function>drm_mode_set_name</function> to fill the mode name from
-                  <structfield>hdisplay</structfield>,
-                  <structfield>vdisplay</structfield> and interlace flag after
-                  filling the corresponding fields.
-                </para>
-              </listitem>
-            </itemizedlist>
-          </para>
-          <para>
-            The <structfield>vrefresh</structfield> value is computed by
-            <function>drm_helper_probe_single_connector_modes</function>.
-          </para>
-          <para>
-            When parsing EDID data, <function>drm_add_edid_modes</function> fills the
-            connector <structfield>display_info</structfield>
-            <structfield>width_mm</structfield> and
-            <structfield>height_mm</structfield> fields. When creating modes
-            manually the <methodname>get_modes</methodname> helper operation must
-            set the <structfield>display_info</structfield>
-            <structfield>width_mm</structfield> and
-            <structfield>height_mm</structfield> fields if they haven't been set
-            already (for instance at initialization time when a fixed-size panel is
-            attached to the connector). The mode <structfield>width_mm</structfield>
-            and <structfield>height_mm</structfield> fields are only used internally
-            during EDID parsing and should not be set when creating modes manually.
-          </para>
-        </listitem>
-        <listitem>
-          <synopsis>int (*mode_valid)(struct drm_connector *connector,
-		  struct drm_display_mode *mode);</synopsis>
-          <para>
-            Verify whether a mode is valid for the connector. Return MODE_OK for
-            supported modes and one of the enum drm_mode_status values (MODE_*)
-            for unsupported modes. This operation is optional.
-          </para>
-          <para>
-            As the mode rejection reason is currently not used beside for
-            immediately removing the unsupported mode, an implementation can
-            return MODE_BAD regardless of the exact reason why the mode is not
-            valid.
-          </para>
-          <note><para>
-            Note that the <methodname>mode_valid</methodname> helper operation is
-            only called for modes detected by the device, and
-            <emphasis>not</emphasis> for modes set by the user through the CRTC
-            <methodname>set_config</methodname> operation.
-          </para></note>
-        </listitem>
-      </itemizedlist>
-    </sect2>
-    <sect2>
       <title>Atomic Modeset Helper Functions Reference</title>
       <sect3>
 	<title>Overview</title>
@@ -2327,8 +1596,12 @@
 !Edrivers/gpu/drm/drm_atomic_helper.c
     </sect2>
     <sect2>
-      <title>Modeset Helper Functions Reference</title>
-!Iinclude/drm/drm_crtc_helper.h
+      <title>Modeset Helper Reference for Common Vtables</title>
+!Iinclude/drm/drm_modeset_helper_vtables.h
+!Pinclude/drm/drm_modeset_helper_vtables.h overview
+    </sect2>
+    <sect2>
+      <title>Legacy CRTC/Modeset Helper Functions Reference</title>
 !Edrivers/gpu/drm/drm_crtc_helper.c
 !Pdrivers/gpu/drm/drm_crtc_helper.c overview
     </sect2>
@@ -4039,92 +3312,6 @@
       <sect2>
         <title>DPIO</title>
 !Pdrivers/gpu/drm/i915/i915_reg.h DPIO
-	<table id="dpiox2">
-	  <title>Dual channel PHY (VLV/CHV/BXT)</title>
-	  <tgroup cols="8">
-	    <colspec colname="c0" />
-	    <colspec colname="c1" />
-	    <colspec colname="c2" />
-	    <colspec colname="c3" />
-	    <colspec colname="c4" />
-	    <colspec colname="c5" />
-	    <colspec colname="c6" />
-	    <colspec colname="c7" />
-	    <spanspec spanname="ch0" namest="c0" nameend="c3" />
-	    <spanspec spanname="ch1" namest="c4" nameend="c7" />
-	    <spanspec spanname="ch0pcs01" namest="c0" nameend="c1" />
-	    <spanspec spanname="ch0pcs23" namest="c2" nameend="c3" />
-	    <spanspec spanname="ch1pcs01" namest="c4" nameend="c5" />
-	    <spanspec spanname="ch1pcs23" namest="c6" nameend="c7" />
-	    <thead>
-	      <row>
-		<entry spanname="ch0">CH0</entry>
-		<entry spanname="ch1">CH1</entry>
-	      </row>
-	    </thead>
-	    <tbody valign="top" align="center">
-	      <row>
-		<entry spanname="ch0">CMN/PLL/REF</entry>
-		<entry spanname="ch1">CMN/PLL/REF</entry>
-	      </row>
-	      <row>
-		<entry spanname="ch0pcs01">PCS01</entry>
-		<entry spanname="ch0pcs23">PCS23</entry>
-		<entry spanname="ch1pcs01">PCS01</entry>
-		<entry spanname="ch1pcs23">PCS23</entry>
-	      </row>
-	      <row>
-		<entry>TX0</entry>
-		<entry>TX1</entry>
-		<entry>TX2</entry>
-		<entry>TX3</entry>
-		<entry>TX0</entry>
-		<entry>TX1</entry>
-		<entry>TX2</entry>
-		<entry>TX3</entry>
-	      </row>
-	      <row>
-		<entry spanname="ch0">DDI0</entry>
-		<entry spanname="ch1">DDI1</entry>
-	      </row>
-	    </tbody>
-	  </tgroup>
-	</table>
-	<table id="dpiox1">
-	  <title>Single channel PHY (CHV/BXT)</title>
-	  <tgroup cols="4">
-	    <colspec colname="c0" />
-	    <colspec colname="c1" />
-	    <colspec colname="c2" />
-	    <colspec colname="c3" />
-	    <spanspec spanname="ch0" namest="c0" nameend="c3" />
-	    <spanspec spanname="ch0pcs01" namest="c0" nameend="c1" />
-	    <spanspec spanname="ch0pcs23" namest="c2" nameend="c3" />
-	    <thead>
-	      <row>
-		<entry spanname="ch0">CH0</entry>
-	      </row>
-	    </thead>
-	    <tbody valign="top" align="center">
-	      <row>
-		<entry spanname="ch0">CMN/PLL/REF</entry>
-	      </row>
-	      <row>
-		<entry spanname="ch0pcs01">PCS01</entry>
-		<entry spanname="ch0pcs23">PCS23</entry>
-	      </row>
-	      <row>
-		<entry>TX0</entry>
-		<entry>TX1</entry>
-		<entry>TX2</entry>
-		<entry>TX3</entry>
-	      </row>
-	      <row>
-		<entry spanname="ch0">DDI2</entry>
-	      </row>
-	    </tbody>
-	  </tgroup>
-	</table>
       </sect2>
 
       <sect2>
@@ -4201,17 +3388,21 @@
       </sect2>
     </sect1>
     <sect1>
-      <title>GuC-based Command Submission</title>
+      <title>GuC</title>
       <sect2>
-        <title>GuC</title>
+        <title>GuC-specific firmware loader</title>
 !Pdrivers/gpu/drm/i915/intel_guc_loader.c GuC-specific firmware loader
 !Idrivers/gpu/drm/i915/intel_guc_loader.c
       </sect2>
       <sect2>
-        <title>GuC Client</title>
-!Pdrivers/gpu/drm/i915/i915_guc_submission.c GuC-based command submissison
+        <title>GuC-based command submission</title>
+!Pdrivers/gpu/drm/i915/i915_guc_submission.c GuC-based command submission
 !Idrivers/gpu/drm/i915/i915_guc_submission.c
       </sect2>
+      <sect2>
+        <title>GuC Firmware Layout</title>
+!Pdrivers/gpu/drm/i915/intel_guc_fwif.h GuC Firmware Layout
+      </sect2>
     </sect1>
 
     <sect1>
@@ -4246,41 +3437,63 @@
 
   <chapter id="modes_of_use">
     <title>Modes of Use</title>
-  <sect1>
-    <title>Manual switching and manual power control</title>
+    <sect1>
+      <title>Manual switching and manual power control</title>
 !Pdrivers/gpu/vga/vga_switcheroo.c Manual switching and manual power control
-  </sect1>
-  <sect1>
-    <title>Driver power control</title>
+    </sect1>
+    <sect1>
+      <title>Driver power control</title>
 !Pdrivers/gpu/vga/vga_switcheroo.c Driver power control
-  </sect1>
+    </sect1>
   </chapter>
 
-  <chapter id="pubfunctions">
-    <title>Public functions</title>
+  <chapter id="api">
+    <title>API</title>
+    <sect1>
+      <title>Public functions</title>
 !Edrivers/gpu/vga/vga_switcheroo.c
-  </chapter>
-
-  <chapter id="pubstructures">
-    <title>Public structures</title>
+    </sect1>
+    <sect1>
+      <title>Public structures</title>
 !Finclude/linux/vga_switcheroo.h vga_switcheroo_handler
 !Finclude/linux/vga_switcheroo.h vga_switcheroo_client_ops
-  </chapter>
-
-  <chapter id="pubconstants">
-    <title>Public constants</title>
+    </sect1>
+    <sect1>
+      <title>Public constants</title>
 !Finclude/linux/vga_switcheroo.h vga_switcheroo_client_id
 !Finclude/linux/vga_switcheroo.h vga_switcheroo_state
-  </chapter>
-
-  <chapter id="privstructures">
-    <title>Private structures</title>
+    </sect1>
+    <sect1>
+      <title>Private structures</title>
 !Fdrivers/gpu/vga/vga_switcheroo.c vgasr_priv
 !Fdrivers/gpu/vga/vga_switcheroo.c vga_switcheroo_client
+    </sect1>
+  </chapter>
+
+  <chapter id="handlers">
+    <title>Handlers</title>
+    <sect1>
+      <title>apple-gmux Handler</title>
+!Pdrivers/platform/x86/apple-gmux.c Overview
+!Pdrivers/platform/x86/apple-gmux.c Interrupt
+      <sect2>
+        <title>Graphics mux</title>
+!Pdrivers/platform/x86/apple-gmux.c Graphics mux
+      </sect2>
+      <sect2>
+        <title>Power control</title>
+!Pdrivers/platform/x86/apple-gmux.c Power control
+      </sect2>
+      <sect2>
+        <title>Backlight control</title>
+!Pdrivers/platform/x86/apple-gmux.c Backlight control
+      </sect2>
+    </sect1>
   </chapter>
 
 !Cdrivers/gpu/vga/vga_switcheroo.c
 !Cinclude/linux/vga_switcheroo.h
+!Cdrivers/platform/x86/apple-gmux.c
 </part>
 
 </book>
diff --git a/Documentation/DocBook/iio.tmpl b/Documentation/DocBook/iio.tmpl
index 98be322..f525bf5 100644
--- a/Documentation/DocBook/iio.tmpl
+++ b/Documentation/DocBook/iio.tmpl
@@ -458,7 +458,7 @@
             .scan_type = {
               .sign = 's',
               .realbits = 12,
-              .storgebits = 16,
+              .storagebits = 16,
               .shift = 4,
               .endianness = IIO_LE,
             },
diff --git a/Documentation/DocBook/media/Makefile b/Documentation/DocBook/media/Makefile
index 08527e7..2840ff4 100644
--- a/Documentation/DocBook/media/Makefile
+++ b/Documentation/DocBook/media/Makefile
@@ -199,8 +199,10 @@
 #
 
 install_media_images = \
-	$(Q)-mkdir $(MEDIA_OBJ_DIR)/media_api; \
-	cp $(OBJIMGFILES) $(MEDIA_SRC_DIR)/*.svg $(MEDIA_SRC_DIR)/v4l/*.svg $(MEDIA_OBJ_DIR)/media_api
+	$(Q)if [ "x$(findstring media_api.xml,$(DOCBOOKS))" != "x" ]; then \
+		mkdir -p $(MEDIA_OBJ_DIR)/media_api; \
+		cp $(OBJIMGFILES) $(MEDIA_SRC_DIR)/*.svg $(MEDIA_SRC_DIR)/v4l/*.svg $(MEDIA_OBJ_DIR)/media_api; \
+	fi
 
 $(MEDIA_OBJ_DIR)/%: $(MEDIA_SRC_DIR)/%.b64
 	$(Q)base64 -d $< >$@
diff --git a/Documentation/DocBook/media/dvb/dvbproperty.xml b/Documentation/DocBook/media/dvb/dvbproperty.xml
index 08227d4..e579ae5 100644
--- a/Documentation/DocBook/media/dvb/dvbproperty.xml
+++ b/Documentation/DocBook/media/dvb/dvbproperty.xml
@@ -76,7 +76,7 @@
 
 <para>NOTE: While it is possible to directly call the Kernel code like the
     above example, it is strongly recommended to use
-    <ulink url="http://linuxtv.org/docs/libdvbv5/index.html">libdvbv5</ulink>,
+    <ulink url="https://linuxtv.org/docs/libdvbv5/index.html">libdvbv5</ulink>,
     as it provides abstraction to work with the supported digital TV standards
     and provides methods for usual operations like program scanning and to
     read/write channel descriptor files.</para>
diff --git a/Documentation/DocBook/media/dvb/examples.xml b/Documentation/DocBook/media/dvb/examples.xml
index c9f68c7..837fb3b 100644
--- a/Documentation/DocBook/media/dvb/examples.xml
+++ b/Documentation/DocBook/media/dvb/examples.xml
@@ -3,7 +3,7 @@
 </para>
 <para>NOTE: This section is out of date, and the code below won't even
     compile. Please refer to the
-    <ulink url="http://linuxtv.org/docs/libdvbv5/index.html">libdvbv5</ulink>
+    <ulink url="https://linuxtv.org/docs/libdvbv5/index.html">libdvbv5</ulink>
     for updated/recommended examples.
 </para>
 
diff --git a/Documentation/DocBook/media/dvb/intro.xml b/Documentation/DocBook/media/dvb/intro.xml
index 51db156..b5b701f 100644
--- a/Documentation/DocBook/media/dvb/intro.xml
+++ b/Documentation/DocBook/media/dvb/intro.xml
@@ -32,7 +32,7 @@
 new standard Linux DVB API. As a commitment to the development of
 terminals based on open standards, Nokia and Convergence made it
 available to all Linux developers and published it on
-<ulink url="http://www.linuxtv.org/" /> in September 2000.
+<ulink url="https://linuxtv.org" /> in September 2000.
 Convergence is the maintainer of the Linux DVB API. Together with the
 LinuxTV community (i.e. you, the reader of this document), the Linux DVB
 API will be constantly reviewed and improved. With the Linux driver for
diff --git a/Documentation/DocBook/media/v4l/capture.c.xml b/Documentation/DocBook/media/v4l/capture.c.xml
index 1c5c49a..22126a9 100644
--- a/Documentation/DocBook/media/v4l/capture.c.xml
+++ b/Documentation/DocBook/media/v4l/capture.c.xml
@@ -5,7 +5,7 @@
  *  This program can be used and distributed without restrictions.
  *
  *      This program is provided with the V4L2 API
- * see http://linuxtv.org/docs.php for more information
+ * see https://linuxtv.org/docs.php for more information
  */
 
 #include &lt;stdio.h&gt;
diff --git a/Documentation/DocBook/media/v4l/compat.xml b/Documentation/DocBook/media/v4l/compat.xml
index 5701a08..5399e89 100644
--- a/Documentation/DocBook/media/v4l/compat.xml
+++ b/Documentation/DocBook/media/v4l/compat.xml
@@ -2666,7 +2666,7 @@
         <para>V4L2 does not support digital terrestrial, cable or
 satellite broadcast. A separate project aiming at digital receivers
 exists. You can find its homepage at <ulink
-url="http://linuxtv.org">http://linuxtv.org</ulink>. The Linux DVB API
+url="https://linuxtv.org">https://linuxtv.org</ulink>. The Linux DVB API
 has no connection to the V4L2 API except that drivers for hybrid
 hardware may support both.</para>
       </section>
diff --git a/Documentation/DocBook/media/v4l/io.xml b/Documentation/DocBook/media/v4l/io.xml
index da65403..144158b 100644
--- a/Documentation/DocBook/media/v4l/io.xml
+++ b/Documentation/DocBook/media/v4l/io.xml
@@ -699,7 +699,7 @@
 buffer. It depends on the negotiated data format and may change with
 each buffer for compressed variable size data like JPEG images.
 Drivers must set this field when <structfield>type</structfield>
-refers to an input stream, applications when it refers to an output stream.
+refers to a capture stream, applications when it refers to an output stream.
 If the application sets this to 0 for an output stream, then
 <structfield>bytesused</structfield> will be set to the size of the
 buffer (see the <structfield>length</structfield> field of this struct) by
@@ -720,14 +720,14 @@
 	    <entry>Indicates the field order of the image in the
 buffer, see <xref linkend="v4l2-field" />. This field is not used when
 the buffer contains VBI data. Drivers must set it when
-<structfield>type</structfield> refers to an input stream,
+<structfield>type</structfield> refers to a capture stream,
 applications when it refers to an output stream.</entry>
 	  </row>
 	  <row>
 	    <entry>struct timeval</entry>
 	    <entry><structfield>timestamp</structfield></entry>
 	    <entry></entry>
-	    <entry><para>For input streams this is time when the first data
+	    <entry><para>For capture streams this is time when the first data
 	    byte was captured, as returned by the
 	    <function>clock_gettime()</function> function for the relevant
 	    clock id; see <constant>V4L2_BUF_FLAG_TIMESTAMP_*</constant> in
@@ -866,7 +866,7 @@
 	    <entry></entry>
 	    <entry>The number of bytes occupied by data in the plane
 	      (its payload). Drivers must set this field when <structfield>type</structfield>
-	      refers to an input stream, applications when it refers to an output stream.
+	      refers to a capture stream, applications when it refers to an output stream.
 	      If the application sets this to 0 for an output stream, then
 	      <structfield>bytesused</structfield> will be set to the size of the
 	      plane (see the <structfield>length</structfield> field of this struct)
@@ -919,7 +919,7 @@
 	    <entry></entry>
 	    <entry>Offset in bytes to video data in the plane.
 	      Drivers must set this field when <structfield>type</structfield>
-	      refers to an input stream, applications when it refers to an output stream.
+	      refers to a capture stream, applications when it refers to an output stream.
 	      Note that data_offset is included in <structfield>bytesused</structfield>.
 	      So the size of the image in the plane is
 	      <structfield>bytesused</structfield>-<structfield>data_offset</structfield> at
diff --git a/Documentation/DocBook/media/v4l/media-controller.xml b/Documentation/DocBook/media/v4l/media-controller.xml
index 873ac3a..5f2fc07 100644
--- a/Documentation/DocBook/media/v4l/media-controller.xml
+++ b/Documentation/DocBook/media/v4l/media-controller.xml
@@ -58,21 +58,36 @@
     <title>Media device model</title>
     <para>Discovering a device internal topology, and configuring it at runtime,
     is one of the goals of the media controller API. To achieve this, hardware
-    devices are modelled as an oriented graph of building blocks called entities
-    connected through pads.</para>
-    <para>An entity is a basic media hardware or software building block. It can
-    correspond to a large variety of logical blocks such as physical hardware
-    devices (CMOS sensor for instance), logical hardware devices (a building
-    block in a System-on-Chip image processing pipeline), DMA channels or
-    physical connectors.</para>
-    <para>A pad is a connection endpoint through which an entity can interact
-    with other entities. Data (not restricted to video) produced by an entity
-    flows from the entity's output to one or more entity inputs. Pads should not
-    be confused with physical pins at chip boundaries.</para>
-    <para>A link is a point-to-point oriented connection between two pads,
-    either on the same entity or on different entities. Data flows from a source
-    pad to a sink pad.</para>
+    devices and Linux Kernel interfaces are modelled as graph objects on
+    an oriented graph. The object types that constitute the graph are:</para>
+    <itemizedlist>
+    <listitem><para>An <emphasis role="bold">entity</emphasis>
+    is a basic media hardware or software building block. It can correspond to
+    a large variety of logical blocks such as physical hardware devices
+    (CMOS sensor for instance), logical hardware devices (a building block in
+    a System-on-Chip image processing pipeline), DMA channels or physical
+    connectors.</para></listitem>
+    <listitem><para>An <emphasis role="bold">interface</emphasis>
+    is a graph representation of a Linux Kernel userspace API interface,
+    like a device node or a sysfs file that controls one or more entities
+    in the graph.</para></listitem>
+    <listitem><para>A <emphasis role="bold">pad</emphasis>
+    is a data connection endpoint through which an entity can interact with
+    other entities. Data (not restricted to video) produced by an entity
+    flows from the entity's output to one or more entity inputs. Pads should
+    not be confused with physical pins at chip boundaries.</para></listitem>
+    <listitem><para>A <emphasis role="bold">data link</emphasis>
+    is a point-to-point oriented connection between two pads, either on the
+    same entity or on different entities. Data flows from a source pad to a
+    sink pad.</para></listitem>
+    <listitem><para>An <emphasis role="bold">interface link</emphasis>
+    is a point-to-point bidirectional control connection between a Linux
+    Kernel interface and an entity.m</para></listitem>
+    </itemizedlist>
   </section>
+
+  <!-- All non-ioctl specific data types go here. -->
+  &sub-media-types;
 </chapter>
 
 <appendix id="media-user-func">
@@ -83,6 +98,7 @@
   &sub-media-func-ioctl;
   <!-- All ioctls go here. -->
   &sub-media-ioc-device-info;
+  &sub-media-ioc-g-topology;
   &sub-media-ioc-enum-entities;
   &sub-media-ioc-enum-links;
   &sub-media-ioc-setup-link;
diff --git a/Documentation/DocBook/media/v4l/media-ioc-enum-entities.xml b/Documentation/DocBook/media/v4l/media-ioc-enum-entities.xml
index 5872f8bb..0c4f96b 100644
--- a/Documentation/DocBook/media/v4l/media-ioc-enum-entities.xml
+++ b/Documentation/DocBook/media/v4l/media-ioc-enum-entities.xml
@@ -59,15 +59,6 @@
     <para>Entity IDs can be non-contiguous. Applications must
     <emphasis>not</emphasis> try to enumerate entities by calling
     MEDIA_IOC_ENUM_ENTITIES with increasing id's until they get an error.</para>
-    <para>Two or more entities that share a common non-zero
-    <structfield>group_id</structfield> value are considered as logically
-    grouped. Groups are used to report
-    <itemizedlist>
-      <listitem><para>ALSA, VBI and video nodes that carry the same media
-      stream</para></listitem>
-      <listitem><para>lens and flash controllers associated with a sensor</para></listitem>
-    </itemizedlist>
-    </para>
 
     <table pgwide="1" frame="none" id="media-entity-desc">
       <title>struct <structname>media_entity_desc</structname></title>
@@ -106,7 +97,7 @@
 	    <entry><structfield>revision</structfield></entry>
 	    <entry></entry>
 	    <entry></entry>
-	    <entry>Entity revision in a driver/hardware specific format.</entry>
+	    <entry>Entity revision. Always zero (obsolete)</entry>
 	  </row>
 	  <row>
 	    <entry>__u32</entry>
@@ -120,7 +111,7 @@
 	    <entry><structfield>group_id</structfield></entry>
 	    <entry></entry>
 	    <entry></entry>
-	    <entry>Entity group ID</entry>
+	    <entry>Entity group ID. Always zero (obsolete)</entry>
 	  </row>
 	  <row>
 	    <entry>__u16</entry>
@@ -171,97 +162,6 @@
 	</tbody>
       </tgroup>
     </table>
-
-    <table frame="none" pgwide="1" id="media-entity-type">
-      <title>Media entity types</title>
-      <tgroup cols="2">
-        <colspec colname="c1"/>
-        <colspec colname="c2"/>
-	<tbody valign="top">
-	  <row>
-	    <entry><constant>MEDIA_ENT_T_DEVNODE</constant></entry>
-	    <entry>Unknown device node</entry>
-	  </row>
-	  <row>
-	    <entry><constant>MEDIA_ENT_T_DEVNODE_V4L</constant></entry>
-	    <entry>V4L video, radio or vbi device node</entry>
-	  </row>
-	  <row>
-	    <entry><constant>MEDIA_ENT_T_DEVNODE_FB</constant></entry>
-	    <entry>Frame buffer device node</entry>
-	  </row>
-	  <row>
-	    <entry><constant>MEDIA_ENT_T_DEVNODE_ALSA</constant></entry>
-	    <entry>ALSA card</entry>
-	  </row>
-	  <row>
-	    <entry><constant>MEDIA_ENT_T_DEVNODE_DVB_FE</constant></entry>
-	    <entry>DVB frontend devnode</entry>
-	  </row>
-	  <row>
-	    <entry><constant>MEDIA_ENT_T_DEVNODE_DVB_DEMUX</constant></entry>
-	    <entry>DVB demux devnode</entry>
-	  </row>
-	  <row>
-	    <entry><constant>MEDIA_ENT_T_DEVNODE_DVB_DVR</constant></entry>
-	    <entry>DVB DVR devnode</entry>
-	  </row>
-	  <row>
-	    <entry><constant>MEDIA_ENT_T_DEVNODE_DVB_CA</constant></entry>
-	    <entry>DVB CAM devnode</entry>
-	  </row>
-	  <row>
-	    <entry><constant>MEDIA_ENT_T_DEVNODE_DVB_NET</constant></entry>
-	    <entry>DVB network devnode</entry>
-	  </row>
-	  <row>
-	    <entry><constant>MEDIA_ENT_T_V4L2_SUBDEV</constant></entry>
-	    <entry>Unknown V4L sub-device</entry>
-	  </row>
-	  <row>
-	    <entry><constant>MEDIA_ENT_T_V4L2_SUBDEV_SENSOR</constant></entry>
-	    <entry>Video sensor</entry>
-	  </row>
-	  <row>
-	    <entry><constant>MEDIA_ENT_T_V4L2_SUBDEV_FLASH</constant></entry>
-	    <entry>Flash controller</entry>
-	  </row>
-	  <row>
-	    <entry><constant>MEDIA_ENT_T_V4L2_SUBDEV_LENS</constant></entry>
-	    <entry>Lens controller</entry>
-	  </row>
-	  <row>
-	    <entry><constant>MEDIA_ENT_T_V4L2_SUBDEV_DECODER</constant></entry>
-	    <entry>Video decoder, the basic function of the video decoder is to
-	    accept analogue video from a wide variety of sources such as
-	    broadcast, DVD players, cameras and video cassette recorders, in
-	    either NTSC, PAL or HD format and still occasionally SECAM, separate
-	    it into its component parts, luminance and chrominance, and output
-	    it in some digital video standard, with appropriate embedded timing
-	    signals.</entry>
-	  </row>
-	  <row>
-	    <entry><constant>MEDIA_ENT_T_V4L2_SUBDEV_TUNER</constant></entry>
-	    <entry>TV and/or radio tuner</entry>
-	  </row>
-	</tbody>
-      </tgroup>
-    </table>
-
-    <table frame="none" pgwide="1" id="media-entity-flag">
-      <title>Media entity flags</title>
-      <tgroup cols="2">
-        <colspec colname="c1"/>
-        <colspec colname="c2"/>
-	<tbody valign="top">
-	  <row>
-	    <entry><constant>MEDIA_ENT_FL_DEFAULT</constant></entry>
-	    <entry>Default entity for its type. Used to discover the default
-	    audio, VBI and video devices, the default camera sensor, ...</entry>
-	  </row>
-	</tbody>
-      </tgroup>
-    </table>
   </refsect1>
 
   <refsect1>
diff --git a/Documentation/DocBook/media/v4l/media-ioc-enum-links.xml b/Documentation/DocBook/media/v4l/media-ioc-enum-links.xml
index 74fb394..2bbeea9 100644
--- a/Documentation/DocBook/media/v4l/media-ioc-enum-links.xml
+++ b/Documentation/DocBook/media/v4l/media-ioc-enum-links.xml
@@ -118,35 +118,6 @@
       </tgroup>
     </table>
 
-    <table frame="none" pgwide="1" id="media-pad-flag">
-      <title>Media pad flags</title>
-      <tgroup cols="2">
-        <colspec colname="c1"/>
-        <colspec colname="c2"/>
-	<tbody valign="top">
-	  <row>
-	    <entry><constant>MEDIA_PAD_FL_SINK</constant></entry>
-	    <entry>Input pad, relative to the entity. Input pads sink data and
-	    are targets of links.</entry>
-	  </row>
-	  <row>
-	    <entry><constant>MEDIA_PAD_FL_SOURCE</constant></entry>
-	    <entry>Output pad, relative to the entity. Output pads source data
-	    and are origins of links.</entry>
-	  </row>
-	  <row>
-	    <entry><constant>MEDIA_PAD_FL_MUST_CONNECT</constant></entry>
-	    <entry>If this flag is set and the pad is linked to any other
-	    pad, then at least one of those links must be enabled for the
-	    entity to be able to stream. There could be temporary reasons
-	    (e.g. device configuration dependent) for the pad to need
-	    enabled links even when this flag isn't set; the absence of the
-	    flag doesn't imply there is none.</entry>
-	  </row>
-	</tbody>
-      </tgroup>
-    </table>
-
     <table pgwide="1" frame="none" id="media-link-desc">
       <title>struct <structname>media_link_desc</structname></title>
       <tgroup cols="3">
@@ -171,33 +142,6 @@
       </tgroup>
     </table>
 
-    <table frame="none" pgwide="1" id="media-link-flag">
-      <title>Media link flags</title>
-      <tgroup cols="2">
-        <colspec colname="c1"/>
-        <colspec colname="c2"/>
-	<tbody valign="top">
-	  <row>
-	    <entry><constant>MEDIA_LNK_FL_ENABLED</constant></entry>
-	    <entry>The link is enabled and can be used to transfer media data.
-	    When two or more links target a sink pad, only one of them can be
-	    enabled at a time.</entry>
-	  </row>
-	  <row>
-	    <entry><constant>MEDIA_LNK_FL_IMMUTABLE</constant></entry>
-	    <entry>The link enabled state can't be modified at runtime. An
-	    immutable link is always enabled.</entry>
-	  </row>
-	  <row>
-	    <entry><constant>MEDIA_LNK_FL_DYNAMIC</constant></entry>
-	    <entry>The link enabled state can be modified during streaming. This
-	    flag is set by drivers and is read-only for applications.</entry>
-	  </row>
-	</tbody>
-      </tgroup>
-    </table>
-    <para>One and only one of <constant>MEDIA_PAD_FL_SINK</constant> and
-    <constant>MEDIA_PAD_FL_SOURCE</constant> must be set for every pad.</para>
   </refsect1>
 
   <refsect1>
diff --git a/Documentation/DocBook/media/v4l/media-ioc-g-topology.xml b/Documentation/DocBook/media/v4l/media-ioc-g-topology.xml
new file mode 100644
index 0000000..63152ab
--- /dev/null
+++ b/Documentation/DocBook/media/v4l/media-ioc-g-topology.xml
@@ -0,0 +1,394 @@
+<refentry id="media-g-topology">
+  <refmeta>
+    <refentrytitle>ioctl MEDIA_IOC_G_TOPOLOGY</refentrytitle>
+    &manvol;
+  </refmeta>
+
+  <refnamediv>
+    <refname>MEDIA_IOC_G_TOPOLOGY</refname>
+    <refpurpose>Enumerate the graph topology and graph element properties</refpurpose>
+  </refnamediv>
+
+  <refsynopsisdiv>
+    <funcsynopsis>
+      <funcprototype>
+	<funcdef>int <function>ioctl</function></funcdef>
+	<paramdef>int <parameter>fd</parameter></paramdef>
+	<paramdef>int <parameter>request</parameter></paramdef>
+	<paramdef>struct media_v2_topology *<parameter>argp</parameter></paramdef>
+      </funcprototype>
+    </funcsynopsis>
+  </refsynopsisdiv>
+
+  <refsect1>
+    <title>Arguments</title>
+
+    <variablelist>
+      <varlistentry>
+	<term><parameter>fd</parameter></term>
+	<listitem>
+	  <para>File descriptor returned by
+	  <link linkend='media-func-open'><function>open()</function></link>.</para>
+	</listitem>
+      </varlistentry>
+      <varlistentry>
+	<term><parameter>request</parameter></term>
+	<listitem>
+	  <para>MEDIA_IOC_G_TOPOLOGY</para>
+	</listitem>
+      </varlistentry>
+      <varlistentry>
+	<term><parameter>argp</parameter></term>
+	<listitem>
+	  <para></para>
+	</listitem>
+      </varlistentry>
+    </variablelist>
+  </refsect1>
+
+  <refsect1>
+    <title>Description</title>
+
+    <para><emphasis role="bold">NOTE:</emphasis> This new ioctl is programmed to be added on Kernel 4.6. Its definition/arguments may change until its final version.</para>
+
+    <para>The typical usage of this ioctl is to call it twice.
+    On the first call, the structure defined at &media-v2-topology; should
+    be zeroed. At return, if no errors happen, this ioctl will return the
+    <constant>topology_version</constant> and the total number of entities,
+    interfaces, pads and links.</para>
+    <para>Before the second call, the userspace should allocate arrays to
+    store the graph elements that are desired, putting the pointers to them
+    at the ptr_entities, ptr_interfaces, ptr_links and/or ptr_pads, keeping
+    the other values untouched.</para>
+    <para>If the <constant>topology_version</constant> remains the same, the
+    ioctl should fill the desired arrays with the media graph elements.</para>
+
+    <table pgwide="1" frame="none" id="media-v2-topology">
+      <title>struct <structname>media_v2_topology</structname></title>
+      <tgroup cols="5">
+	<colspec colname="c1" />
+	<colspec colname="c2" />
+	<colspec colname="c3" />
+	<colspec colname="c4" />
+	<colspec colname="c5" />
+	<tbody valign="top">
+	  <row>
+	    <entry>__u64</entry>
+	    <entry><structfield>topology_version</structfield></entry>
+	    <entry></entry>
+	    <entry></entry>
+	    <entry>Version of the media graph topology. When the graph is
+		    created, this field starts with zero. Every time a graph
+		    element is added or removed, this field is
+		    incremented.</entry>
+	  </row>
+	  <row>
+	    <entry>__u64</entry>
+	    <entry><structfield>num_entities</structfield></entry>
+	    <entry></entry>
+	    <entry></entry>
+	    <entry>Number of entities in the graph</entry>
+	  </row>
+	  <row>
+	    <entry>__u64</entry>
+	    <entry><structfield>ptr_entities</structfield></entry>
+	    <entry></entry>
+	    <entry></entry>
+	    <entry>A pointer to a memory area where the entities array
+		   will be stored, converted to a 64-bits integer.
+		   It can be zero. if zero, the ioctl won't store the
+		   entities. It will just update
+		   <constant>num_entities</constant></entry>
+	  </row>
+	  <row>
+	    <entry>__u64</entry>
+	    <entry><structfield>num_interfaces</structfield></entry>
+	    <entry></entry>
+	    <entry></entry>
+	    <entry>Number of interfaces in the graph</entry>
+	  </row>
+	  <row>
+	    <entry>__u64</entry>
+	    <entry><structfield>ptr_interfaces</structfield></entry>
+	    <entry></entry>
+	    <entry></entry>
+	    <entry>A pointer to a memory area where the interfaces array
+		   will be stored, converted to a 64-bits integer.
+		   It can be zero. if zero, the ioctl won't store the
+		   interfaces. It will just update
+		   <constant>num_interfaces</constant></entry>
+	  </row>
+	  <row>
+	    <entry>__u64</entry>
+	    <entry><structfield>num_pads</structfield></entry>
+	    <entry></entry>
+	    <entry></entry>
+	    <entry>Total number of pads in the graph</entry>
+	  </row>
+	  <row>
+	    <entry>__u64</entry>
+	    <entry><structfield>ptr_pads</structfield></entry>
+	    <entry></entry>
+	    <entry></entry>
+	    <entry>A pointer to a memory area where the pads array
+		   will be stored, converted to a 64-bits integer.
+		   It can be zero. if zero, the ioctl won't store the
+		   pads. It will just update
+		   <constant>num_pads</constant></entry>
+	  </row>
+	  <row>
+	    <entry>__u64</entry>
+	    <entry><structfield>num_links</structfield></entry>
+	    <entry></entry>
+	    <entry></entry>
+	    <entry>Total number of data and interface links in the graph</entry>
+	  </row>
+	  <row>
+	    <entry>__u64</entry>
+	    <entry><structfield>ptr_links</structfield></entry>
+	    <entry></entry>
+	    <entry></entry>
+	    <entry>A pointer to a memory area where the links array
+		   will be stored, converted to a 64-bits integer.
+		   It can be zero. if zero, the ioctl won't store the
+		   links. It will just update
+		   <constant>num_links</constant></entry>
+	  </row>
+	</tbody>
+      </tgroup>
+    </table>
+
+    <table pgwide="1" frame="none" id="media-v2-entity">
+      <title>struct <structname>media_v2_entity</structname></title>
+      <tgroup cols="5">
+	<colspec colname="c1" />
+	<colspec colname="c2" />
+	<colspec colname="c3" />
+	<colspec colname="c4" />
+	<colspec colname="c5" />
+	<tbody valign="top">
+	  <row>
+	    <entry>__u32</entry>
+	    <entry><structfield>id</structfield></entry>
+	    <entry></entry>
+	    <entry></entry>
+	    <entry>Unique ID for the entity.</entry>
+	  </row>
+	  <row>
+	    <entry>char</entry>
+	    <entry><structfield>name</structfield>[64]</entry>
+	    <entry></entry>
+	    <entry></entry>
+	    <entry>Entity name as an UTF-8 NULL-terminated string.</entry>
+	  </row>
+	  <row>
+	    <entry>__u32</entry>
+	    <entry><structfield>function</structfield></entry>
+	    <entry></entry>
+	    <entry></entry>
+	    <entry>Entity main function, see <xref linkend="media-entity-type" /> for details.</entry>
+	  </row>
+	  <row>
+	    <entry>__u32</entry>
+	    <entry><structfield>reserved</structfield>[12]</entry>
+	    <entry>Reserved for future extensions. Drivers and applications must
+	    set this array to zero.</entry>
+	  </row>
+	</tbody>
+      </tgroup>
+    </table>
+
+    <table pgwide="1" frame="none" id="media-v2-interface">
+      <title>struct <structname>media_v2_interface</structname></title>
+      <tgroup cols="5">
+	<colspec colname="c1" />
+	<colspec colname="c2" />
+	<colspec colname="c3" />
+	<colspec colname="c4" />
+	<colspec colname="c5" />
+	<tbody valign="top">
+	  <row>
+	    <entry>__u32</entry>
+	    <entry><structfield>id</structfield></entry>
+	    <entry></entry>
+	    <entry></entry>
+	    <entry>Unique ID for the interface.</entry>
+	  </row>
+	  <row>
+	    <entry>__u32</entry>
+	    <entry><structfield>intf_type</structfield></entry>
+	    <entry></entry>
+	    <entry></entry>
+	    <entry>Interface type, see <xref linkend="media-intf-type" /> for details.</entry>
+	  </row>
+	  <row>
+	    <entry>__u32</entry>
+	    <entry><structfield>flags</structfield></entry>
+	    <entry></entry>
+	    <entry></entry>
+	    <entry>Interface flags. Currently unused.</entry>
+	  </row>
+	  <row>
+	    <entry>__u32</entry>
+	    <entry><structfield>reserved</structfield>[9]</entry>
+	    <entry></entry>
+	    <entry></entry>
+	    <entry>Reserved for future extensions. Drivers and applications must
+	    set this array to zero.</entry>
+	  </row>
+	  <row>
+	    <entry>struct media_v2_intf_devnode</entry>
+	    <entry><structfield>devnode</structfield></entry>
+	    <entry></entry>
+	    <entry></entry>
+	    <entry>Used only for device node interfaces. See <xref linkend="media-v2-intf-devnode" /> for details..</entry>
+	  </row>
+	</tbody>
+      </tgroup>
+    </table>
+
+    <table pgwide="1" frame="none" id="media-v2-intf-devnode">
+      <title>struct <structname>media_v2_interface</structname></title>
+      <tgroup cols="5">
+	<colspec colname="c1" />
+	<colspec colname="c2" />
+	<colspec colname="c3" />
+	<colspec colname="c4" />
+	<colspec colname="c5" />
+	<tbody valign="top">
+	  <row>
+	    <entry>__u32</entry>
+	    <entry><structfield>major</structfield></entry>
+	    <entry></entry>
+	    <entry></entry>
+	    <entry>Device node major number.</entry>
+	  </row>
+	  <row>
+	    <entry>__u32</entry>
+	    <entry><structfield>minor</structfield></entry>
+	    <entry></entry>
+	    <entry></entry>
+	    <entry>Device node minor number.</entry>
+	  </row>
+	</tbody>
+      </tgroup>
+    </table>
+
+    <table pgwide="1" frame="none" id="media-v2-pad">
+      <title>struct <structname>media_v2_pad</structname></title>
+      <tgroup cols="5">
+	<colspec colname="c1" />
+	<colspec colname="c2" />
+	<colspec colname="c3" />
+	<colspec colname="c4" />
+	<colspec colname="c5" />
+	<tbody valign="top">
+	  <row>
+	    <entry>__u32</entry>
+	    <entry><structfield>id</structfield></entry>
+	    <entry></entry>
+	    <entry></entry>
+	    <entry>Unique ID for the pad.</entry>
+	  </row>
+	  <row>
+	    <entry>__u32</entry>
+	    <entry><structfield>entity_id</structfield></entry>
+	    <entry></entry>
+	    <entry></entry>
+	    <entry>Unique ID for the entity where this pad belongs.</entry>
+	  </row>
+	  <row>
+	    <entry>__u32</entry>
+	    <entry><structfield>flags</structfield></entry>
+	    <entry></entry>
+	    <entry></entry>
+	    <entry>Pad flags, see <xref linkend="media-pad-flag" /> for more details.</entry>
+	  </row>
+	  <row>
+	    <entry>__u32</entry>
+	    <entry><structfield>reserved</structfield>[9]</entry>
+	    <entry></entry>
+	    <entry></entry>
+	    <entry>Reserved for future extensions. Drivers and applications must
+	    set this array to zero.</entry>
+	  </row>
+	</tbody>
+      </tgroup>
+    </table>
+
+    <table pgwide="1" frame="none" id="media-v2-link">
+      <title>struct <structname>media_v2_pad</structname></title>
+      <tgroup cols="5">
+	<colspec colname="c1" />
+	<colspec colname="c2" />
+	<colspec colname="c3" />
+	<colspec colname="c4" />
+	<colspec colname="c5" />
+	<tbody valign="top">
+	  <row>
+	    <entry>__u32</entry>
+	    <entry><structfield>id</structfield></entry>
+	    <entry></entry>
+	    <entry></entry>
+	    <entry>Unique ID for the pad.</entry>
+	  </row>
+	  <row>
+	    <entry>__u32</entry>
+	    <entry><structfield>source_id</structfield></entry>
+	    <entry></entry>
+	    <entry></entry>
+	    <entry>
+	       <para>On pad to pad links: unique ID for the source pad.</para>
+	       <para>On interface to entity links: unique ID for the interface.</para>
+	    </entry>
+	  </row>
+	  <row>
+	    <entry>__u32</entry>
+	    <entry><structfield>sink_id</structfield></entry>
+	    <entry></entry>
+	    <entry></entry>
+	    <entry>
+	       <para>On pad to pad links: unique ID for the sink pad.</para>
+	       <para>On interface to entity links: unique ID for the entity.</para>
+	    </entry>
+	  </row>
+	  <row>
+	    <entry>__u32</entry>
+	    <entry><structfield>flags</structfield></entry>
+	    <entry></entry>
+	    <entry></entry>
+	    <entry>Link flags, see <xref linkend="media-link-flag" /> for more details.</entry>
+	  </row>
+	  <row>
+	    <entry>__u32</entry>
+	    <entry><structfield>reserved</structfield>[5]</entry>
+	    <entry></entry>
+	    <entry></entry>
+	    <entry>Reserved for future extensions. Drivers and applications must
+	    set this array to zero.</entry>
+	  </row>
+	</tbody>
+      </tgroup>
+    </table>
+
+  </refsect1>
+
+  <refsect1>
+    &return-value;
+
+    <variablelist>
+      <varlistentry>
+	<term><errorcode>ENOSPC</errorcode></term>
+	<listitem>
+	  <para>This is returned when either one or more of the num_entities,
+	  num_interfaces, num_links or num_pads are non-zero and are smaller
+	  than the actual number of elements inside the graph. This may happen
+	  if the <constant>topology_version</constant> changed when compared
+	  to the last time this ioctl was called. Userspace should usually
+	  free the area for the pointers, zero the struct elements and call
+	  this ioctl again.</para>
+	</listitem>
+      </varlistentry>
+    </variablelist>
+  </refsect1>
+</refentry>
diff --git a/Documentation/DocBook/media/v4l/media-types.xml b/Documentation/DocBook/media/v4l/media-types.xml
new file mode 100644
index 0000000..1af3842
--- /dev/null
+++ b/Documentation/DocBook/media/v4l/media-types.xml
@@ -0,0 +1,240 @@
+<section id="media-controller-types">
+<title>Types and flags used to represent the media graph elements</title>
+
+    <table frame="none" pgwide="1" id="media-entity-type">
+      <title>Media entity types</title>
+      <tgroup cols="2">
+	<colspec colname="c1"/>
+	<colspec colname="c2"/>
+	<tbody valign="top">
+	  <row>
+	    <entry><constant>MEDIA_ENT_F_UNKNOWN</constant> and <constant>MEDIA_ENT_F_V4L2_SUBDEV_UNKNOWN</constant></entry>
+	    <entry>Unknown entity. That generally indicates that
+	    a driver didn't initialize properly the entity, with is a Kernel bug</entry>
+	  </row>
+	  <row>
+	    <entry><constant>MEDIA_ENT_F_IO_V4L</constant></entry>
+	    <entry>Data streaming input and/or output entity.</entry>
+	  </row>
+	  <row>
+	    <entry><constant>MEDIA_ENT_F_IO_VBI</constant></entry>
+	    <entry>V4L VBI streaming input or output entity</entry>
+	  </row>
+	  <row>
+	    <entry><constant>MEDIA_ENT_F_IO_SWRADIO</constant></entry>
+	    <entry>V4L Software Digital Radio (SDR) streaming input or output entity</entry>
+	  </row>
+	  <row>
+	    <entry><constant>MEDIA_ENT_F_IO_DTV</constant></entry>
+	    <entry>DVB Digital TV streaming input or output entity</entry>
+	  </row>
+	  <row>
+	    <entry><constant>MEDIA_ENT_F_DTV_DEMOD</constant></entry>
+	    <entry>Digital TV demodulator entity.</entry>
+	  </row>
+	  <row>
+	    <entry><constant>MEDIA_ENT_F_TS_DEMUX</constant></entry>
+	    <entry>MPEG Transport stream demux entity. Could be implemented on hardware or in Kernelspace by the Linux DVB subsystem.</entry>
+	  </row>
+	  <row>
+	    <entry><constant>MEDIA_ENT_F_DTV_CA</constant></entry>
+	    <entry>Digital TV Conditional Access module (CAM) entity</entry>
+	  </row>
+	  <row>
+	    <entry><constant>MEDIA_ENT_F_DTV_NET_DECAP</constant></entry>
+	    <entry>Digital TV network ULE/MLE desencapsulation entity. Could be implemented on hardware or in Kernelspace</entry>
+	  </row>
+	  <row>
+	    <entry><constant>MEDIA_ENT_F_CONN_RF</constant></entry>
+	    <entry>Connector for a Radio Frequency (RF) signal.</entry>
+	  </row>
+	  <row>
+	    <entry><constant>MEDIA_ENT_F_CONN_SVIDEO</constant></entry>
+	    <entry>Connector for a S-Video signal.</entry>
+	  </row>
+	  <row>
+	    <entry><constant>MEDIA_ENT_F_CONN_COMPOSITE</constant></entry>
+	    <entry>Connector for a RGB composite signal.</entry>
+	  </row>
+	  <row>
+	    <entry><constant>MEDIA_ENT_F_CONN_TEST</constant></entry>
+	    <entry>Connector for a test generator.</entry>
+	  </row>
+	  <row>
+	    <entry><constant>MEDIA_ENT_F_CAM_SENSOR</constant></entry>
+	    <entry>Camera video sensor entity.</entry>
+	  </row>
+	  <row>
+	    <entry><constant>MEDIA_ENT_F_FLASH</constant></entry>
+	    <entry>Flash controller entity.</entry>
+	  </row>
+	  <row>
+	    <entry><constant>MEDIA_ENT_F_LENS</constant></entry>
+	    <entry>Lens controller entity.</entry>
+	  </row>
+	  <row>
+	    <entry><constant>MEDIA_ENT_F_ATV_DECODER</constant></entry>
+	    <entry>Analog video decoder, the basic function of the video decoder
+	    is to accept analogue video from a wide variety of sources such as
+	    broadcast, DVD players, cameras and video cassette recorders, in
+	    either NTSC, PAL, SECAM or HD format, separating the stream
+	    into its component parts, luminance and chrominance, and output
+	    it in some digital video standard, with appropriate timing
+	    signals.</entry>
+	  </row>
+	  <row>
+	    <entry><constant>MEDIA_ENT_F_TUNER</constant></entry>
+	    <entry>Digital TV, analog TV, radio and/or software radio tuner.</entry>
+	  </row>
+	</tbody>
+      </tgroup>
+    </table>
+
+    <table frame="none" pgwide="1" id="media-entity-flag">
+      <title>Media entity flags</title>
+      <tgroup cols="2">
+	<colspec colname="c1"/>
+	<colspec colname="c2"/>
+	<tbody valign="top">
+	  <row>
+	    <entry><constant>MEDIA_ENT_FL_DEFAULT</constant></entry>
+	    <entry>Default entity for its type. Used to discover the default
+	    audio, VBI and video devices, the default camera sensor, ...</entry>
+	  </row>
+	  <row>
+	    <entry><constant>MEDIA_ENT_FL_CONNECTOR</constant></entry>
+	    <entry>The entity represents a data conector</entry>
+	  </row>
+	</tbody>
+      </tgroup>
+    </table>
+
+    <table frame="none" pgwide="1" id="media-intf-type">
+      <title>Media interface types</title>
+      <tgroup cols="3">
+	<colspec colname="c1"/>
+	<colspec colname="c2"/>
+	<colspec colname="c3"/>
+	<tbody valign="top">
+	  <row>
+	    <entry><constant>MEDIA_INTF_T_DVB_FE</constant></entry>
+	    <entry>Device node interface for the Digital TV frontend</entry>
+	    <entry>typically, /dev/dvb/adapter?/frontend?</entry>
+	  </row>
+	  <row>
+	    <entry><constant>MEDIA_INTF_T_DVB_DEMUX</constant></entry>
+	    <entry>Device node interface for the Digital TV demux</entry>
+	    <entry>typically, /dev/dvb/adapter?/demux?</entry>
+	  </row>
+	  <row>
+	    <entry><constant>MEDIA_INTF_T_DVB_DVR</constant></entry>
+	    <entry>Device node interface for the Digital TV DVR</entry>
+	    <entry>typically, /dev/dvb/adapter?/dvr?</entry>
+	  </row>
+	  <row>
+	    <entry><constant>MEDIA_INTF_T_DVB_CA</constant></entry>
+	    <entry>Device node interface for the Digital TV Conditional Access</entry>
+	    <entry>typically, /dev/dvb/adapter?/ca?</entry>
+	  </row>
+	  <row>
+	    <entry><constant>MEDIA_INTF_T_DVB_FE</constant></entry>
+	    <entry>Device node interface for the Digital TV network control</entry>
+	    <entry>typically, /dev/dvb/adapter?/net?</entry>
+	  </row>
+	  <row>
+	    <entry><constant>MEDIA_INTF_T_V4L_VIDEO</constant></entry>
+	    <entry>Device node interface for video (V4L)</entry>
+	    <entry>typically, /dev/video?</entry>
+	  </row>
+	  <row>
+	    <entry><constant>MEDIA_INTF_T_V4L_VBI</constant></entry>
+	    <entry>Device node interface for VBI (V4L)</entry>
+	    <entry>typically, /dev/vbi?</entry>
+	  </row>
+	  <row>
+	    <entry><constant>MEDIA_INTF_T_V4L_RADIO</constant></entry>
+	    <entry>Device node interface for radio (V4L)</entry>
+	    <entry>typically, /dev/vbi?</entry>
+	  </row>
+	  <row>
+	    <entry><constant>MEDIA_INTF_T_V4L_SUBDEV</constant></entry>
+	    <entry>Device node interface for a V4L subdevice</entry>
+	    <entry>typically, /dev/v4l-subdev?</entry>
+	  </row>
+	  <row>
+	    <entry><constant>MEDIA_INTF_T_V4L_SWRADIO</constant></entry>
+	    <entry>Device node interface for Software Defined Radio (V4L)</entry>
+	    <entry>typically, /dev/swradio?</entry>
+	  </row>
+	</tbody>
+      </tgroup>
+    </table>
+
+    <table frame="none" pgwide="1" id="media-pad-flag">
+      <title>Media pad flags</title>
+      <tgroup cols="2">
+	<colspec colname="c1"/>
+	<colspec colname="c2"/>
+	<tbody valign="top">
+	  <row>
+	    <entry><constant>MEDIA_PAD_FL_SINK</constant></entry>
+	    <entry>Input pad, relative to the entity. Input pads sink data and
+	    are targets of links.</entry>
+	  </row>
+	  <row>
+	    <entry><constant>MEDIA_PAD_FL_SOURCE</constant></entry>
+	    <entry>Output pad, relative to the entity. Output pads source data
+	    and are origins of links.</entry>
+	  </row>
+	  <row>
+	    <entry><constant>MEDIA_PAD_FL_MUST_CONNECT</constant></entry>
+	    <entry>If this flag is set and the pad is linked to any other
+	    pad, then at least one of those links must be enabled for the
+	    entity to be able to stream. There could be temporary reasons
+	    (e.g. device configuration dependent) for the pad to need
+	    enabled links even when this flag isn't set; the absence of the
+	    flag doesn't imply there is none.</entry>
+	  </row>
+	</tbody>
+      </tgroup>
+    </table>
+
+    <para>One and only one of <constant>MEDIA_PAD_FL_SINK</constant> and
+    <constant>MEDIA_PAD_FL_SOURCE</constant> must be set for every pad.</para>
+
+    <table frame="none" pgwide="1" id="media-link-flag">
+      <title>Media link flags</title>
+      <tgroup cols="2">
+	<colspec colname="c1"/>
+	<colspec colname="c2"/>
+	<tbody valign="top">
+	  <row>
+	    <entry><constant>MEDIA_LNK_FL_ENABLED</constant></entry>
+	    <entry>The link is enabled and can be used to transfer media data.
+	    When two or more links target a sink pad, only one of them can be
+	    enabled at a time.</entry>
+	  </row>
+	  <row>
+	    <entry><constant>MEDIA_LNK_FL_IMMUTABLE</constant></entry>
+	    <entry>The link enabled state can't be modified at runtime. An
+	    immutable link is always enabled.</entry>
+	  </row>
+	  <row>
+	    <entry><constant>MEDIA_LNK_FL_DYNAMIC</constant></entry>
+	    <entry>The link enabled state can be modified during streaming. This
+	    flag is set by drivers and is read-only for applications.</entry>
+	  </row>
+	  <row>
+	    <entry><constant>MEDIA_LNK_FL_LINK_TYPE</constant></entry>
+	    <entry><para>This is a bitmask that defines the type of the link.
+		   Currently, two types of links are supported:</para>
+	    <para><constant>MEDIA_LNK_FL_DATA_LINK</constant>
+	    if the link is between two pads</para>
+	    <para><constant>MEDIA_LNK_FL_INTERFACE_LINK</constant>
+	    if the link is between an interface and an entity</para></entry>
+	  </row>
+	</tbody>
+      </tgroup>
+    </table>
+
+</section>
diff --git a/Documentation/DocBook/media/v4l/v4l2.xml b/Documentation/DocBook/media/v4l/v4l2.xml
index 7e61643..42e626d 100644
--- a/Documentation/DocBook/media/v4l/v4l2.xml
+++ b/Documentation/DocBook/media/v4l/v4l2.xml
@@ -152,6 +152,16 @@
 (compat.xml), along with the possible impact on existing drivers and
 applications. -->
       <revision>
+	<revnumber>4.5</revnumber>
+	<date>2015-10-29</date>
+	<authorinitials>rr</authorinitials>
+	<revremark>Extend vidioc-g-ext-ctrls;. Replace ctrl_class with a new
+union with ctrl_class and which. Which is used to select the current value of
+the control or the default value.
+	</revremark>
+      </revision>
+
+      <revision>
 	<revnumber>4.4</revnumber>
 	<date>2015-05-26</date>
 	<authorinitials>ap</authorinitials>
diff --git a/Documentation/DocBook/media/v4l/vidioc-create-bufs.xml b/Documentation/DocBook/media/v4l/vidioc-create-bufs.xml
index 8ffe74f..d81fa0d 100644
--- a/Documentation/DocBook/media/v4l/vidioc-create-bufs.xml
+++ b/Documentation/DocBook/media/v4l/vidioc-create-bufs.xml
@@ -58,7 +58,7 @@
     <para>This ioctl is used to create buffers for <link linkend="mmap">memory
 mapped</link> or <link linkend="userp">user pointer</link> or <link
 linkend="dmabuf">DMA buffer</link> I/O. It can be used as an alternative or in
-addition to the <constant>VIDIOC_REQBUFS</constant> ioctl, when a tighter
+addition to the &VIDIOC-REQBUFS; ioctl, when a tighter
 control over buffers is required. This ioctl can be called multiple times to
 create buffers of different sizes.</para>
 
@@ -71,30 +71,28 @@
 
     <para>The <structfield>format</structfield> field specifies the image format
 that the buffers must be able to handle. The application has to fill in this
-&v4l2-format;. Usually this will be done using the
-<constant>VIDIOC_TRY_FMT</constant> or <constant>VIDIOC_G_FMT</constant> ioctl()
-to ensure that the requested format is supported by the driver. Unsupported
-formats will result in an error.</para>
+&v4l2-format;. Usually this will be done using the &VIDIOC-TRY-FMT; or &VIDIOC-G-FMT; ioctls
+to ensure that the requested format is supported by the driver.
+Based on the format's <structfield>type</structfield> field the requested buffer
+size (for single-planar) or plane sizes (for multi-planar formats) will be
+used for the allocated buffers. The driver may return an error if the size(s)
+are not supported by the hardware (usually because they are too small).</para>
 
     <para>The buffers created by this ioctl will have as minimum size the size
-defined by the <structfield>format.pix.sizeimage</structfield> field. If the
+defined by the <structfield>format.pix.sizeimage</structfield> field (or the
+corresponding fields for other format types). Usually if the
 <structfield>format.pix.sizeimage</structfield> field is less than the minimum
-required for the given format, then <structfield>sizeimage</structfield> will be
-increased by the driver to that minimum to allocate the buffers. If it is
-larger, then the value will be used as-is. The same applies to the
-<structfield>sizeimage</structfield> field of the
-<structname>v4l2_plane_pix_format</structname> structure in the case of
-multiplanar formats.</para>
+required for the given format, then an error will be returned since drivers will
+typically not allow this. If it is larger, then the value will be used as-is.
+In other words, the driver may reject the requested size, but if it is accepted
+the driver will use it unchanged.</para>
 
     <para>When the ioctl is called with a pointer to this structure the driver
 will attempt to allocate up to the requested number of buffers and store the
 actual number allocated and the starting index in the
 <structfield>count</structfield> and the <structfield>index</structfield> fields
 respectively. On return <structfield>count</structfield> can be smaller than
-the number requested. The driver may also increase buffer sizes if required,
-however, it will not update <structfield>sizeimage</structfield> field values.
-The user has to use <constant>VIDIOC_QUERYBUF</constant> to retrieve that
-information.</para>
+the number requested.</para>
 
     <table pgwide="1" frame="none" id="v4l2-create-buffers">
       <title>struct <structname>v4l2_create_buffers</structname></title>
diff --git a/Documentation/DocBook/media/v4l/vidioc-dbg-g-chip-info.xml b/Documentation/DocBook/media/v4l/vidioc-dbg-g-chip-info.xml
index 4c4603c..f14a3bb 100644
--- a/Documentation/DocBook/media/v4l/vidioc-dbg-g-chip-info.xml
+++ b/Documentation/DocBook/media/v4l/vidioc-dbg-g-chip-info.xml
@@ -99,7 +99,7 @@
     <para>We recommended the <application>v4l2-dbg</application>
 utility over calling this ioctl directly. It is available from the
 LinuxTV v4l-dvb repository; see <ulink
-url="http://linuxtv.org/repo/">http://linuxtv.org/repo/</ulink> for
+url="https://linuxtv.org/repo/">https://linuxtv.org/repo/</ulink> for
 access instructions.</para>
 
     <!-- Note for convenience vidioc-dbg-g-register.sgml
diff --git a/Documentation/DocBook/media/v4l/vidioc-dbg-g-register.xml b/Documentation/DocBook/media/v4l/vidioc-dbg-g-register.xml
index 3d038e7..5877f68 100644
--- a/Documentation/DocBook/media/v4l/vidioc-dbg-g-register.xml
+++ b/Documentation/DocBook/media/v4l/vidioc-dbg-g-register.xml
@@ -117,7 +117,7 @@
     <para>We recommended the <application>v4l2-dbg</application>
 utility over calling these ioctls directly. It is available from the
 LinuxTV v4l-dvb repository; see <ulink
-url="http://linuxtv.org/repo/">http://linuxtv.org/repo/</ulink> for
+url="https://linuxtv.org/repo/">https://linuxtv.org/repo/</ulink> for
 access instructions.</para>
 
     <!-- Note for convenience vidioc-dbg-g-chip-info.sgml
diff --git a/Documentation/DocBook/media/v4l/vidioc-enumstd.xml b/Documentation/DocBook/media/v4l/vidioc-enumstd.xml
index 8065099..f18454e 100644
--- a/Documentation/DocBook/media/v4l/vidioc-enumstd.xml
+++ b/Documentation/DocBook/media/v4l/vidioc-enumstd.xml
@@ -198,7 +198,7 @@
 <constant>V4L2_STD_ATSC_16_VSB</constant> are U.S. terrestrial digital
 TV standards. Presently the V4L2 API does not support digital TV. See
 also the Linux DVB API at <ulink
-url="http://linuxtv.org">http://linuxtv.org</ulink>.</para>
+url="https://linuxtv.org">https://linuxtv.org</ulink>.</para>
 <para><programlisting>
 #define V4L2_STD_PAL_BG         (V4L2_STD_PAL_B         |\
 				 V4L2_STD_PAL_B1        |\
diff --git a/Documentation/DocBook/media/v4l/vidioc-g-ext-ctrls.xml b/Documentation/DocBook/media/v4l/vidioc-g-ext-ctrls.xml
index 842536a..eb82f7e 100644
--- a/Documentation/DocBook/media/v4l/vidioc-g-ext-ctrls.xml
+++ b/Documentation/DocBook/media/v4l/vidioc-g-ext-ctrls.xml
@@ -61,7 +61,7 @@
 
     <para>Applications must always fill in the
 <structfield>count</structfield>,
-<structfield>ctrl_class</structfield>,
+<structfield>which</structfield>,
 <structfield>controls</structfield> and
 <structfield>reserved</structfield> fields of &v4l2-ext-controls;, and
 initialize the &v4l2-ext-control; array pointed to by the
@@ -109,7 +109,7 @@
 value or if an error is returned.</para>
 
     <para>When the <structfield>id</structfield> or
-<structfield>ctrl_class</structfield> is invalid drivers return an
+<structfield>which</structfield> is invalid drivers return an
 &EINVAL;. When the value is out of bounds drivers can choose to take
 the closest valid value or return an &ERANGE;, whatever seems more
 appropriate. In the first case the new value is set in
@@ -223,7 +223,12 @@
       <tgroup cols="3">
 	&cs-str;
 	<tbody valign="top">
+	 <row>
+	    <entry>union</entry>
+	    <entry>(anonymous)</entry>
+	  </row>
 	  <row>
+	    <entry></entry>
 	    <entry>__u32</entry>
 	    <entry><structfield>ctrl_class</structfield></entry>
 	    <entry>The control class to which all controls belong, see
@@ -235,6 +240,23 @@
 supports this feature.</entry>
 	  </row>
 	  <row>
+	    <entry></entry>
+	    <entry>__u32</entry>
+	    <entry><structfield>which</structfield></entry>
+	    <entry><para>Which value of the control to get/set/try. <constant>V4L2_CTRL_WHICH_CUR_VAL</constant>
+will return the current value of the control and <constant>V4L2_CTRL_WHICH_DEF_VAL</constant> will
+return the default value of the control. Please note that you can only get the default value of the
+control, you cannot set or try it.</para>
+<para>For backwards compatibility you can also use a control class here (see
+<xref linkend="ctrl-class" />). In that case all controls have to belong to that
+control class. This usage is deprecated, instead just use <constant>V4L2_CTRL_WHICH_CUR_VAL</constant>.
+There are some very old drivers that do not yet support <constant>V4L2_CTRL_WHICH_CUR_VAL</constant>
+and that require a control class here. You can test for such drivers by setting ctrl_class to
+<constant>V4L2_CTRL_WHICH_CUR_VAL</constant> and calling VIDIOC_TRY_EXT_CTRLS with a count of 0.
+If that fails, then the driver does not support <constant>V4L2_CTRL_WHICH_CUR_VAL</constant>.</para>
+</entry>
+	  </row>
+	  <row>
 	    <entry>__u32</entry>
 	    <entry><structfield>count</structfield></entry>
 	    <entry>The number of controls in the controls array. May
@@ -390,7 +412,7 @@
 	<listitem>
 	  <para>The &v4l2-ext-control; <structfield>id</structfield>
 is invalid, the &v4l2-ext-controls;
-<structfield>ctrl_class</structfield> is invalid, or the &v4l2-ext-control;
+<structfield>which</structfield> is invalid, or the &v4l2-ext-control;
 <structfield>value</structfield> was inappropriate (e.g. the given menu
 index is not supported by the driver). This error code is
 also returned by the <constant>VIDIOC_S_EXT_CTRLS</constant> and
diff --git a/Documentation/DocBook/media_api.tmpl b/Documentation/DocBook/media_api.tmpl
index 9203703..7b77e0f 100644
--- a/Documentation/DocBook/media_api.tmpl
+++ b/Documentation/DocBook/media_api.tmpl
@@ -19,10 +19,10 @@
 <!ENTITY cs-def                 "<colspec colname='c1' colwidth='3*' /><colspec colname='c2' colwidth='1*' /><colspec colname='c3' colwidth='4*' /><spanspec spanname='hspan' namest='c1' nameend='c3' />">
 
 <!-- Video for Linux mailing list address. -->
-<!ENTITY v4l-ml                 "<ulink url='http://www.linuxtv.org/lists.php'>http://www.linuxtv.org/lists.php</ulink>">
+<!ENTITY v4l-ml                 "<ulink url='https://linuxtv.org/lists.php'>https://linuxtv.org/lists.php</ulink>">
 
 <!-- LinuxTV v4l-dvb repository. -->
-<!ENTITY v4l-dvb		"<ulink url='http://linuxtv.org/repo/'>http://linuxtv.org/repo/</ulink>">
+<!ENTITY v4l-dvb		"<ulink url='https://linuxtv.org/repo/'>https://linuxtv.org/repo/</ulink>">
 <!ENTITY dash-ent-8             "<entry>-</entry><entry>-</entry><entry>-</entry><entry>-</entry><entry>-</entry><entry>-</entry><entry>-</entry><entry>-</entry>">
 <!ENTITY dash-ent-10            "<entry>-</entry><entry>-</entry><entry>-</entry><entry>-</entry><entry>-</entry><entry>-</entry><entry>-</entry><entry>-</entry><entry>-</entry><entry>-</entry>">
 <!ENTITY dash-ent-12            "<entry>-</entry><entry>-</entry><entry>-</entry><entry>-</entry><entry>-</entry><entry>-</entry><entry>-</entry><entry>-</entry><entry>-</entry><entry>-</entry><entry>-</entry><entry>-</entry>">
@@ -91,7 +91,7 @@
 	      components, like mixers, PCM capture, PCM playback, etc, which
 	      are controlled via ALSA API.</para>
 	<para>For additional information and for the latest development code,
-		see: <ulink url="http://linuxtv.org">http://linuxtv.org</ulink>.</para>
+		see: <ulink url="https://linuxtv.org">https://linuxtv.org</ulink>.</para>
 	<para>For discussing improvements, reporting troubles, sending new drivers, etc, please mail to: <ulink url="http://vger.kernel.org/vger-lists.html#linux-media">Linux Media Mailing List (LMML).</ulink>.</para>
 </preface>
 
diff --git a/Documentation/DocBook/mtdnand.tmpl b/Documentation/DocBook/mtdnand.tmpl
index 7da8f04..b442921 100644
--- a/Documentation/DocBook/mtdnand.tmpl
+++ b/Documentation/DocBook/mtdnand.tmpl
@@ -162,12 +162,15 @@
 	<sect1 id="Basic_defines">
 		<title>Basic defines</title>
 		<para>
-			At least you have to provide a mtd structure and
-			a storage for the ioremap'ed chip address.
-			You can allocate the mtd structure using kmalloc
-			or you can allocate it statically.
-			In case of static allocation you have to allocate
-			a nand_chip structure too.
+			At least you have to provide a nand_chip structure
+			and a storage for the ioremap'ed chip address.
+			You can allocate the nand_chip structure using
+			kmalloc or you can allocate it statically.
+			The NAND chip structure embeds an mtd structure
+			which will be registered to the MTD subsystem.
+			You can extract a pointer to the mtd structure
+			from a nand_chip pointer using the nand_to_mtd()
+			helper.
 		</para>
 		<para>
 			Kmalloc based example
@@ -180,7 +183,6 @@
 			Static example
 		</para>
 		<programlisting>
-static struct mtd_info board_mtd;
 static struct nand_chip board_chip;
 static void __iomem *baseaddr;
 		</programlisting>
@@ -235,7 +237,7 @@
 		<programlisting>
 static void board_hwcontrol(struct mtd_info *mtd, int cmd)
 {
-	struct nand_chip *this = (struct nand_chip *) mtd->priv;
+	struct nand_chip *this = mtd_to_nand(mtd);
 	switch(cmd){
 		case NAND_CTL_SETCLE: this->IO_ADDR_W |= CLE_ADRR_BIT;  break;
 		case NAND_CTL_CLRCLE: this->IO_ADDR_W &amp;= ~CLE_ADRR_BIT; break;
@@ -274,13 +276,15 @@
 	int err = 0;
 
 	/* Allocate memory for MTD device structure and private data */
-	board_mtd = kzalloc(sizeof(struct mtd_info) + sizeof(struct nand_chip), GFP_KERNEL);
-	if (!board_mtd) {
+	this = kzalloc(sizeof(struct nand_chip), GFP_KERNEL);
+	if (!this) {
 		printk ("Unable to allocate NAND MTD device structure.\n");
 		err = -ENOMEM;
 		goto out;
 	}
 
+	board_mtd = nand_to_mtd(this);
+
 	/* map physical address */
 	baseaddr = ioremap(CHIP_PHYSICAL_ADDRESS, 1024);
 	if (!baseaddr) {
@@ -289,11 +293,6 @@
 		goto out_mtd;
 	}
 
-	/* Get pointer to private data */
-	this = (struct nand_chip *) ();
-	/* Link the private data with the MTD structure */
-	board_mtd->priv = this;
-
 	/* Set address of NAND IO lines */
 	this->IO_ADDR_R = baseaddr;
 	this->IO_ADDR_W = baseaddr;
@@ -317,7 +316,7 @@
 out_ior:
 	iounmap(baseaddr);
 out_mtd:
-	kfree (board_mtd);
+	kfree (this);
 out:
 	return err;
 }
@@ -343,7 +342,7 @@
 	iounmap(baseaddr);
 	
 	/* Free the MTD device structure */
-	kfree (board_mtd);
+	kfree (mtd_to_nand(board_mtd));
 }
 module_exit(board_cleanup);
 #endif
@@ -399,7 +398,7 @@
 		<programlisting>
 static void board_select_chip (struct mtd_info *mtd, int chip)
 {
-	struct nand_chip *this = (struct nand_chip *) mtd->priv;
+	struct nand_chip *this = mtd_to_nand(mtd);
 	
 	/* Deselect all chips */
 	this->IO_ADDR_R &amp;= ~BOARD_NAND_ADDR_MASK;
diff --git a/Documentation/HOWTO b/Documentation/HOWTO
index 21152d3..d5a699d 100644
--- a/Documentation/HOWTO
+++ b/Documentation/HOWTO
@@ -209,7 +209,7 @@
 Cross-Reference project, which is able to present source code in a
 self-referential, indexed webpage format. An excellent up-to-date
 repository of the kernel code may be found at:
-	http://lxr.linux.no/+trees
+	http://lxr.free-electrons.com/
 
 
 The development process
diff --git a/Documentation/IPMI.txt b/Documentation/IPMI.txt
index 31d1d65..c0d8788 100644
--- a/Documentation/IPMI.txt
+++ b/Documentation/IPMI.txt
@@ -587,7 +587,7 @@
 
   modprobe ipmi_watchdog timeout=<t> pretimeout=<t> action=<action type>
       preaction=<preaction type> preop=<preop type> start_now=x
-      nowayout=x ifnum_to_use=n
+      nowayout=x ifnum_to_use=n panic_wdt_timeout=<t>
 
 ifnum_to_use specifies which interface the watchdog timer should use.
 The default is -1, which means to pick the first one registered.
@@ -597,7 +597,9 @@
 occur (if pretimeout is zero, then pretimeout will not be enabled).  Note
 that the pretimeout is the time before the final timeout.  So if the
 timeout is 50 seconds and the pretimeout is 10 seconds, then the pretimeout
-will occur in 40 second (10 seconds before the timeout).
+will occur in 40 second (10 seconds before the timeout). The panic_wdt_timeout
+is the value of timeout which is set on kernel panic, in order to let actions
+such as kdump to occur during panic.
 
 The action may be "reset", "power_cycle", or "power_off", and
 specifies what to do when the timer times out, and defaults to
@@ -634,6 +636,7 @@
 	ipmi_watchdog.preop=<preop type>
 	ipmi_watchdog.start_now=x
 	ipmi_watchdog.nowayout=x
+	ipmi_watchdog.panic_wdt_timeout=<t>
 
 The options are the same as the module parameter options.
 
diff --git a/Documentation/Intel-IOMMU.txt b/Documentation/Intel-IOMMU.txt
index 7b57fc0..49585b6 100644
--- a/Documentation/Intel-IOMMU.txt
+++ b/Documentation/Intel-IOMMU.txt
@@ -3,7 +3,7 @@
 
 The architecture spec can be obtained from the below location.
 
-http://www.intel.com/technology/virtualization/
+http://www.intel.com/content/dam/www/public/us/en/documents/product-specifications/vt-directed-io-spec.pdf
 
 This guide gives a quick cheat sheet for some basic understanding.
 
diff --git a/Documentation/Makefile b/Documentation/Makefile
index bc05482..1207d79 100644
--- a/Documentation/Makefile
+++ b/Documentation/Makefile
@@ -1,4 +1,4 @@
 subdir-y := accounting auxdisplay blackfin connector \
 	filesystems filesystems ia64 laptops mic misc-devices \
-	networking pcmcia prctl ptp spi timers vDSO video4linux \
+	networking pcmcia prctl ptp timers vDSO video4linux \
 	watchdog
diff --git a/Documentation/RCU/Design/Requirements/2013-08-is-it-dead.png b/Documentation/RCU/Design/Requirements/2013-08-is-it-dead.png
new file mode 100644
index 0000000..7496a55
--- /dev/null
+++ b/Documentation/RCU/Design/Requirements/2013-08-is-it-dead.png
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diff --git a/Documentation/RCU/Design/Requirements/GPpartitionReaders1.svg b/Documentation/RCU/Design/Requirements/GPpartitionReaders1.svg
new file mode 100644
index 0000000..4b4014f
--- /dev/null
+++ b/Documentation/RCU/Design/Requirements/GPpartitionReaders1.svg
@@ -0,0 +1,374 @@
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+       xml:space="preserve"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
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+    <text
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+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
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+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
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+    <text
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+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
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+         sodipodi:role="line"
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+       xml:space="preserve"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
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+       xml:space="preserve"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
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+         sodipodi:role="line"
+         id="tspan4431-9-4-4"
+         x="396.10254"
+         y="597.40289">rcu_read_unlock();</tspan></text>
+    <text
+       xml:space="preserve"
+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
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+         sodipodi:role="line"
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diff --git a/Documentation/RCU/Design/Requirements/RCUApplicability.svg b/Documentation/RCU/Design/Requirements/RCUApplicability.svg
new file mode 100644
index 0000000..ebcbeee
--- /dev/null
+++ b/Documentation/RCU/Design/Requirements/RCUApplicability.svg
@@ -0,0 +1,237 @@
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+      <cc:Work
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+        <dc:format>image/svg+xml</dc:format>
+        <dc:type
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+        <dc:title />
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+       y="0"
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+       rx="0"
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+       id="rect12" />
+    <!-- Text -->
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+       xml:space="preserve"
+       x="7200"
+       y="900"
+       font-style="normal"
+       font-weight="normal"
+       font-size="324"
+       id="text14"
+       sodipodi:linespacing="125%"
+       style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"><tspan
+         style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"
+         id="tspan3017">Read-Mostly, Stale &amp;</tspan></text>
+    <!-- Text -->
+    <text
+       xml:space="preserve"
+       x="7200"
+       y="1350"
+       font-style="normal"
+       font-weight="normal"
+       font-size="324"
+       id="text16"
+       sodipodi:linespacing="125%"
+       style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"><tspan
+         style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"
+         id="tspan3019">Inconsistent Data OK</tspan></text>
+    <!-- Text -->
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+       font-weight="normal"
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+       style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"><tspan
+         style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"
+         id="tspan3021">(RCU Works Great!!!)</tspan></text>
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+       style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"><tspan
+         style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"
+         id="tspan3023">(RCU Works Well)</tspan></text>
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+    <text
+       xml:space="preserve"
+       x="7200"
+       y="3375"
+       font-style="normal"
+       font-weight="normal"
+       font-size="324"
+       id="text22"
+       sodipodi:linespacing="125%"
+       style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"><tspan
+         style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"
+         id="tspan3025">Read-Mostly, Need Consistent Data</tspan></text>
+    <!-- Text -->
+    <text
+       xml:space="preserve"
+       x="7200"
+       y="5175"
+       font-style="normal"
+       font-weight="normal"
+       font-size="324"
+       id="text24"
+       sodipodi:linespacing="125%"
+       style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"><tspan
+         style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"
+         id="tspan3027">Read-Write, Need Consistent Data</tspan></text>
+    <!-- Text -->
+    <text
+       xml:space="preserve"
+       x="7200"
+       y="6975"
+       font-style="normal"
+       font-weight="normal"
+       font-size="324"
+       id="text26"
+       style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"
+       sodipodi:linespacing="125%">Update-Mostly, Need Consistent Data</text>
+    <!-- Text -->
+    <text
+       xml:space="preserve"
+       x="7200"
+       y="5625"
+       font-style="normal"
+       font-weight="normal"
+       font-size="324"
+       id="text28"
+       sodipodi:linespacing="125%"
+       style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"><tspan
+         style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"
+         id="tspan3029">(RCU Might Be OK...)</tspan></text>
+    <!-- Text -->
+    <text
+       xml:space="preserve"
+       x="7200"
+       y="7875"
+       font-style="normal"
+       font-weight="normal"
+       font-size="324"
+       id="text30"
+       style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"
+       sodipodi:linespacing="125%">(1) Provide Existence Guarantees For Update-Friendly Mechanisms</text>
+    <!-- Text -->
+    <text
+       xml:space="preserve"
+       x="7200"
+       y="8325"
+       font-style="normal"
+       font-weight="normal"
+       font-size="324"
+       id="text32"
+       style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"
+       sodipodi:linespacing="125%">(2) Provide Wait-Free Read-Side Primitives for Real-Time Use)</text>
+    <!-- Text -->
+    <text
+       xml:space="preserve"
+       x="7200"
+       y="7425"
+       font-style="normal"
+       font-weight="normal"
+       font-size="324"
+       id="text34"
+       style="font-size:427.63009644px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;writing-mode:lr-tb;text-anchor:middle;fill:#000000;font-family:Nimbus Sans L;-inkscape-font-specification:Nimbus Sans L"
+       sodipodi:linespacing="125%">(RCU is Very Unlikely to be the Right Tool For The Job, But it Can:</text>
+  </g>
+</svg>
diff --git a/Documentation/RCU/Design/Requirements/ReadersPartitionGP1.svg b/Documentation/RCU/Design/Requirements/ReadersPartitionGP1.svg
new file mode 100644
index 0000000..48cd162
--- /dev/null
+++ b/Documentation/RCU/Design/Requirements/ReadersPartitionGP1.svg
@@ -0,0 +1,639 @@
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+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
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+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
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+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
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+    <text
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+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
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+    <text
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+       style="font-size:10px;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;text-align:center;line-height:125%;letter-spacing:0px;word-spacing:0px;writing-mode:lr-tb;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Symbol;-inkscape-font-specification:Symbol"
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+         sodipodi:role="line"
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diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html
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+<!-- DO NOT HAND EDIT. -->
+<!-- Instead, edit Documentation/RCU/Design/Requirements/Requirements.htmlx and run 'sh htmlqqz.sh Documentation/RCU/Design/Requirements/Requirements' -->
+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
+        "http://www.w3.org/TR/html4/loose.dtd">
+        <html>
+        <head><title>A Tour Through RCU's Requirements [LWN.net]</title>
+        <meta HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=utf-8">
+
+<h1>A Tour Through RCU's Requirements</h1>
+
+<p>Copyright IBM Corporation, 2015</p>
+<p>Author: Paul E.&nbsp;McKenney</p>
+<p><i>The initial version of this document appeared in the
+<a href="https://lwn.net/">LWN</a> articles
+<a href="https://lwn.net/Articles/652156/">here</a>,
+<a href="https://lwn.net/Articles/652677/">here</a>, and
+<a href="https://lwn.net/Articles/653326/">here</a>.</i></p>
+
+<h2>Introduction</h2>
+
+<p>
+Read-copy update (RCU) is a synchronization mechanism that is often
+used as a replacement for reader-writer locking.
+RCU is unusual in that updaters do not block readers,
+which means that RCU's read-side primitives can be exceedingly fast
+and scalable.
+In addition, updaters can make useful forward progress concurrently
+with readers.
+However, all this concurrency between RCU readers and updaters does raise
+the question of exactly what RCU readers are doing, which in turn
+raises the question of exactly what RCU's requirements are.
+
+<p>
+This document therefore summarizes RCU's requirements, and can be thought
+of as an informal, high-level specification for RCU.
+It is important to understand that RCU's specification is primarily
+empirical in nature;
+in fact, I learned about many of these requirements the hard way.
+This situation might cause some consternation, however, not only
+has this learning process been a lot of fun, but it has also been
+a great privilege to work with so many people willing to apply
+technologies in interesting new ways.
+
+<p>
+All that aside, here are the categories of currently known RCU requirements:
+</p>
+
+<ol>
+<li>	<a href="#Fundamental Requirements">
+	Fundamental Requirements</a>
+<li>	<a href="#Fundamental Non-Requirements">Fundamental Non-Requirements</a>
+<li>	<a href="#Parallelism Facts of Life">
+	Parallelism Facts of Life</a>
+<li>	<a href="#Quality-of-Implementation Requirements">
+	Quality-of-Implementation Requirements</a>
+<li>	<a href="#Linux Kernel Complications">
+	Linux Kernel Complications</a>
+<li>	<a href="#Software-Engineering Requirements">
+	Software-Engineering Requirements</a>
+<li>	<a href="#Other RCU Flavors">
+	Other RCU Flavors</a>
+<li>	<a href="#Possible Future Changes">
+	Possible Future Changes</a>
+</ol>
+
+<p>
+This is followed by a <a href="#Summary">summary</a>,
+which is in turn followed by the inevitable
+<a href="#Answers to Quick Quizzes">answers to the quick quizzes</a>.
+
+<h2><a name="Fundamental Requirements">Fundamental Requirements</a></h2>
+
+<p>
+RCU's fundamental requirements are the closest thing RCU has to hard
+mathematical requirements.
+These are:
+
+<ol>
+<li>	<a href="#Grace-Period Guarantee">
+	Grace-Period Guarantee</a>
+<li>	<a href="#Publish-Subscribe Guarantee">
+	Publish-Subscribe Guarantee</a>
+<li>	<a href="#Memory-Barrier Guarantees">
+	Memory-Barrier Guarantees</a>
+<li>	<a href="#RCU Primitives Guaranteed to Execute Unconditionally">
+	RCU Primitives Guaranteed to Execute Unconditionally</a>
+<li>	<a href="#Guaranteed Read-to-Write Upgrade">
+	Guaranteed Read-to-Write Upgrade</a>
+</ol>
+
+<h3><a name="Grace-Period Guarantee">Grace-Period Guarantee</a></h3>
+
+<p>
+RCU's grace-period guarantee is unusual in being premeditated:
+Jack Slingwine and I had this guarantee firmly in mind when we started
+work on RCU (then called &ldquo;rclock&rdquo;) in the early 1990s.
+That said, the past two decades of experience with RCU have produced
+a much more detailed understanding of this guarantee.
+
+<p>
+RCU's grace-period guarantee allows updaters to wait for the completion
+of all pre-existing RCU read-side critical sections.
+An RCU read-side critical section
+begins with the marker <tt>rcu_read_lock()</tt> and ends with
+the marker <tt>rcu_read_unlock()</tt>.
+These markers may be nested, and RCU treats a nested set as one
+big RCU read-side critical section.
+Production-quality implementations of <tt>rcu_read_lock()</tt> and
+<tt>rcu_read_unlock()</tt> are extremely lightweight, and in
+fact have exactly zero overhead in Linux kernels built for production
+use with <tt>CONFIG_PREEMPT=n</tt>.
+
+<p>
+This guarantee allows ordering to be enforced with extremely low
+overhead to readers, for example:
+
+<blockquote>
+<pre>
+ 1 int x, y;
+ 2
+ 3 void thread0(void)
+ 4 {
+ 5   rcu_read_lock();
+ 6   r1 = READ_ONCE(x);
+ 7   r2 = READ_ONCE(y);
+ 8   rcu_read_unlock();
+ 9 }
+10
+11 void thread1(void)
+12 {
+13   WRITE_ONCE(x, 1);
+14   synchronize_rcu();
+15   WRITE_ONCE(y, 1);
+16 }
+</pre>
+</blockquote>
+
+<p>
+Because the <tt>synchronize_rcu()</tt> on line&nbsp;14 waits for
+all pre-existing readers, any instance of <tt>thread0()</tt> that
+loads a value of zero from <tt>x</tt> must complete before
+<tt>thread1()</tt> stores to <tt>y</tt>, so that instance must
+also load a value of zero from <tt>y</tt>.
+Similarly, any instance of <tt>thread0()</tt> that loads a value of
+one from <tt>y</tt> must have started after the
+<tt>synchronize_rcu()</tt> started, and must therefore also load
+a value of one from <tt>x</tt>.
+Therefore, the outcome:
+<blockquote>
+<pre>
+(r1 == 0 &amp;&amp; r2 == 1)
+</pre>
+</blockquote>
+cannot happen.
+
+<p><a name="Quick Quiz 1"><b>Quick Quiz 1</b>:</a>
+Wait a minute!
+You said that updaters can make useful forward progress concurrently
+with readers, but pre-existing readers will block
+<tt>synchronize_rcu()</tt>!!!
+Just who are you trying to fool???
+<br><a href="#qq1answer">Answer</a>
+
+<p>
+This scenario resembles one of the first uses of RCU in
+<a href="https://en.wikipedia.org/wiki/DYNIX">DYNIX/ptx</a>,
+which managed a distributed lock manager's transition into
+a state suitable for handling recovery from node failure,
+more or less as follows:
+
+<blockquote>
+<pre>
+ 1 #define STATE_NORMAL        0
+ 2 #define STATE_WANT_RECOVERY 1
+ 3 #define STATE_RECOVERING    2
+ 4 #define STATE_WANT_NORMAL   3
+ 5
+ 6 int state = STATE_NORMAL;
+ 7
+ 8 void do_something_dlm(void)
+ 9 {
+10   int state_snap;
+11
+12   rcu_read_lock();
+13   state_snap = READ_ONCE(state);
+14   if (state_snap == STATE_NORMAL)
+15     do_something();
+16   else
+17     do_something_carefully();
+18   rcu_read_unlock();
+19 }
+20
+21 void start_recovery(void)
+22 {
+23   WRITE_ONCE(state, STATE_WANT_RECOVERY);
+24   synchronize_rcu();
+25   WRITE_ONCE(state, STATE_RECOVERING);
+26   recovery();
+27   WRITE_ONCE(state, STATE_WANT_NORMAL);
+28   synchronize_rcu();
+29   WRITE_ONCE(state, STATE_NORMAL);
+30 }
+</pre>
+</blockquote>
+
+<p>
+The RCU read-side critical section in <tt>do_something_dlm()</tt>
+works with the <tt>synchronize_rcu()</tt> in <tt>start_recovery()</tt>
+to guarantee that <tt>do_something()</tt> never runs concurrently
+with <tt>recovery()</tt>, but with little or no synchronization
+overhead in <tt>do_something_dlm()</tt>.
+
+<p><a name="Quick Quiz 2"><b>Quick Quiz 2</b>:</a>
+Why is the <tt>synchronize_rcu()</tt> on line&nbsp;28 needed?
+<br><a href="#qq2answer">Answer</a>
+
+<p>
+In order to avoid fatal problems such as deadlocks,
+an RCU read-side critical section must not contain calls to
+<tt>synchronize_rcu()</tt>.
+Similarly, an RCU read-side critical section must not
+contain anything that waits, directly or indirectly, on completion of
+an invocation of <tt>synchronize_rcu()</tt>.
+
+<p>
+Although RCU's grace-period guarantee is useful in and of itself, with
+<a href="https://lwn.net/Articles/573497/">quite a few use cases</a>,
+it would be good to be able to use RCU to coordinate read-side
+access to linked data structures.
+For this, the grace-period guarantee is not sufficient, as can
+be seen in function <tt>add_gp_buggy()</tt> below.
+We will look at the reader's code later, but in the meantime, just think of
+the reader as locklessly picking up the <tt>gp</tt> pointer,
+and, if the value loaded is non-<tt>NULL</tt>, locklessly accessing the
+<tt>-&gt;a</tt> and <tt>-&gt;b</tt> fields.
+
+<blockquote>
+<pre>
+ 1 bool add_gp_buggy(int a, int b)
+ 2 {
+ 3   p = kmalloc(sizeof(*p), GFP_KERNEL);
+ 4   if (!p)
+ 5     return -ENOMEM;
+ 6   spin_lock(&amp;gp_lock);
+ 7   if (rcu_access_pointer(gp)) {
+ 8     spin_unlock(&amp;gp_lock);
+ 9     return false;
+10   }
+11   p-&gt;a = a;
+12   p-&gt;b = a;
+13   gp = p; /* ORDERING BUG */
+14   spin_unlock(&amp;gp_lock);
+15   return true;
+16 }
+</pre>
+</blockquote>
+
+<p>
+The problem is that both the compiler and weakly ordered CPUs are within
+their rights to reorder this code as follows:
+
+<blockquote>
+<pre>
+ 1 bool add_gp_buggy_optimized(int a, int b)
+ 2 {
+ 3   p = kmalloc(sizeof(*p), GFP_KERNEL);
+ 4   if (!p)
+ 5     return -ENOMEM;
+ 6   spin_lock(&amp;gp_lock);
+ 7   if (rcu_access_pointer(gp)) {
+ 8     spin_unlock(&amp;gp_lock);
+ 9     return false;
+10   }
+<b>11   gp = p; /* ORDERING BUG */
+12   p-&gt;a = a;
+13   p-&gt;b = a;</b>
+14   spin_unlock(&amp;gp_lock);
+15   return true;
+16 }
+</pre>
+</blockquote>
+
+<p>
+If an RCU reader fetches <tt>gp</tt> just after
+<tt>add_gp_buggy_optimized</tt> executes line&nbsp;11,
+it will see garbage in the <tt>-&gt;a</tt> and <tt>-&gt;b</tt>
+fields.
+And this is but one of many ways in which compiler and hardware optimizations
+could cause trouble.
+Therefore, we clearly need some way to prevent the compiler and the CPU from
+reordering in this manner, which brings us to the publish-subscribe
+guarantee discussed in the next section.
+
+<h3><a name="Publish-Subscribe Guarantee">Publish/Subscribe Guarantee</a></h3>
+
+<p>
+RCU's publish-subscribe guarantee allows data to be inserted
+into a linked data structure without disrupting RCU readers.
+The updater uses <tt>rcu_assign_pointer()</tt> to insert the
+new data, and readers use <tt>rcu_dereference()</tt> to
+access data, whether new or old.
+The following shows an example of insertion:
+
+<blockquote>
+<pre>
+ 1 bool add_gp(int a, int b)
+ 2 {
+ 3   p = kmalloc(sizeof(*p), GFP_KERNEL);
+ 4   if (!p)
+ 5     return -ENOMEM;
+ 6   spin_lock(&amp;gp_lock);
+ 7   if (rcu_access_pointer(gp)) {
+ 8     spin_unlock(&amp;gp_lock);
+ 9     return false;
+10   }
+11   p-&gt;a = a;
+12   p-&gt;b = a;
+13   rcu_assign_pointer(gp, p);
+14   spin_unlock(&amp;gp_lock);
+15   return true;
+16 }
+</pre>
+</blockquote>
+
+<p>
+The <tt>rcu_assign_pointer()</tt> on line&nbsp;13 is conceptually
+equivalent to a simple assignment statement, but also guarantees
+that its assignment will
+happen after the two assignments in lines&nbsp;11 and&nbsp;12,
+similar to the C11 <tt>memory_order_release</tt> store operation.
+It also prevents any number of &ldquo;interesting&rdquo; compiler
+optimizations, for example, the use of <tt>gp</tt> as a scratch
+location immediately preceding the assignment.
+
+<p><a name="Quick Quiz 3"><b>Quick Quiz 3</b>:</a>
+But <tt>rcu_assign_pointer()</tt> does nothing to prevent the
+two assignments to <tt>p-&gt;a</tt> and <tt>p-&gt;b</tt>
+from being reordered.
+Can't that also cause problems?
+<br><a href="#qq3answer">Answer</a>
+
+<p>
+It is tempting to assume that the reader need not do anything special
+to control its accesses to the RCU-protected data,
+as shown in <tt>do_something_gp_buggy()</tt> below:
+
+<blockquote>
+<pre>
+ 1 bool do_something_gp_buggy(void)
+ 2 {
+ 3   rcu_read_lock();
+ 4   p = gp;  /* OPTIMIZATIONS GALORE!!! */
+ 5   if (p) {
+ 6     do_something(p-&gt;a, p-&gt;b);
+ 7     rcu_read_unlock();
+ 8     return true;
+ 9   }
+10   rcu_read_unlock();
+11   return false;
+12 }
+</pre>
+</blockquote>
+
+<p>
+However, this temptation must be resisted because there are a
+surprisingly large number of ways that the compiler
+(to say nothing of
+<a href="https://h71000.www7.hp.com/wizard/wiz_2637.html">DEC Alpha CPUs</a>)
+can trip this code up.
+For but one example, if the compiler were short of registers, it
+might choose to refetch from <tt>gp</tt> rather than keeping
+a separate copy in <tt>p</tt> as follows:
+
+<blockquote>
+<pre>
+ 1 bool do_something_gp_buggy_optimized(void)
+ 2 {
+ 3   rcu_read_lock();
+ 4   if (gp) { /* OPTIMIZATIONS GALORE!!! */
+<b> 5     do_something(gp-&gt;a, gp-&gt;b);</b>
+ 6     rcu_read_unlock();
+ 7     return true;
+ 8   }
+ 9   rcu_read_unlock();
+10   return false;
+11 }
+</pre>
+</blockquote>
+
+<p>
+If this function ran concurrently with a series of updates that
+replaced the current structure with a new one,
+the fetches of <tt>gp-&gt;a</tt>
+and <tt>gp-&gt;b</tt> might well come from two different structures,
+which could cause serious confusion.
+To prevent this (and much else besides), <tt>do_something_gp()</tt> uses
+<tt>rcu_dereference()</tt> to fetch from <tt>gp</tt>:
+
+<blockquote>
+<pre>
+ 1 bool do_something_gp(void)
+ 2 {
+ 3   rcu_read_lock();
+ 4   p = rcu_dereference(gp);
+ 5   if (p) {
+ 6     do_something(p-&gt;a, p-&gt;b);
+ 7     rcu_read_unlock();
+ 8     return true;
+ 9   }
+10   rcu_read_unlock();
+11   return false;
+12 }
+</pre>
+</blockquote>
+
+<p>
+The <tt>rcu_dereference()</tt> uses volatile casts and (for DEC Alpha)
+memory barriers in the Linux kernel.
+Should a
+<a href="http://www.rdrop.com/users/paulmck/RCU/consume.2015.07.13a.pdf">high-quality implementation of C11 <tt>memory_order_consume</tt> [PDF]</a>
+ever appear, then <tt>rcu_dereference()</tt> could be implemented
+as a <tt>memory_order_consume</tt> load.
+Regardless of the exact implementation, a pointer fetched by
+<tt>rcu_dereference()</tt> may not be used outside of the
+outermost RCU read-side critical section containing that
+<tt>rcu_dereference()</tt>, unless protection of
+the corresponding data element has been passed from RCU to some
+other synchronization mechanism, most commonly locking or
+<a href="https://www.kernel.org/doc/Documentation/RCU/rcuref.txt">reference counting</a>.
+
+<p>
+In short, updaters use <tt>rcu_assign_pointer()</tt> and readers
+use <tt>rcu_dereference()</tt>, and these two RCU API elements
+work together to ensure that readers have a consistent view of
+newly added data elements.
+
+<p>
+Of course, it is also necessary to remove elements from RCU-protected
+data structures, for example, using the following process:
+
+<ol>
+<li>	Remove the data element from the enclosing structure.
+<li>	Wait for all pre-existing RCU read-side critical sections
+	to complete (because only pre-existing readers can possibly have
+	a reference to the newly removed data element).
+<li>	At this point, only the updater has a reference to the
+	newly removed data element, so it can safely reclaim
+	the data element, for example, by passing it to <tt>kfree()</tt>.
+</ol>
+
+This process is implemented by <tt>remove_gp_synchronous()</tt>:
+
+<blockquote>
+<pre>
+ 1 bool remove_gp_synchronous(void)
+ 2 {
+ 3   struct foo *p;
+ 4
+ 5   spin_lock(&amp;gp_lock);
+ 6   p = rcu_access_pointer(gp);
+ 7   if (!p) {
+ 8     spin_unlock(&amp;gp_lock);
+ 9     return false;
+10   }
+11   rcu_assign_pointer(gp, NULL);
+12   spin_unlock(&amp;gp_lock);
+13   synchronize_rcu();
+14   kfree(p);
+15   return true;
+16 }
+</pre>
+</blockquote>
+
+<p>
+This function is straightforward, with line&nbsp;13 waiting for a grace
+period before line&nbsp;14 frees the old data element.
+This waiting ensures that readers will reach line&nbsp;7 of
+<tt>do_something_gp()</tt> before the data element referenced by
+<tt>p</tt> is freed.
+The <tt>rcu_access_pointer()</tt> on line&nbsp;6 is similar to
+<tt>rcu_dereference()</tt>, except that:
+
+<ol>
+<li>	The value returned by <tt>rcu_access_pointer()</tt>
+	cannot be dereferenced.
+	If you want to access the value pointed to as well as
+	the pointer itself, use <tt>rcu_dereference()</tt>
+	instead of <tt>rcu_access_pointer()</tt>.
+<li>	The call to <tt>rcu_access_pointer()</tt> need not be
+	protected.
+	In contrast, <tt>rcu_dereference()</tt> must either be
+	within an RCU read-side critical section or in a code
+	segment where the pointer cannot change, for example, in
+	code protected by the corresponding update-side lock.
+</ol>
+
+<p><a name="Quick Quiz 4"><b>Quick Quiz 4</b>:</a>
+Without the <tt>rcu_dereference()</tt> or the
+<tt>rcu_access_pointer()</tt>, what destructive optimizations
+might the compiler make use of?
+<br><a href="#qq4answer">Answer</a>
+
+<p>
+In short, RCU's publish-subscribe guarantee is provided by the combination
+of <tt>rcu_assign_pointer()</tt> and <tt>rcu_dereference()</tt>.
+This guarantee allows data elements to be safely added to RCU-protected
+linked data structures without disrupting RCU readers.
+This guarantee can be used in combination with the grace-period
+guarantee to also allow data elements to be removed from RCU-protected
+linked data structures, again without disrupting RCU readers.
+
+<p>
+This guarantee was only partially premeditated.
+DYNIX/ptx used an explicit memory barrier for publication, but had nothing
+resembling <tt>rcu_dereference()</tt> for subscription, nor did it
+have anything resembling the <tt>smp_read_barrier_depends()</tt>
+that was later subsumed into <tt>rcu_dereference()</tt>.
+The need for these operations made itself known quite suddenly at a
+late-1990s meeting with the DEC Alpha architects, back in the days when
+DEC was still a free-standing company.
+It took the Alpha architects a good hour to convince me that any sort
+of barrier would ever be needed, and it then took me a good <i>two</i> hours
+to convince them that their documentation did not make this point clear.
+More recent work with the C and C++ standards committees have provided
+much education on tricks and traps from the compiler.
+In short, compilers were much less tricky in the early 1990s, but in
+2015, don't even think about omitting <tt>rcu_dereference()</tt>!
+
+<h3><a name="Memory-Barrier Guarantees">Memory-Barrier Guarantees</a></h3>
+
+<p>
+The previous section's simple linked-data-structure scenario clearly
+demonstrates the need for RCU's stringent memory-ordering guarantees on
+systems with more than one CPU:
+
+<ol>
+<li>	Each CPU that has an RCU read-side critical section that
+	begins before <tt>synchronize_rcu()</tt> starts is
+	guaranteed to execute a full memory barrier between the time
+	that the RCU read-side critical section ends and the time that
+	<tt>synchronize_rcu()</tt> returns.
+	Without this guarantee, a pre-existing RCU read-side critical section
+	might hold a reference to the newly removed <tt>struct foo</tt>
+	after the <tt>kfree()</tt> on line&nbsp;14 of
+	<tt>remove_gp_synchronous()</tt>.
+<li>	Each CPU that has an RCU read-side critical section that ends
+	after <tt>synchronize_rcu()</tt> returns is guaranteed
+	to execute a full memory barrier between the time that
+	<tt>synchronize_rcu()</tt> begins and the time that the RCU
+	read-side critical section begins.
+	Without this guarantee, a later RCU read-side critical section
+	running after the <tt>kfree()</tt> on line&nbsp;14 of
+	<tt>remove_gp_synchronous()</tt> might
+	later run <tt>do_something_gp()</tt> and find the
+	newly deleted <tt>struct foo</tt>.
+<li>	If the task invoking <tt>synchronize_rcu()</tt> remains
+	on a given CPU, then that CPU is guaranteed to execute a full
+	memory barrier sometime during the execution of
+	<tt>synchronize_rcu()</tt>.
+	This guarantee ensures that the <tt>kfree()</tt> on
+	line&nbsp;14 of <tt>remove_gp_synchronous()</tt> really does
+	execute after the removal on line&nbsp;11.
+<li>	If the task invoking <tt>synchronize_rcu()</tt> migrates
+	among a group of CPUs during that invocation, then each of the
+	CPUs in that group is guaranteed to execute a full memory barrier
+	sometime during the execution of <tt>synchronize_rcu()</tt>.
+	This guarantee also ensures that the <tt>kfree()</tt> on
+	line&nbsp;14 of <tt>remove_gp_synchronous()</tt> really does
+	execute after the removal on
+	line&nbsp;11, but also in the case where the thread executing the
+	<tt>synchronize_rcu()</tt> migrates in the meantime.
+</ol>
+
+<p><a name="Quick Quiz 5"><b>Quick Quiz 5</b>:</a>
+Given that multiple CPUs can start RCU read-side critical sections
+at any time without any ordering whatsoever, how can RCU possibly tell whether
+or not a given RCU read-side critical section starts before a
+given instance of <tt>synchronize_rcu()</tt>?
+<br><a href="#qq5answer">Answer</a>
+
+<p><a name="Quick Quiz 6"><b>Quick Quiz 6</b>:</a>
+The first and second guarantees require unbelievably strict ordering!
+Are all these memory barriers <i> really</i> required?
+<br><a href="#qq6answer">Answer</a>
+
+<p>
+Note that these memory-barrier requirements do not replace the fundamental
+RCU requirement that a grace period wait for all pre-existing readers.
+On the contrary, the memory barriers called out in this section must operate in
+such a way as to <i>enforce</i> this fundamental requirement.
+Of course, different implementations enforce this requirement in different
+ways, but enforce it they must.
+
+<h3><a name="RCU Primitives Guaranteed to Execute Unconditionally">RCU Primitives Guaranteed to Execute Unconditionally</a></h3>
+
+<p>
+The common-case RCU primitives are unconditional.
+They are invoked, they do their job, and they return, with no possibility
+of error, and no need to retry.
+This is a key RCU design philosophy.
+
+<p>
+However, this philosophy is pragmatic rather than pigheaded.
+If someone comes up with a good justification for a particular conditional
+RCU primitive, it might well be implemented and added.
+After all, this guarantee was reverse-engineered, not premeditated.
+The unconditional nature of the RCU primitives was initially an
+accident of implementation, and later experience with synchronization
+primitives with conditional primitives caused me to elevate this
+accident to a guarantee.
+Therefore, the justification for adding a conditional primitive to
+RCU would need to be based on detailed and compelling use cases.
+
+<h3><a name="Guaranteed Read-to-Write Upgrade">Guaranteed Read-to-Write Upgrade</a></h3>
+
+<p>
+As far as RCU is concerned, it is always possible to carry out an
+update within an RCU read-side critical section.
+For example, that RCU read-side critical section might search for
+a given data element, and then might acquire the update-side
+spinlock in order to update that element, all while remaining
+in that RCU read-side critical section.
+Of course, it is necessary to exit the RCU read-side critical section
+before invoking <tt>synchronize_rcu()</tt>, however, this
+inconvenience can be avoided through use of the
+<tt>call_rcu()</tt> and <tt>kfree_rcu()</tt> API members
+described later in this document.
+
+<p><a name="Quick Quiz 7"><b>Quick Quiz 7</b>:</a>
+But how does the upgrade-to-write operation exclude other readers?
+<br><a href="#qq7answer">Answer</a>
+
+<p>
+This guarantee allows lookup code to be shared between read-side
+and update-side code, and was premeditated, appearing in the earliest
+DYNIX/ptx RCU documentation.
+
+<h2><a name="Fundamental Non-Requirements">Fundamental Non-Requirements</a></h2>
+
+<p>
+RCU provides extremely lightweight readers, and its read-side guarantees,
+though quite useful, are correspondingly lightweight.
+It is therefore all too easy to assume that RCU is guaranteeing more
+than it really is.
+Of course, the list of things that RCU does not guarantee is infinitely
+long, however, the following sections list a few non-guarantees that
+have caused confusion.
+Except where otherwise noted, these non-guarantees were premeditated.
+
+<ol>
+<li>	<a href="#Readers Impose Minimal Ordering">
+	Readers Impose Minimal Ordering</a>
+<li>	<a href="#Readers Do Not Exclude Updaters">
+	Readers Do Not Exclude Updaters</a>
+<li>	<a href="#Updaters Only Wait For Old Readers">
+	Updaters Only Wait For Old Readers</a>
+<li>	<a href="#Grace Periods Don't Partition Read-Side Critical Sections">
+	Grace Periods Don't Partition Read-Side Critical Sections</a>
+<li>	<a href="#Read-Side Critical Sections Don't Partition Grace Periods">
+	Read-Side Critical Sections Don't Partition Grace Periods</a>
+<li>	<a href="#Disabling Preemption Does Not Block Grace Periods">
+	Disabling Preemption Does Not Block Grace Periods</a>
+</ol>
+
+<h3><a name="Readers Impose Minimal Ordering">Readers Impose Minimal Ordering</a></h3>
+
+<p>
+Reader-side markers such as <tt>rcu_read_lock()</tt> and
+<tt>rcu_read_unlock()</tt> provide absolutely no ordering guarantees
+except through their interaction with the grace-period APIs such as
+<tt>synchronize_rcu()</tt>.
+To see this, consider the following pair of threads:
+
+<blockquote>
+<pre>
+ 1 void thread0(void)
+ 2 {
+ 3   rcu_read_lock();
+ 4   WRITE_ONCE(x, 1);
+ 5   rcu_read_unlock();
+ 6   rcu_read_lock();
+ 7   WRITE_ONCE(y, 1);
+ 8   rcu_read_unlock();
+ 9 }
+10
+11 void thread1(void)
+12 {
+13   rcu_read_lock();
+14   r1 = READ_ONCE(y);
+15   rcu_read_unlock();
+16   rcu_read_lock();
+17   r2 = READ_ONCE(x);
+18   rcu_read_unlock();
+19 }
+</pre>
+</blockquote>
+
+<p>
+After <tt>thread0()</tt> and <tt>thread1()</tt> execute
+concurrently, it is quite possible to have
+
+<blockquote>
+<pre>
+(r1 == 1 &amp;&amp; r2 == 0)
+</pre>
+</blockquote>
+
+(that is, <tt>y</tt> appears to have been assigned before <tt>x</tt>),
+which would not be possible if <tt>rcu_read_lock()</tt> and
+<tt>rcu_read_unlock()</tt> had much in the way of ordering
+properties.
+But they do not, so the CPU is within its rights
+to do significant reordering.
+This is by design:  Any significant ordering constraints would slow down
+these fast-path APIs.
+
+<p><a name="Quick Quiz 8"><b>Quick Quiz 8</b>:</a>
+Can't the compiler also reorder this code?
+<br><a href="#qq8answer">Answer</a>
+
+<h3><a name="Readers Do Not Exclude Updaters">Readers Do Not Exclude Updaters</a></h3>
+
+<p>
+Neither <tt>rcu_read_lock()</tt> nor <tt>rcu_read_unlock()</tt>
+exclude updates.
+All they do is to prevent grace periods from ending.
+The following example illustrates this:
+
+<blockquote>
+<pre>
+ 1 void thread0(void)
+ 2 {
+ 3   rcu_read_lock();
+ 4   r1 = READ_ONCE(y);
+ 5   if (r1) {
+ 6     do_something_with_nonzero_x();
+ 7     r2 = READ_ONCE(x);
+ 8     WARN_ON(!r2); /* BUG!!! */
+ 9   }
+10   rcu_read_unlock();
+11 }
+12
+13 void thread1(void)
+14 {
+15   spin_lock(&amp;my_lock);
+16   WRITE_ONCE(x, 1);
+17   WRITE_ONCE(y, 1);
+18   spin_unlock(&amp;my_lock);
+19 }
+</pre>
+</blockquote>
+
+<p>
+If the <tt>thread0()</tt> function's <tt>rcu_read_lock()</tt>
+excluded the <tt>thread1()</tt> function's update,
+the <tt>WARN_ON()</tt> could never fire.
+But the fact is that <tt>rcu_read_lock()</tt> does not exclude
+much of anything aside from subsequent grace periods, of which
+<tt>thread1()</tt> has none, so the
+<tt>WARN_ON()</tt> can and does fire.
+
+<h3><a name="Updaters Only Wait For Old Readers">Updaters Only Wait For Old Readers</a></h3>
+
+<p>
+It might be tempting to assume that after <tt>synchronize_rcu()</tt>
+completes, there are no readers executing.
+This temptation must be avoided because
+new readers can start immediately after <tt>synchronize_rcu()</tt>
+starts, and <tt>synchronize_rcu()</tt> is under no
+obligation to wait for these new readers.
+
+<p><a name="Quick Quiz 9"><b>Quick Quiz 9</b>:</a>
+Suppose that synchronize_rcu() did wait until all readers had completed.
+Would the updater be able to rely on this?
+<br><a href="#qq9answer">Answer</a>
+
+<h3><a name="Grace Periods Don't Partition Read-Side Critical Sections">
+Grace Periods Don't Partition Read-Side Critical Sections</a></h3>
+
+<p>
+It is tempting to assume that if any part of one RCU read-side critical
+section precedes a given grace period, and if any part of another RCU
+read-side critical section follows that same grace period, then all of
+the first RCU read-side critical section must precede all of the second.
+However, this just isn't the case: A single grace period does not
+partition the set of RCU read-side critical sections.
+An example of this situation can be illustrated as follows, where
+<tt>x</tt>, <tt>y</tt>, and <tt>z</tt> are initially all zero:
+
+<blockquote>
+<pre>
+ 1 void thread0(void)
+ 2 {
+ 3   rcu_read_lock();
+ 4   WRITE_ONCE(a, 1);
+ 5   WRITE_ONCE(b, 1);
+ 6   rcu_read_unlock();
+ 7 }
+ 8
+ 9 void thread1(void)
+10 {
+11   r1 = READ_ONCE(a);
+12   synchronize_rcu();
+13   WRITE_ONCE(c, 1);
+14 }
+15
+16 void thread2(void)
+17 {
+18   rcu_read_lock();
+19   r2 = READ_ONCE(b);
+20   r3 = READ_ONCE(c);
+21   rcu_read_unlock();
+22 }
+</pre>
+</blockquote>
+
+<p>
+It turns out that the outcome:
+
+<blockquote>
+<pre>
+(r1 == 1 &amp;&amp; r2 == 0 &amp;&amp; r3 == 1)
+</pre>
+</blockquote>
+
+is entirely possible.
+The following figure show how this can happen, with each circled
+<tt>QS</tt> indicating the point at which RCU recorded a
+<i>quiescent state</i> for each thread, that is, a state in which
+RCU knows that the thread cannot be in the midst of an RCU read-side
+critical section that started before the current grace period:
+
+<p><img src="GPpartitionReaders1.svg" alt="GPpartitionReaders1.svg" width="60%"></p>
+
+<p>
+If it is necessary to partition RCU read-side critical sections in this
+manner, it is necessary to use two grace periods, where the first
+grace period is known to end before the second grace period starts:
+
+<blockquote>
+<pre>
+ 1 void thread0(void)
+ 2 {
+ 3   rcu_read_lock();
+ 4   WRITE_ONCE(a, 1);
+ 5   WRITE_ONCE(b, 1);
+ 6   rcu_read_unlock();
+ 7 }
+ 8
+ 9 void thread1(void)
+10 {
+11   r1 = READ_ONCE(a);
+12   synchronize_rcu();
+13   WRITE_ONCE(c, 1);
+14 }
+15
+16 void thread2(void)
+17 {
+18   r2 = READ_ONCE(c);
+19   synchronize_rcu();
+20   WRITE_ONCE(d, 1);
+21 }
+22
+23 void thread3(void)
+24 {
+25   rcu_read_lock();
+26   r3 = READ_ONCE(b);
+27   r4 = READ_ONCE(d);
+28   rcu_read_unlock();
+29 }
+</pre>
+</blockquote>
+
+<p>
+Here, if <tt>(r1 == 1)</tt>, then
+<tt>thread0()</tt>'s write to <tt>b</tt> must happen
+before the end of <tt>thread1()</tt>'s grace period.
+If in addition <tt>(r4 == 1)</tt>, then
+<tt>thread3()</tt>'s read from <tt>b</tt> must happen
+after the beginning of <tt>thread2()</tt>'s grace period.
+If it is also the case that <tt>(r2 == 1)</tt>, then the
+end of <tt>thread1()</tt>'s grace period must precede the
+beginning of <tt>thread2()</tt>'s grace period.
+This mean that the two RCU read-side critical sections cannot overlap,
+guaranteeing that <tt>(r3 == 1)</tt>.
+As a result, the outcome:
+
+<blockquote>
+<pre>
+(r1 == 1 &amp;&amp; r2 == 1 &amp;&amp; r3 == 0 &amp;&amp; r4 == 1)
+</pre>
+</blockquote>
+
+cannot happen.
+
+<p>
+This non-requirement was also non-premeditated, but became apparent
+when studying RCU's interaction with memory ordering.
+
+<h3><a name="Read-Side Critical Sections Don't Partition Grace Periods">
+Read-Side Critical Sections Don't Partition Grace Periods</a></h3>
+
+<p>
+It is also tempting to assume that if an RCU read-side critical section
+happens between a pair of grace periods, then those grace periods cannot
+overlap.
+However, this temptation leads nowhere good, as can be illustrated by
+the following, with all variables initially zero:
+
+<blockquote>
+<pre>
+ 1 void thread0(void)
+ 2 {
+ 3   rcu_read_lock();
+ 4   WRITE_ONCE(a, 1);
+ 5   WRITE_ONCE(b, 1);
+ 6   rcu_read_unlock();
+ 7 }
+ 8
+ 9 void thread1(void)
+10 {
+11   r1 = READ_ONCE(a);
+12   synchronize_rcu();
+13   WRITE_ONCE(c, 1);
+14 }
+15
+16 void thread2(void)
+17 {
+18   rcu_read_lock();
+19   WRITE_ONCE(d, 1);
+20   r2 = READ_ONCE(c);
+21   rcu_read_unlock();
+22 }
+23
+24 void thread3(void)
+25 {
+26   r3 = READ_ONCE(d);
+27   synchronize_rcu();
+28   WRITE_ONCE(e, 1);
+29 }
+30
+31 void thread4(void)
+32 {
+33   rcu_read_lock();
+34   r4 = READ_ONCE(b);
+35   r5 = READ_ONCE(e);
+36   rcu_read_unlock();
+37 }
+</pre>
+</blockquote>
+
+<p>
+In this case, the outcome:
+
+<blockquote>
+<pre>
+(r1 == 1 &amp;&amp; r2 == 1 &amp;&amp; r3 == 1 &amp;&amp; r4 == 0 &amp&amp; r5 == 1)
+</pre>
+</blockquote>
+
+is entirely possible, as illustrated below:
+
+<p><img src="ReadersPartitionGP1.svg" alt="ReadersPartitionGP1.svg" width="100%"></p>
+
+<p>
+Again, an RCU read-side critical section can overlap almost all of a
+given grace period, just so long as it does not overlap the entire
+grace period.
+As a result, an RCU read-side critical section cannot partition a pair
+of RCU grace periods.
+
+<p><a name="Quick Quiz 10"><b>Quick Quiz 10</b>:</a>
+How long a sequence of grace periods, each separated by an RCU read-side
+critical section, would be required to partition the RCU read-side
+critical sections at the beginning and end of the chain?
+<br><a href="#qq10answer">Answer</a>
+
+<h3><a name="Disabling Preemption Does Not Block Grace Periods">
+Disabling Preemption Does Not Block Grace Periods</a></h3>
+
+<p>
+There was a time when disabling preemption on any given CPU would block
+subsequent grace periods.
+However, this was an accident of implementation and is not a requirement.
+And in the current Linux-kernel implementation, disabling preemption
+on a given CPU in fact does not block grace periods, as Oleg Nesterov
+<a href="https://lkml.kernel.org/g/20150614193825.GA19582@redhat.com">demonstrated</a>.
+
+<p>
+If you need a preempt-disable region to block grace periods, you need to add
+<tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>, for example
+as follows:
+
+<blockquote>
+<pre>
+ 1 preempt_disable();
+ 2 rcu_read_lock();
+ 3 do_something();
+ 4 rcu_read_unlock();
+ 5 preempt_enable();
+ 6
+ 7 /* Spinlocks implicitly disable preemption. */
+ 8 spin_lock(&amp;mylock);
+ 9 rcu_read_lock();
+10 do_something();
+11 rcu_read_unlock();
+12 spin_unlock(&amp;mylock);
+</pre>
+</blockquote>
+
+<p>
+In theory, you could enter the RCU read-side critical section first,
+but it is more efficient to keep the entire RCU read-side critical
+section contained in the preempt-disable region as shown above.
+Of course, RCU read-side critical sections that extend outside of
+preempt-disable regions will work correctly, but such critical sections
+can be preempted, which forces <tt>rcu_read_unlock()</tt> to do
+more work.
+And no, this is <i>not</i> an invitation to enclose all of your RCU
+read-side critical sections within preempt-disable regions, because
+doing so would degrade real-time response.
+
+<p>
+This non-requirement appeared with preemptible RCU.
+If you need a grace period that waits on non-preemptible code regions, use
+<a href="#Sched Flavor">RCU-sched</a>.
+
+<h2><a name="Parallelism Facts of Life">Parallelism Facts of Life</a></h2>
+
+<p>
+These parallelism facts of life are by no means specific to RCU, but
+the RCU implementation must abide by them.
+They therefore bear repeating:
+
+<ol>
+<li>	Any CPU or task may be delayed at any time,
+	and any attempts to avoid these delays by disabling
+	preemption, interrupts, or whatever are completely futile.
+	This is most obvious in preemptible user-level
+	environments and in virtualized environments (where
+	a given guest OS's VCPUs can be preempted at any time by
+	the underlying hypervisor), but can also happen in bare-metal
+	environments due to ECC errors, NMIs, and other hardware
+	events.
+	Although a delay of more than about 20 seconds can result
+	in splats, the RCU implementation is obligated to use
+	algorithms that can tolerate extremely long delays, but where
+	&ldquo;extremely long&rdquo; is not long enough to allow
+	wrap-around when incrementing a 64-bit counter.
+<li>	Both the compiler and the CPU can reorder memory accesses.
+	Where it matters, RCU must use compiler directives and
+	memory-barrier instructions to preserve ordering.
+<li>	Conflicting writes to memory locations in any given cache line
+	will result in expensive cache misses.
+	Greater numbers of concurrent writes and more-frequent
+	concurrent writes will result in more dramatic slowdowns.
+	RCU is therefore obligated to use algorithms that have
+	sufficient locality to avoid significant performance and
+	scalability problems.
+<li>	As a rough rule of thumb, only one CPU's worth of processing
+	may be carried out under the protection of any given exclusive
+	lock.
+	RCU must therefore use scalable locking designs.
+<li>	Counters are finite, especially on 32-bit systems.
+	RCU's use of counters must therefore tolerate counter wrap,
+	or be designed such that counter wrap would take way more
+	time than a single system is likely to run.
+	An uptime of ten years is quite possible, a runtime
+	of a century much less so.
+	As an example of the latter, RCU's dyntick-idle nesting counter
+	allows 54 bits for interrupt nesting level (this counter
+	is 64 bits even on a 32-bit system).
+	Overflowing this counter requires 2<sup>54</sup>
+	half-interrupts on a given CPU without that CPU ever going idle.
+	If a half-interrupt happened every microsecond, it would take
+	570 years of runtime to overflow this counter, which is currently
+	believed to be an acceptably long time.
+<li>	Linux systems can have thousands of CPUs running a single
+	Linux kernel in a single shared-memory environment.
+	RCU must therefore pay close attention to high-end scalability.
+</ol>
+
+<p>
+This last parallelism fact of life means that RCU must pay special
+attention to the preceding facts of life.
+The idea that Linux might scale to systems with thousands of CPUs would
+have been met with some skepticism in the 1990s, but these requirements
+would have otherwise have been unsurprising, even in the early 1990s.
+
+<h2><a name="Quality-of-Implementation Requirements">Quality-of-Implementation Requirements</a></h2>
+
+<p>
+These sections list quality-of-implementation requirements.
+Although an RCU implementation that ignores these requirements could
+still be used, it would likely be subject to limitations that would
+make it inappropriate for industrial-strength production use.
+Classes of quality-of-implementation requirements are as follows:
+
+<ol>
+<li>	<a href="#Specialization">Specialization</a>
+<li>	<a href="#Performance and Scalability">Performance and Scalability</a>
+<li>	<a href="#Composability">Composability</a>
+<li>	<a href="#Corner Cases">Corner Cases</a>
+</ol>
+
+<p>
+These classes is covered in the following sections.
+
+<h3><a name="Specialization">Specialization</a></h3>
+
+<p>
+RCU is and always has been intended primarily for read-mostly situations, as
+illustrated by the following figure.
+This means that RCU's read-side primitives are optimized, often at the
+expense of its update-side primitives.
+
+<p><img src="RCUApplicability.svg" alt="RCUApplicability.svg" width="70%"></p>
+
+<p>
+This focus on read-mostly situations means that RCU must interoperate
+with other synchronization primitives.
+For example, the <tt>add_gp()</tt> and <tt>remove_gp_synchronous()</tt>
+examples discussed earlier use RCU to protect readers and locking to
+coordinate updaters.
+However, the need extends much farther, requiring that a variety of
+synchronization primitives be legal within RCU read-side critical sections,
+including spinlocks, sequence locks, atomic operations, reference
+counters, and memory barriers.
+
+<p><a name="Quick Quiz 11"><b>Quick Quiz 11</b>:</a>
+What about sleeping locks?
+<br><a href="#qq11answer">Answer</a>
+
+<p>
+It often comes as a surprise that many algorithms do not require a
+consistent view of data, but many can function in that mode,
+with network routing being the poster child.
+Internet routing algorithms take significant time to propagate
+updates, so that by the time an update arrives at a given system,
+that system has been sending network traffic the wrong way for
+a considerable length of time.
+Having a few threads continue to send traffic the wrong way for a
+few more milliseconds is clearly not a problem:  In the worst case,
+TCP retransmissions will eventually get the data where it needs to go.
+In general, when tracking the state of the universe outside of the
+computer, some level of inconsistency must be tolerated due to
+speed-of-light delays if nothing else.
+
+<p>
+Furthermore, uncertainty about external state is inherent in many cases.
+For example, a pair of veternarians might use heartbeat to determine
+whether or not a given cat was alive.
+But how long should they wait after the last heartbeat to decide that
+the cat is in fact dead?
+Waiting less than 400 milliseconds makes no sense because this would
+mean that a relaxed cat would be considered to cycle between death
+and life more than 100 times per minute.
+Moreover, just as with human beings, a cat's heart might stop for
+some period of time, so the exact wait period is a judgment call.
+One of our pair of veternarians might wait 30 seconds before pronouncing
+the cat dead, while the other might insist on waiting a full minute.
+The two veternarians would then disagree on the state of the cat during
+the final 30 seconds of the minute following the last heartbeat, as
+fancifully illustrated below:
+
+<p><img src="2013-08-is-it-dead.png" alt="2013-08-is-it-dead.png" width="431"></p>
+
+<p>
+Interestingly enough, this same situation applies to hardware.
+When push comes to shove, how do we tell whether or not some
+external server has failed?
+We send messages to it periodically, and declare it failed if we
+don't receive a response within a given period of time.
+Policy decisions can usually tolerate short
+periods of inconsistency.
+The policy was decided some time ago, and is only now being put into
+effect, so a few milliseconds of delay is normally inconsequential.
+
+<p>
+However, there are algorithms that absolutely must see consistent data.
+For example, the translation between a user-level SystemV semaphore
+ID to the corresponding in-kernel data structure is protected by RCU,
+but it is absolutely forbidden to update a semaphore that has just been
+removed.
+In the Linux kernel, this need for consistency is accommodated by acquiring
+spinlocks located in the in-kernel data structure from within
+the RCU read-side critical section, and this is indicated by the
+green box in the figure above.
+Many other techniques may be used, and are in fact used within the
+Linux kernel.
+
+<p>
+In short, RCU is not required to maintain consistency, and other
+mechanisms may be used in concert with RCU when consistency is required.
+RCU's specialization allows it to do its job extremely well, and its
+ability to interoperate with other synchronization mechanisms allows
+the right mix of synchronization tools to be used for a given job.
+
+<h3><a name="Performance and Scalability">Performance and Scalability</a></h3>
+
+<p>
+Energy efficiency is a critical component of performance today,
+and Linux-kernel RCU implementations must therefore avoid unnecessarily
+awakening idle CPUs.
+I cannot claim that this requirement was premeditated.
+In fact, I learned of it during a telephone conversation in which I
+was given &ldquo;frank and open&rdquo; feedback on the importance
+of energy efficiency in battery-powered systems and on specific
+energy-efficiency shortcomings of the Linux-kernel RCU implementation.
+In my experience, the battery-powered embedded community will consider
+any unnecessary wakeups to be extremely unfriendly acts.
+So much so that mere Linux-kernel-mailing-list posts are
+insufficient to vent their ire.
+
+<p>
+Memory consumption is not particularly important for in most
+situations, and has become decreasingly
+so as memory sizes have expanded and memory
+costs have plummeted.
+However, as I learned from Matt Mackall's
+<a href="http://elinux.org/Linux_Tiny-FAQ">bloatwatch</a>
+efforts, memory footprint is critically important on single-CPU systems with
+non-preemptible (<tt>CONFIG_PREEMPT=n</tt>) kernels, and thus
+<a href="https://lkml.kernel.org/g/20090113221724.GA15307@linux.vnet.ibm.com">tiny RCU</a>
+was born.
+Josh Triplett has since taken over the small-memory banner with his
+<a href="https://tiny.wiki.kernel.org/">Linux kernel tinification</a>
+project, which resulted in
+<a href="#Sleepable RCU">SRCU</a>
+becoming optional for those kernels not needing it.
+
+<p>
+The remaining performance requirements are, for the most part,
+unsurprising.
+For example, in keeping with RCU's read-side specialization,
+<tt>rcu_dereference()</tt> should have negligible overhead (for
+example, suppression of a few minor compiler optimizations).
+Similarly, in non-preemptible environments, <tt>rcu_read_lock()</tt> and
+<tt>rcu_read_unlock()</tt> should have exactly zero overhead.
+
+<p>
+In preemptible environments, in the case where the RCU read-side
+critical section was not preempted (as will be the case for the
+highest-priority real-time process), <tt>rcu_read_lock()</tt> and
+<tt>rcu_read_unlock()</tt> should have minimal overhead.
+In particular, they should not contain atomic read-modify-write
+operations, memory-barrier instructions, preemption disabling,
+interrupt disabling, or backwards branches.
+However, in the case where the RCU read-side critical section was preempted,
+<tt>rcu_read_unlock()</tt> may acquire spinlocks and disable interrupts.
+This is why it is better to nest an RCU read-side critical section
+within a preempt-disable region than vice versa, at least in cases
+where that critical section is short enough to avoid unduly degrading
+real-time latencies.
+
+<p>
+The <tt>synchronize_rcu()</tt> grace-period-wait primitive is
+optimized for throughput.
+It may therefore incur several milliseconds of latency in addition to
+the duration of the longest RCU read-side critical section.
+On the other hand, multiple concurrent invocations of
+<tt>synchronize_rcu()</tt> are required to use batching optimizations
+so that they can be satisfied by a single underlying grace-period-wait
+operation.
+For example, in the Linux kernel, it is not unusual for a single
+grace-period-wait operation to serve more than
+<a href="https://www.usenix.org/conference/2004-usenix-annual-technical-conference/making-rcu-safe-deep-sub-millisecond-response">1,000 separate invocations</a>
+of <tt>synchronize_rcu()</tt>, thus amortizing the per-invocation
+overhead down to nearly zero.
+However, the grace-period optimization is also required to avoid
+measurable degradation of real-time scheduling and interrupt latencies.
+
+<p>
+In some cases, the multi-millisecond <tt>synchronize_rcu()</tt>
+latencies are unacceptable.
+In these cases, <tt>synchronize_rcu_expedited()</tt> may be used
+instead, reducing the grace-period latency down to a few tens of
+microseconds on small systems, at least in cases where the RCU read-side
+critical sections are short.
+There are currently no special latency requirements for
+<tt>synchronize_rcu_expedited()</tt> on large systems, but,
+consistent with the empirical nature of the RCU specification,
+that is subject to change.
+However, there most definitely are scalability requirements:
+A storm of <tt>synchronize_rcu_expedited()</tt> invocations on 4096
+CPUs should at least make reasonable forward progress.
+In return for its shorter latencies, <tt>synchronize_rcu_expedited()</tt>
+is permitted to impose modest degradation of real-time latency
+on non-idle online CPUs.
+That said, it will likely be necessary to take further steps to reduce this
+degradation, hopefully to roughly that of a scheduling-clock interrupt.
+
+<p>
+There are a number of situations where even
+<tt>synchronize_rcu_expedited()</tt>'s reduced grace-period
+latency is unacceptable.
+In these situations, the asynchronous <tt>call_rcu()</tt> can be
+used in place of <tt>synchronize_rcu()</tt> as follows:
+
+<blockquote>
+<pre>
+ 1 struct foo {
+ 2   int a;
+ 3   int b;
+ 4   struct rcu_head rh;
+ 5 };
+ 6
+ 7 static void remove_gp_cb(struct rcu_head *rhp)
+ 8 {
+ 9   struct foo *p = container_of(rhp, struct foo, rh);
+10
+11   kfree(p);
+12 }
+13
+14 bool remove_gp_asynchronous(void)
+15 {
+16   struct foo *p;
+17
+18   spin_lock(&amp;gp_lock);
+19   p = rcu_dereference(gp);
+20   if (!p) {
+21     spin_unlock(&amp;gp_lock);
+22     return false;
+23   }
+24   rcu_assign_pointer(gp, NULL);
+25   call_rcu(&amp;p-&gt;rh, remove_gp_cb);
+26   spin_unlock(&amp;gp_lock);
+27   return true;
+28 }
+</pre>
+</blockquote>
+
+<p>
+A definition of <tt>struct foo</tt> is finally needed, and appears
+on lines&nbsp;1-5.
+The function <tt>remove_gp_cb()</tt> is passed to <tt>call_rcu()</tt>
+on line&nbsp;25, and will be invoked after the end of a subsequent
+grace period.
+This gets the same effect as <tt>remove_gp_synchronous()</tt>,
+but without forcing the updater to wait for a grace period to elapse.
+The <tt>call_rcu()</tt> function may be used in a number of
+situations where neither <tt>synchronize_rcu()</tt> nor
+<tt>synchronize_rcu_expedited()</tt> would be legal,
+including within preempt-disable code, <tt>local_bh_disable()</tt> code,
+interrupt-disable code, and interrupt handlers.
+However, even <tt>call_rcu()</tt> is illegal within NMI handlers.
+The callback function (<tt>remove_gp_cb()</tt> in this case) will be
+executed within softirq (software interrupt) environment within the
+Linux kernel,
+either within a real softirq handler or under the protection
+of <tt>local_bh_disable()</tt>.
+In both the Linux kernel and in userspace, it is bad practice to
+write an RCU callback function that takes too long.
+Long-running operations should be relegated to separate threads or
+(in the Linux kernel) workqueues.
+
+<p><a name="Quick Quiz 12"><b>Quick Quiz 12</b>:</a>
+Why does line&nbsp;19 use <tt>rcu_access_pointer()</tt>?
+After all, <tt>call_rcu()</tt> on line&nbsp;25 stores into the
+structure, which would interact badly with concurrent insertions.
+Doesn't this mean that <tt>rcu_dereference()</tt> is required?
+<br><a href="#qq12answer">Answer</a>
+
+<p>
+However, all that <tt>remove_gp_cb()</tt> is doing is
+invoking <tt>kfree()</tt> on the data element.
+This is a common idiom, and is supported by <tt>kfree_rcu()</tt>,
+which allows &ldquo;fire and forget&rdquo; operation as shown below:
+
+<blockquote>
+<pre>
+ 1 struct foo {
+ 2   int a;
+ 3   int b;
+ 4   struct rcu_head rh;
+ 5 };
+ 6
+ 7 bool remove_gp_faf(void)
+ 8 {
+ 9   struct foo *p;
+10
+11   spin_lock(&amp;gp_lock);
+12   p = rcu_dereference(gp);
+13   if (!p) {
+14     spin_unlock(&amp;gp_lock);
+15     return false;
+16   }
+17   rcu_assign_pointer(gp, NULL);
+18   kfree_rcu(p, rh);
+19   spin_unlock(&amp;gp_lock);
+20   return true;
+21 }
+</pre>
+</blockquote>
+
+<p>
+Note that <tt>remove_gp_faf()</tt> simply invokes
+<tt>kfree_rcu()</tt> and proceeds, without any need to pay any
+further attention to the subsequent grace period and <tt>kfree()</tt>.
+It is permissible to invoke <tt>kfree_rcu()</tt> from the same
+environments as for <tt>call_rcu()</tt>.
+Interestingly enough, DYNIX/ptx had the equivalents of
+<tt>call_rcu()</tt> and <tt>kfree_rcu()</tt>, but not
+<tt>synchronize_rcu()</tt>.
+This was due to the fact that RCU was not heavily used within DYNIX/ptx,
+so the very few places that needed something like
+<tt>synchronize_rcu()</tt> simply open-coded it.
+
+<p><a name="Quick Quiz 13"><b>Quick Quiz 13</b>:</a>
+Earlier it was claimed that <tt>call_rcu()</tt> and
+<tt>kfree_rcu()</tt> allowed updaters to avoid being blocked
+by readers.
+But how can that be correct, given that the invocation of the callback
+and the freeing of the memory (respectively) must still wait for
+a grace period to elapse?
+<br><a href="#qq13answer">Answer</a>
+
+<p>
+But what if the updater must wait for the completion of code to be
+executed after the end of the grace period, but has other tasks
+that can be carried out in the meantime?
+The polling-style <tt>get_state_synchronize_rcu()</tt> and
+<tt>cond_synchronize_rcu()</tt> functions may be used for this
+purpose, as shown below:
+
+<blockquote>
+<pre>
+ 1 bool remove_gp_poll(void)
+ 2 {
+ 3   struct foo *p;
+ 4   unsigned long s;
+ 5
+ 6   spin_lock(&amp;gp_lock);
+ 7   p = rcu_access_pointer(gp);
+ 8   if (!p) {
+ 9     spin_unlock(&amp;gp_lock);
+10     return false;
+11   }
+12   rcu_assign_pointer(gp, NULL);
+13   spin_unlock(&amp;gp_lock);
+14   s = get_state_synchronize_rcu();
+15   do_something_while_waiting();
+16   cond_synchronize_rcu(s);
+17   kfree(p);
+18   return true;
+19 }
+</pre>
+</blockquote>
+
+<p>
+On line&nbsp;14, <tt>get_state_synchronize_rcu()</tt> obtains a
+&ldquo;cookie&rdquo; from RCU,
+then line&nbsp;15 carries out other tasks,
+and finally, line&nbsp;16 returns immediately if a grace period has
+elapsed in the meantime, but otherwise waits as required.
+The need for <tt>get_state_synchronize_rcu</tt> and
+<tt>cond_synchronize_rcu()</tt> has appeared quite recently,
+so it is too early to tell whether they will stand the test of time.
+
+<p>
+RCU thus provides a range of tools to allow updaters to strike the
+required tradeoff between latency, flexibility and CPU overhead.
+
+<h3><a name="Composability">Composability</a></h3>
+
+<p>
+Composability has received much attention in recent years, perhaps in part
+due to the collision of multicore hardware with object-oriented techniques
+designed in single-threaded environments for single-threaded use.
+And in theory, RCU read-side critical sections may be composed, and in
+fact may be nested arbitrarily deeply.
+In practice, as with all real-world implementations of composable
+constructs, there are limitations.
+
+<p>
+Implementations of RCU for which <tt>rcu_read_lock()</tt>
+and <tt>rcu_read_unlock()</tt> generate no code, such as
+Linux-kernel RCU when <tt>CONFIG_PREEMPT=n</tt>, can be
+nested arbitrarily deeply.
+After all, there is no overhead.
+Except that if all these instances of <tt>rcu_read_lock()</tt>
+and <tt>rcu_read_unlock()</tt> are visible to the compiler,
+compilation will eventually fail due to exhausting memory,
+mass storage, or user patience, whichever comes first.
+If the nesting is not visible to the compiler, as is the case with
+mutually recursive functions each in its own translation unit,
+stack overflow will result.
+If the nesting takes the form of loops, either the control variable
+will overflow or (in the Linux kernel) you will get an RCU CPU stall warning.
+Nevertheless, this class of RCU implementations is one
+of the most composable constructs in existence.
+
+<p>
+RCU implementations that explicitly track nesting depth
+are limited by the nesting-depth counter.
+For example, the Linux kernel's preemptible RCU limits nesting to
+<tt>INT_MAX</tt>.
+This should suffice for almost all practical purposes.
+That said, a consecutive pair of RCU read-side critical sections
+between which there is an operation that waits for a grace period
+cannot be enclosed in another RCU read-side critical section.
+This is because it is not legal to wait for a grace period within
+an RCU read-side critical section:  To do so would result either
+in deadlock or
+in RCU implicitly splitting the enclosing RCU read-side critical
+section, neither of which is conducive to a long-lived and prosperous
+kernel.
+
+<p>
+It is worth noting that RCU is not alone in limiting composability.
+For example, many transactional-memory implementations prohibit
+composing a pair of transactions separated by an irrevocable
+operation (for example, a network receive operation).
+For another example, lock-based critical sections can be composed
+surprisingly freely, but only if deadlock is avoided.
+
+<p>
+In short, although RCU read-side critical sections are highly composable,
+care is required in some situations, just as is the case for any other
+composable synchronization mechanism.
+
+<h3><a name="Corner Cases">Corner Cases</a></h3>
+
+<p>
+A given RCU workload might have an endless and intense stream of
+RCU read-side critical sections, perhaps even so intense that there
+was never a point in time during which there was not at least one
+RCU read-side critical section in flight.
+RCU cannot allow this situation to block grace periods:  As long as
+all the RCU read-side critical sections are finite, grace periods
+must also be finite.
+
+<p>
+That said, preemptible RCU implementations could potentially result
+in RCU read-side critical sections being preempted for long durations,
+which has the effect of creating a long-duration RCU read-side
+critical section.
+This situation can arise only in heavily loaded systems, but systems using
+real-time priorities are of course more vulnerable.
+Therefore, RCU priority boosting is provided to help deal with this
+case.
+That said, the exact requirements on RCU priority boosting will likely
+evolve as more experience accumulates.
+
+<p>
+Other workloads might have very high update rates.
+Although one can argue that such workloads should instead use
+something other than RCU, the fact remains that RCU must
+handle such workloads gracefully.
+This requirement is another factor driving batching of grace periods,
+but it is also the driving force behind the checks for large numbers
+of queued RCU callbacks in the <tt>call_rcu()</tt> code path.
+Finally, high update rates should not delay RCU read-side critical
+sections, although some read-side delays can occur when using
+<tt>synchronize_rcu_expedited()</tt>, courtesy of this function's use
+of <tt>try_stop_cpus()</tt>.
+(In the future, <tt>synchronize_rcu_expedited()</tt> will be
+converted to use lighter-weight inter-processor interrupts (IPIs),
+but this will still disturb readers, though to a much smaller degree.)
+
+<p>
+Although all three of these corner cases were understood in the early
+1990s, a simple user-level test consisting of <tt>close(open(path))</tt>
+in a tight loop
+in the early 2000s suddenly provided a much deeper appreciation of the
+high-update-rate corner case.
+This test also motivated addition of some RCU code to react to high update
+rates, for example, if a given CPU finds itself with more than 10,000
+RCU callbacks queued, it will cause RCU to take evasive action by
+more aggressively starting grace periods and more aggressively forcing
+completion of grace-period processing.
+This evasive action causes the grace period to complete more quickly,
+but at the cost of restricting RCU's batching optimizations, thus
+increasing the CPU overhead incurred by that grace period.
+
+<h2><a name="Software-Engineering Requirements">
+Software-Engineering Requirements</a></h2>
+
+<p>
+Between Murphy's Law and &ldquo;To err is human&rdquo;, it is necessary to
+guard against mishaps and misuse:
+
+<ol>
+<li>	It is all too easy to forget to use <tt>rcu_read_lock()</tt>
+	everywhere that it is needed, so kernels built with
+	<tt>CONFIG_PROVE_RCU=y</tt> will spat if
+	<tt>rcu_dereference()</tt> is used outside of an
+	RCU read-side critical section.
+	Update-side code can use <tt>rcu_dereference_protected()</tt>,
+	which takes a
+	<a href="https://lwn.net/Articles/371986/">lockdep expression</a>
+	to indicate what is providing the protection.
+	If the indicated protection is not provided, a lockdep splat
+	is emitted.
+
+	<p>
+	Code shared between readers and updaters can use
+	<tt>rcu_dereference_check()</tt>, which also takes a
+	lockdep expression, and emits a lockdep splat if neither
+	<tt>rcu_read_lock()</tt> nor the indicated protection
+	is in place.
+	In addition, <tt>rcu_dereference_raw()</tt> is used in those
+	(hopefully rare) cases where the required protection cannot
+	be easily described.
+	Finally, <tt>rcu_read_lock_held()</tt> is provided to
+	allow a function to verify that it has been invoked within
+	an RCU read-side critical section.
+	I was made aware of this set of requirements shortly after Thomas
+	Gleixner audited a number of RCU uses.
+<li>	A given function might wish to check for RCU-related preconditions
+	upon entry, before using any other RCU API.
+	The <tt>rcu_lockdep_assert()</tt> does this job,
+	asserting the expression in kernels having lockdep enabled
+	and doing nothing otherwise.
+<li>	It is also easy to forget to use <tt>rcu_assign_pointer()</tt>
+	and <tt>rcu_dereference()</tt>, perhaps (incorrectly)
+	substituting a simple assignment.
+	To catch this sort of error, a given RCU-protected pointer may be
+	tagged with <tt>__rcu</tt>, after which running sparse
+	with <tt>CONFIG_SPARSE_RCU_POINTER=y</tt> will complain
+	about simple-assignment accesses to that pointer.
+	Arnd Bergmann made me aware of this requirement, and also
+	supplied the needed
+	<a href="https://lwn.net/Articles/376011/">patch series</a>.
+<li>	Kernels built with <tt>CONFIG_DEBUG_OBJECTS_RCU_HEAD=y</tt>
+	will splat if a data element is passed to <tt>call_rcu()</tt>
+	twice in a row, without a grace period in between.
+	(This error is similar to a double free.)
+	The corresponding <tt>rcu_head</tt> structures that are
+	dynamically allocated are automatically tracked, but
+	<tt>rcu_head</tt> structures allocated on the stack
+	must be initialized with <tt>init_rcu_head_on_stack()</tt>
+	and cleaned up with <tt>destroy_rcu_head_on_stack()</tt>.
+	Similarly, statically allocated non-stack <tt>rcu_head</tt>
+	structures must be initialized with <tt>init_rcu_head()</tt>
+	and cleaned up with <tt>destroy_rcu_head()</tt>.
+	Mathieu Desnoyers made me aware of this requirement, and also
+	supplied the needed
+	<a href="https://lkml.kernel.org/g/20100319013024.GA28456@Krystal">patch</a>.
+<li>	An infinite loop in an RCU read-side critical section will
+	eventually trigger an RCU CPU stall warning splat, with
+	the duration of &ldquo;eventually&rdquo; being controlled by the
+	<tt>RCU_CPU_STALL_TIMEOUT</tt> <tt>Kconfig</tt> option, or,
+	alternatively, by the
+	<tt>rcupdate.rcu_cpu_stall_timeout</tt> boot/sysfs
+	parameter.
+	However, RCU is not obligated to produce this splat
+	unless there is a grace period waiting on that particular
+	RCU read-side critical section.
+	<p>
+	Some extreme workloads might intentionally delay
+	RCU grace periods, and systems running those workloads can
+	be booted with <tt>rcupdate.rcu_cpu_stall_suppress</tt>
+	to suppress the splats.
+	This kernel parameter may also be set via <tt>sysfs</tt>.
+	Furthermore, RCU CPU stall warnings are counter-productive
+	during sysrq dumps and during panics.
+	RCU therefore supplies the <tt>rcu_sysrq_start()</tt> and
+	<tt>rcu_sysrq_end()</tt> API members to be called before
+	and after long sysrq dumps.
+	RCU also supplies the <tt>rcu_panic()</tt> notifier that is
+	automatically invoked at the beginning of a panic to suppress
+	further RCU CPU stall warnings.
+
+	<p>
+	This requirement made itself known in the early 1990s, pretty
+	much the first time that it was necessary to debug a CPU stall.
+	That said, the initial implementation in DYNIX/ptx was quite
+	generic in comparison with that of Linux.
+<li>	Although it would be very good to detect pointers leaking out
+	of RCU read-side critical sections, there is currently no
+	good way of doing this.
+	One complication is the need to distinguish between pointers
+	leaking and pointers that have been handed off from RCU to
+	some other synchronization mechanism, for example, reference
+	counting.
+<li>	In kernels built with <tt>CONFIG_RCU_TRACE=y</tt>, RCU-related
+	information is provided via both debugfs and event tracing.
+<li>	Open-coded use of <tt>rcu_assign_pointer()</tt> and
+	<tt>rcu_dereference()</tt> to create typical linked
+	data structures can be surprisingly error-prone.
+	Therefore, RCU-protected
+	<a href="https://lwn.net/Articles/609973/#RCU List APIs">linked lists</a>
+	and, more recently, RCU-protected
+	<a href="https://lwn.net/Articles/612100/">hash tables</a>
+	are available.
+	Many other special-purpose RCU-protected data structures are
+	available in the Linux kernel and the userspace RCU library.
+<li>	Some linked structures are created at compile time, but still
+	require <tt>__rcu</tt> checking.
+	The <tt>RCU_POINTER_INITIALIZER()</tt> macro serves this
+	purpose.
+<li>	It is not necessary to use <tt>rcu_assign_pointer()</tt>
+	when creating linked structures that are to be published via
+	a single external pointer.
+	The <tt>RCU_INIT_POINTER()</tt> macro is provided for
+	this task and also for assigning <tt>NULL</tt> pointers
+	at runtime.
+</ol>
+
+<p>
+This not a hard-and-fast list:  RCU's diagnostic capabilities will
+continue to be guided by the number and type of usage bugs found
+in real-world RCU usage.
+
+<h2><a name="Linux Kernel Complications">Linux Kernel Complications</a></h2>
+
+<p>
+The Linux kernel provides an interesting environment for all kinds of
+software, including RCU.
+Some of the relevant points of interest are as follows:
+
+<ol>
+<li>	<a href="#Configuration">Configuration</a>.
+<li>	<a href="#Firmware Interface">Firmware Interface</a>.
+<li>	<a href="#Early Boot">Early Boot</a>.
+<li>	<a href="#Interrupts and NMIs">
+	Interrupts and non-maskable interrupts (NMIs)</a>.
+<li>	<a href="#Loadable Modules">Loadable Modules</a>.
+<li>	<a href="#Hotplug CPU">Hotplug CPU</a>.
+<li>	<a href="#Scheduler and RCU">Scheduler and RCU</a>.
+<li>	<a href="#Tracing and RCU">Tracing and RCU</a>.
+<li>	<a href="#Energy Efficiency">Energy Efficiency</a>.
+<li>	<a href="#Memory Efficiency">Memory Efficiency</a>.
+<li>	<a href="#Performance, Scalability, Response Time, and Reliability">
+	Performance, Scalability, Response Time, and Reliability</a>.
+</ol>
+
+<p>
+This list is probably incomplete, but it does give a feel for the
+most notable Linux-kernel complications.
+Each of the following sections covers one of the above topics.
+
+<h3><a name="Configuration">Configuration</a></h3>
+
+<p>
+RCU's goal is automatic configuration, so that almost nobody
+needs to worry about RCU's <tt>Kconfig</tt> options.
+And for almost all users, RCU does in fact work well
+&ldquo;out of the box.&rdquo;
+
+<p>
+However, there are specialized use cases that are handled by
+kernel boot parameters and <tt>Kconfig</tt> options.
+Unfortunately, the <tt>Kconfig</tt> system will explicitly ask users
+about new <tt>Kconfig</tt> options, which requires almost all of them
+be hidden behind a <tt>CONFIG_RCU_EXPERT</tt> <tt>Kconfig</tt> option.
+
+<p>
+This all should be quite obvious, but the fact remains that
+Linus Torvalds recently had to
+<a href="https://lkml.kernel.org/g/CA+55aFy4wcCwaL4okTs8wXhGZ5h-ibecy_Meg9C4MNQrUnwMcg@mail.gmail.com">remind</a>
+me of this requirement.
+
+<h3><a name="Firmware Interface">Firmware Interface</a></h3>
+
+<p>
+In many cases, kernel obtains information about the system from the
+firmware, and sometimes things are lost in translation.
+Or the translation is accurate, but the original message is bogus.
+
+<p>
+For example, some systems' firmware overreports the number of CPUs,
+sometimes by a large factor.
+If RCU naively believed the firmware, as it used to do,
+it would create too many per-CPU kthreads.
+Although the resulting system will still run correctly, the extra
+kthreads needlessly consume memory and can cause confusion
+when they show up in <tt>ps</tt> listings.
+
+<p>
+RCU must therefore wait for a given CPU to actually come online before
+it can allow itself to believe that the CPU actually exists.
+The resulting &ldquo;ghost CPUs&rdquo; (which are never going to
+come online) cause a number of
+<a href="https://paulmck.livejournal.com/37494.html">interesting complications</a>.
+
+<h3><a name="Early Boot">Early Boot</a></h3>
+
+<p>
+The Linux kernel's boot sequence is an interesting process,
+and RCU is used early, even before <tt>rcu_init()</tt>
+is invoked.
+In fact, a number of RCU's primitives can be used as soon as the
+initial task's <tt>task_struct</tt> is available and the
+boot CPU's per-CPU variables are set up.
+The read-side primitives (<tt>rcu_read_lock()</tt>,
+<tt>rcu_read_unlock()</tt>, <tt>rcu_dereference()</tt>,
+and <tt>rcu_access_pointer()</tt>) will operate normally very early on,
+as will <tt>rcu_assign_pointer()</tt>.
+
+<p>
+Although <tt>call_rcu()</tt> may be invoked at any
+time during boot, callbacks are not guaranteed to be invoked until after
+the scheduler is fully up and running.
+This delay in callback invocation is due to the fact that RCU does not
+invoke callbacks until it is fully initialized, and this full initialization
+cannot occur until after the scheduler has initialized itself to the
+point where RCU can spawn and run its kthreads.
+In theory, it would be possible to invoke callbacks earlier,
+however, this is not a panacea because there would be severe restrictions
+on what operations those callbacks could invoke.
+
+<p>
+Perhaps surprisingly, <tt>synchronize_rcu()</tt>,
+<a href="#Bottom-Half Flavor"><tt>synchronize_rcu_bh()</tt></a>
+(<a href="#Bottom-Half Flavor">discussed below</a>),
+and
+<a href="#Sched Flavor"><tt>synchronize_sched()</tt></a>
+will all operate normally
+during very early boot, the reason being that there is only one CPU
+and preemption is disabled.
+This means that the call <tt>synchronize_rcu()</tt> (or friends)
+itself is a quiescent
+state and thus a grace period, so the early-boot implementation can
+be a no-op.
+
+<p>
+Both <tt>synchronize_rcu_bh()</tt> and <tt>synchronize_sched()</tt>
+continue to operate normally through the remainder of boot, courtesy
+of the fact that preemption is disabled across their RCU read-side
+critical sections and also courtesy of the fact that there is still
+only one CPU.
+However, once the scheduler starts initializing, preemption is enabled.
+There is still only a single CPU, but the fact that preemption is enabled
+means that the no-op implementation of <tt>synchronize_rcu()</tt> no
+longer works in <tt>CONFIG_PREEMPT=y</tt> kernels.
+Therefore, as soon as the scheduler starts initializing, the early-boot
+fastpath is disabled.
+This means that <tt>synchronize_rcu()</tt> switches to its runtime
+mode of operation where it posts callbacks, which in turn means that
+any call to <tt>synchronize_rcu()</tt> will block until the corresponding
+callback is invoked.
+Unfortunately, the callback cannot be invoked until RCU's runtime
+grace-period machinery is up and running, which cannot happen until
+the scheduler has initialized itself sufficiently to allow RCU's
+kthreads to be spawned.
+Therefore, invoking <tt>synchronize_rcu()</tt> during scheduler
+initialization can result in deadlock.
+
+<p><a name="Quick Quiz 14"><b>Quick Quiz 14</b>:</a>
+So what happens with <tt>synchronize_rcu()</tt> during
+scheduler initialization for <tt>CONFIG_PREEMPT=n</tt>
+kernels?
+<br><a href="#qq14answer">Answer</a>
+
+<p>
+I learned of these boot-time requirements as a result of a series of
+system hangs.
+
+<h3><a name="Interrupts and NMIs">Interrupts and NMIs</a></h3>
+
+<p>
+The Linux kernel has interrupts, and RCU read-side critical sections are
+legal within interrupt handlers and within interrupt-disabled regions
+of code, as are invocations of <tt>call_rcu()</tt>.
+
+<p>
+Some Linux-kernel architectures can enter an interrupt handler from
+non-idle process context, and then just never leave it, instead stealthily
+transitioning back to process context.
+This trick is sometimes used to invoke system calls from inside the kernel.
+These &ldquo;half-interrupts&rdquo; mean that RCU has to be very careful
+about how it counts interrupt nesting levels.
+I learned of this requirement the hard way during a rewrite
+of RCU's dyntick-idle code.
+
+<p>
+The Linux kernel has non-maskable interrupts (NMIs), and
+RCU read-side critical sections are legal within NMI handlers.
+Thankfully, RCU update-side primitives, including
+<tt>call_rcu()</tt>, are prohibited within NMI handlers.
+
+<p>
+The name notwithstanding, some Linux-kernel architectures
+can have nested NMIs, which RCU must handle correctly.
+Andy Lutomirski
+<a href="https://lkml.kernel.org/g/CALCETrXLq1y7e_dKFPgou-FKHB6Pu-r8+t-6Ds+8=va7anBWDA@mail.gmail.com">surprised me</a>
+with this requirement;
+he also kindly surprised me with
+<a href="https://lkml.kernel.org/g/CALCETrXSY9JpW3uE6H8WYk81sg56qasA2aqmjMPsq5dOtzso=g@mail.gmail.com">an algorithm</a>
+that meets this requirement.
+
+<h3><a name="Loadable Modules">Loadable Modules</a></h3>
+
+<p>
+The Linux kernel has loadable modules, and these modules can
+also be unloaded.
+After a given module has been unloaded, any attempt to call
+one of its functions results in a segmentation fault.
+The module-unload functions must therefore cancel any
+delayed calls to loadable-module functions, for example,
+any outstanding <tt>mod_timer()</tt> must be dealt with
+via <tt>del_timer_sync()</tt> or similar.
+
+<p>
+Unfortunately, there is no way to cancel an RCU callback;
+once you invoke <tt>call_rcu()</tt>, the callback function is
+going to eventually be invoked, unless the system goes down first.
+Because it is normally considered socially irresponsible to crash the system
+in response to a module unload request, we need some other way
+to deal with in-flight RCU callbacks.
+
+<p>
+RCU therefore provides
+<tt><a href="https://lwn.net/Articles/217484/">rcu_barrier()</a></tt>,
+which waits until all in-flight RCU callbacks have been invoked.
+If a module uses <tt>call_rcu()</tt>, its exit function should therefore
+prevent any future invocation of <tt>call_rcu()</tt>, then invoke
+<tt>rcu_barrier()</tt>.
+In theory, the underlying module-unload code could invoke
+<tt>rcu_barrier()</tt> unconditionally, but in practice this would
+incur unacceptable latencies.
+
+<p>
+Nikita Danilov noted this requirement for an analogous filesystem-unmount
+situation, and Dipankar Sarma incorporated <tt>rcu_barrier()</tt> into RCU.
+The need for <tt>rcu_barrier()</tt> for module unloading became
+apparent later.
+
+<h3><a name="Hotplug CPU">Hotplug CPU</a></h3>
+
+<p>
+The Linux kernel supports CPU hotplug, which means that CPUs
+can come and go.
+It is of course illegal to use any RCU API member from an offline CPU.
+This requirement was present from day one in DYNIX/ptx, but
+on the other hand, the Linux kernel's CPU-hotplug implementation
+is &ldquo;interesting.&rdquo;
+
+<p>
+The Linux-kernel CPU-hotplug implementation has notifiers that
+are used to allow the various kernel subsystems (including RCU)
+to respond appropriately to a given CPU-hotplug operation.
+Most RCU operations may be invoked from CPU-hotplug notifiers,
+including even normal synchronous grace-period operations
+such as <tt>synchronize_rcu()</tt>.
+However, expedited grace-period operations such as
+<tt>synchronize_rcu_expedited()</tt> are not supported,
+due to the fact that current implementations block CPU-hotplug
+operations, which could result in deadlock.
+
+<p>
+In addition, all-callback-wait operations such as
+<tt>rcu_barrier()</tt> are also not supported, due to the
+fact that there are phases of CPU-hotplug operations where
+the outgoing CPU's callbacks will not be invoked until after
+the CPU-hotplug operation ends, which could also result in deadlock.
+
+<h3><a name="Scheduler and RCU">Scheduler and RCU</a></h3>
+
+<p>
+RCU depends on the scheduler, and the scheduler uses RCU to
+protect some of its data structures.
+This means the scheduler is forbidden from acquiring
+the runqueue locks and the priority-inheritance locks
+in the middle of an outermost RCU read-side critical section unless either
+(1)&nbsp;it releases them before exiting that same
+RCU read-side critical section, or
+(2)&nbsp;interrupts are disabled across
+that entire RCU read-side critical section.
+This same prohibition also applies (recursively!) to any lock that is acquired
+while holding any lock to which this prohibition applies.
+Adhering to this rule prevents preemptible RCU from invoking
+<tt>rcu_read_unlock_special()</tt> while either runqueue or
+priority-inheritance locks are held, thus avoiding deadlock.
+
+<p>
+Prior to v4.4, it was only necessary to disable preemption across
+RCU read-side critical sections that acquired scheduler locks.
+In v4.4, expedited grace periods started using IPIs, and these
+IPIs could force a <tt>rcu_read_unlock()</tt> to take the slowpath.
+Therefore, this expedited-grace-period change required disabling of
+interrupts, not just preemption.
+
+<p>
+For RCU's part, the preemptible-RCU <tt>rcu_read_unlock()</tt>
+implementation must be written carefully to avoid similar deadlocks.
+In particular, <tt>rcu_read_unlock()</tt> must tolerate an
+interrupt where the interrupt handler invokes both
+<tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>.
+This possibility requires <tt>rcu_read_unlock()</tt> to use
+negative nesting levels to avoid destructive recursion via
+interrupt handler's use of RCU.
+
+<p>
+This pair of mutual scheduler-RCU requirements came as a
+<a href="https://lwn.net/Articles/453002/">complete surprise</a>.
+
+<p>
+As noted above, RCU makes use of kthreads, and it is necessary to
+avoid excessive CPU-time accumulation by these kthreads.
+This requirement was no surprise, but RCU's violation of it
+when running context-switch-heavy workloads when built with
+<tt>CONFIG_NO_HZ_FULL=y</tt>
+<a href="http://www.rdrop.com/users/paulmck/scalability/paper/BareMetal.2015.01.15b.pdf">did come as a surprise [PDF]</a>.
+RCU has made good progress towards meeting this requirement, even
+for context-switch-have <tt>CONFIG_NO_HZ_FULL=y</tt> workloads,
+but there is room for further improvement.
+
+<h3><a name="Tracing and RCU">Tracing and RCU</a></h3>
+
+<p>
+It is possible to use tracing on RCU code, but tracing itself
+uses RCU.
+For this reason, <tt>rcu_dereference_raw_notrace()</tt>
+is provided for use by tracing, which avoids the destructive
+recursion that could otherwise ensue.
+This API is also used by virtualization in some architectures,
+where RCU readers execute in environments in which tracing
+cannot be used.
+The tracing folks both located the requirement and provided the
+needed fix, so this surprise requirement was relatively painless.
+
+<h3><a name="Energy Efficiency">Energy Efficiency</a></h3>
+
+<p>
+Interrupting idle CPUs is considered socially unacceptable,
+especially by people with battery-powered embedded systems.
+RCU therefore conserves energy by detecting which CPUs are
+idle, including tracking CPUs that have been interrupted from idle.
+This is a large part of the energy-efficiency requirement,
+so I learned of this via an irate phone call.
+
+<p>
+Because RCU avoids interrupting idle CPUs, it is illegal to
+execute an RCU read-side critical section on an idle CPU.
+(Kernels built with <tt>CONFIG_PROVE_RCU=y</tt> will splat
+if you try it.)
+The <tt>RCU_NONIDLE()</tt> macro and <tt>_rcuidle</tt>
+event tracing is provided to work around this restriction.
+In addition, <tt>rcu_is_watching()</tt> may be used to
+test whether or not it is currently legal to run RCU read-side
+critical sections on this CPU.
+I learned of the need for diagnostics on the one hand
+and <tt>RCU_NONIDLE()</tt> on the other while inspecting
+idle-loop code.
+Steven Rostedt supplied <tt>_rcuidle</tt> event tracing,
+which is used quite heavily in the idle loop.
+
+<p>
+It is similarly socially unacceptable to interrupt an
+<tt>nohz_full</tt> CPU running in userspace.
+RCU must therefore track <tt>nohz_full</tt> userspace
+execution.
+And in
+<a href="https://lwn.net/Articles/558284/"><tt>CONFIG_NO_HZ_FULL_SYSIDLE=y</tt></a>
+kernels, RCU must separately track idle CPUs on the one hand and
+CPUs that are either idle or executing in userspace on the other.
+In both cases, RCU must be able to sample state at two points in
+time, and be able to determine whether or not some other CPU spent
+any time idle and/or executing in userspace.
+
+<p>
+These energy-efficiency requirements have proven quite difficult to
+understand and to meet, for example, there have been more than five
+clean-sheet rewrites of RCU's energy-efficiency code, the last of
+which was finally able to demonstrate
+<a href="http://www.rdrop.com/users/paulmck/realtime/paper/AMPenergy.2013.04.19a.pdf">real energy savings running on real hardware [PDF]</a>.
+As noted earlier,
+I learned of many of these requirements via angry phone calls:
+Flaming me on the Linux-kernel mailing list was apparently not
+sufficient to fully vent their ire at RCU's energy-efficiency bugs!
+
+<h3><a name="Memory Efficiency">Memory Efficiency</a></h3>
+
+<p>
+Although small-memory non-realtime systems can simply use Tiny RCU,
+code size is only one aspect of memory efficiency.
+Another aspect is the size of the <tt>rcu_head</tt> structure
+used by <tt>call_rcu()</tt> and <tt>kfree_rcu()</tt>.
+Although this structure contains nothing more than a pair of pointers,
+it does appear in many RCU-protected data structures, including
+some that are size critical.
+The <tt>page</tt> structure is a case in point, as evidenced by
+the many occurrences of the <tt>union</tt> keyword within that structure.
+
+<p>
+This need for memory efficiency is one reason that RCU uses hand-crafted
+singly linked lists to track the <tt>rcu_head</tt> structures that
+are waiting for a grace period to elapse.
+It is also the reason why <tt>rcu_head</tt> structures do not contain
+debug information, such as fields tracking the file and line of the
+<tt>call_rcu()</tt> or <tt>kfree_rcu()</tt> that posted them.
+Although this information might appear in debug-only kernel builds at some
+point, in the meantime, the <tt>-&gt;func</tt> field will often provide
+the needed debug information.
+
+<p>
+However, in some cases, the need for memory efficiency leads to even
+more extreme measures.
+Returning to the <tt>page</tt> structure, the <tt>rcu_head</tt> field
+shares storage with a great many other structures that are used at
+various points in the corresponding page's lifetime.
+In order to correctly resolve certain
+<a href="https://lkml.kernel.org/g/1439976106-137226-1-git-send-email-kirill.shutemov@linux.intel.com">race conditions</a>,
+the Linux kernel's memory-management subsystem needs a particular bit
+to remain zero during all phases of grace-period processing,
+and that bit happens to map to the bottom bit of the
+<tt>rcu_head</tt> structure's <tt>-&gt;next</tt> field.
+RCU makes this guarantee as long as <tt>call_rcu()</tt>
+is used to post the callback, as opposed to <tt>kfree_rcu()</tt>
+or some future &ldquo;lazy&rdquo;
+variant of <tt>call_rcu()</tt> that might one day be created for
+energy-efficiency purposes.
+
+<h3><a name="Performance, Scalability, Response Time, and Reliability">
+Performance, Scalability, Response Time, and Reliability</a></h3>
+
+<p>
+Expanding on the
+<a href="#Performance and Scalability">earlier discussion</a>,
+RCU is used heavily by hot code paths in performance-critical
+portions of the Linux kernel's networking, security, virtualization,
+and scheduling code paths.
+RCU must therefore use efficient implementations, especially in its
+read-side primitives.
+To that end, it would be good if preemptible RCU's implementation
+of <tt>rcu_read_lock()</tt> could be inlined, however, doing
+this requires resolving <tt>#include</tt> issues with the
+<tt>task_struct</tt> structure.
+
+<p>
+The Linux kernel supports hardware configurations with up to
+4096 CPUs, which means that RCU must be extremely scalable.
+Algorithms that involve frequent acquisitions of global locks or
+frequent atomic operations on global variables simply cannot be
+tolerated within the RCU implementation.
+RCU therefore makes heavy use of a combining tree based on the
+<tt>rcu_node</tt> structure.
+RCU is required to tolerate all CPUs continuously invoking any
+combination of RCU's runtime primitives with minimal per-operation
+overhead.
+In fact, in many cases, increasing load must <i>decrease</i> the
+per-operation overhead, witness the batching optimizations for
+<tt>synchronize_rcu()</tt>, <tt>call_rcu()</tt>,
+<tt>synchronize_rcu_expedited()</tt>, and <tt>rcu_barrier()</tt>.
+As a general rule, RCU must cheerfully accept whatever the
+rest of the Linux kernel decides to throw at it.
+
+<p>
+The Linux kernel is used for real-time workloads, especially
+in conjunction with the
+<a href="https://rt.wiki.kernel.org/index.php/Main_Page">-rt patchset</a>.
+The real-time-latency response requirements are such that the
+traditional approach of disabling preemption across RCU
+read-side critical sections is inappropriate.
+Kernels built with <tt>CONFIG_PREEMPT=y</tt> therefore
+use an RCU implementation that allows RCU read-side critical
+sections to be preempted.
+This requirement made its presence known after users made it
+clear that an earlier
+<a href="https://lwn.net/Articles/107930/">real-time patch</a>
+did not meet their needs, in conjunction with some
+<a href="https://lkml.kernel.org/g/20050318002026.GA2693@us.ibm.com">RCU issues</a>
+encountered by a very early version of the -rt patchset.
+
+<p>
+In addition, RCU must make do with a sub-100-microsecond real-time latency
+budget.
+In fact, on smaller systems with the -rt patchset, the Linux kernel
+provides sub-20-microsecond real-time latencies for the whole kernel,
+including RCU.
+RCU's scalability and latency must therefore be sufficient for
+these sorts of configurations.
+To my surprise, the sub-100-microsecond real-time latency budget
+<a href="http://www.rdrop.com/users/paulmck/realtime/paper/bigrt.2013.01.31a.LCA.pdf">
+applies to even the largest systems [PDF]</a>,
+up to and including systems with 4096 CPUs.
+This real-time requirement motivated the grace-period kthread, which
+also simplified handling of a number of race conditions.
+
+<p>
+Finally, RCU's status as a synchronization primitive means that
+any RCU failure can result in arbitrary memory corruption that can be
+extremely difficult to debug.
+This means that RCU must be extremely reliable, which in
+practice also means that RCU must have an aggressive stress-test
+suite.
+This stress-test suite is called <tt>rcutorture</tt>.
+
+<p>
+Although the need for <tt>rcutorture</tt> was no surprise,
+the current immense popularity of the Linux kernel is posing
+interesting&mdash;and perhaps unprecedented&mdash;validation
+challenges.
+To see this, keep in mind that there are well over one billion
+instances of the Linux kernel running today, given Android
+smartphones, Linux-powered televisions, and servers.
+This number can be expected to increase sharply with the advent of
+the celebrated Internet of Things.
+
+<p>
+Suppose that RCU contains a race condition that manifests on average
+once per million years of runtime.
+This bug will be occurring about three times per <i>day</i> across
+the installed base.
+RCU could simply hide behind hardware error rates, given that no one
+should really expect their smartphone to last for a million years.
+However, anyone taking too much comfort from this thought should
+consider the fact that in most jurisdictions, a successful multi-year
+test of a given mechanism, which might include a Linux kernel,
+suffices for a number of types of safety-critical certifications.
+In fact, rumor has it that the Linux kernel is already being used
+in production for safety-critical applications.
+I don't know about you, but I would feel quite bad if a bug in RCU
+killed someone.
+Which might explain my recent focus on validation and verification.
+
+<h2><a name="Other RCU Flavors">Other RCU Flavors</a></h2>
+
+<p>
+One of the more surprising things about RCU is that there are now
+no fewer than five <i>flavors</i>, or API families.
+In addition, the primary flavor that has been the sole focus up to
+this point has two different implementations, non-preemptible and
+preemptible.
+The other four flavors are listed below, with requirements for each
+described in a separate section.
+
+<ol>
+<li>	<a href="#Bottom-Half Flavor">Bottom-Half Flavor</a>
+<li>	<a href="#Sched Flavor">Sched Flavor</a>
+<li>	<a href="#Sleepable RCU">Sleepable RCU</a>
+<li>	<a href="#Tasks RCU">Tasks RCU</a>
+</ol>
+
+<h3><a name="Bottom-Half Flavor">Bottom-Half Flavor</a></h3>
+
+<p>
+The softirq-disable (AKA &ldquo;bottom-half&rdquo;,
+hence the &ldquo;_bh&rdquo; abbreviations)
+flavor of RCU, or <i>RCU-bh</i>, was developed by
+Dipankar Sarma to provide a flavor of RCU that could withstand the
+network-based denial-of-service attacks researched by Robert
+Olsson.
+These attacks placed so much networking load on the system
+that some of the CPUs never exited softirq execution,
+which in turn prevented those CPUs from ever executing a context switch,
+which, in the RCU implementation of that time, prevented grace periods
+from ever ending.
+The result was an out-of-memory condition and a system hang.
+
+<p>
+The solution was the creation of RCU-bh, which does
+<tt>local_bh_disable()</tt>
+across its read-side critical sections, and which uses the transition
+from one type of softirq processing to another as a quiescent state
+in addition to context switch, idle, user mode, and offline.
+This means that RCU-bh grace periods can complete even when some of
+the CPUs execute in softirq indefinitely, thus allowing algorithms
+based on RCU-bh to withstand network-based denial-of-service attacks.
+
+<p>
+Because
+<tt>rcu_read_lock_bh()</tt> and <tt>rcu_read_unlock_bh()</tt>
+disable and re-enable softirq handlers, any attempt to start a softirq
+handlers during the
+RCU-bh read-side critical section will be deferred.
+In this case, <tt>rcu_read_unlock_bh()</tt>
+will invoke softirq processing, which can take considerable time.
+One can of course argue that this softirq overhead should be associated
+with the code following the RCU-bh read-side critical section rather
+than <tt>rcu_read_unlock_bh()</tt>, but the fact
+is that most profiling tools cannot be expected to make this sort
+of fine distinction.
+For example, suppose that a three-millisecond-long RCU-bh read-side
+critical section executes during a time of heavy networking load.
+There will very likely be an attempt to invoke at least one softirq
+handler during that three milliseconds, but any such invocation will
+be delayed until the time of the <tt>rcu_read_unlock_bh()</tt>.
+This can of course make it appear at first glance as if
+<tt>rcu_read_unlock_bh()</tt> was executing very slowly.
+
+<p>
+The
+<a href="https://lwn.net/Articles/609973/#RCU Per-Flavor API Table">RCU-bh API</a>
+includes
+<tt>rcu_read_lock_bh()</tt>,
+<tt>rcu_read_unlock_bh()</tt>,
+<tt>rcu_dereference_bh()</tt>,
+<tt>rcu_dereference_bh_check()</tt>,
+<tt>synchronize_rcu_bh()</tt>,
+<tt>synchronize_rcu_bh_expedited()</tt>,
+<tt>call_rcu_bh()</tt>,
+<tt>rcu_barrier_bh()</tt>, and
+<tt>rcu_read_lock_bh_held()</tt>.
+
+<h3><a name="Sched Flavor">Sched Flavor</a></h3>
+
+<p>
+Before preemptible RCU, waiting for an RCU grace period had the
+side effect of also waiting for all pre-existing interrupt
+and NMI handlers.
+However, there are legitimate preemptible-RCU implementations that
+do not have this property, given that any point in the code outside
+of an RCU read-side critical section can be a quiescent state.
+Therefore, <i>RCU-sched</i> was created, which follows &ldquo;classic&rdquo;
+RCU in that an RCU-sched grace period waits for for pre-existing
+interrupt and NMI handlers.
+In kernels built with <tt>CONFIG_PREEMPT=n</tt>, the RCU and RCU-sched
+APIs have identical implementations, while kernels built with
+<tt>CONFIG_PREEMPT=y</tt> provide a separate implementation for each.
+
+<p>
+Note well that in <tt>CONFIG_PREEMPT=y</tt> kernels,
+<tt>rcu_read_lock_sched()</tt> and <tt>rcu_read_unlock_sched()</tt>
+disable and re-enable preemption, respectively.
+This means that if there was a preemption attempt during the
+RCU-sched read-side critical section, <tt>rcu_read_unlock_sched()</tt>
+will enter the scheduler, with all the latency and overhead entailed.
+Just as with <tt>rcu_read_unlock_bh()</tt>, this can make it look
+as if <tt>rcu_read_unlock_sched()</tt> was executing very slowly.
+However, the highest-priority task won't be preempted, so that task
+will enjoy low-overhead <tt>rcu_read_unlock_sched()</tt> invocations.
+
+<p>
+The
+<a href="https://lwn.net/Articles/609973/#RCU Per-Flavor API Table">RCU-sched API</a>
+includes
+<tt>rcu_read_lock_sched()</tt>,
+<tt>rcu_read_unlock_sched()</tt>,
+<tt>rcu_read_lock_sched_notrace()</tt>,
+<tt>rcu_read_unlock_sched_notrace()</tt>,
+<tt>rcu_dereference_sched()</tt>,
+<tt>rcu_dereference_sched_check()</tt>,
+<tt>synchronize_sched()</tt>,
+<tt>synchronize_rcu_sched_expedited()</tt>,
+<tt>call_rcu_sched()</tt>,
+<tt>rcu_barrier_sched()</tt>, and
+<tt>rcu_read_lock_sched_held()</tt>.
+However, anything that disables preemption also marks an RCU-sched
+read-side critical section, including
+<tt>preempt_disable()</tt> and <tt>preempt_enable()</tt>,
+<tt>local_irq_save()</tt> and <tt>local_irq_restore()</tt>,
+and so on.
+
+<h3><a name="Sleepable RCU">Sleepable RCU</a></h3>
+
+<p>
+For well over a decade, someone saying &ldquo;I need to block within
+an RCU read-side critical section&rdquo; was a reliable indication
+that this someone did not understand RCU.
+After all, if you are always blocking in an RCU read-side critical
+section, you can probably afford to use a higher-overhead synchronization
+mechanism.
+However, that changed with the advent of the Linux kernel's notifiers,
+whose RCU read-side critical
+sections almost never sleep, but sometimes need to.
+This resulted in the introduction of
+<a href="https://lwn.net/Articles/202847/">sleepable RCU</a>,
+or <i>SRCU</i>.
+
+<p>
+SRCU allows different domains to be defined, with each such domain
+defined by an instance of an <tt>srcu_struct</tt> structure.
+A pointer to this structure must be passed in to each SRCU function,
+for example, <tt>synchronize_srcu(&amp;ss)</tt>, where
+<tt>ss</tt> is the <tt>srcu_struct</tt> structure.
+The key benefit of these domains is that a slow SRCU reader in one
+domain does not delay an SRCU grace period in some other domain.
+That said, one consequence of these domains is that read-side code
+must pass a &ldquo;cookie&rdquo; from <tt>srcu_read_lock()</tt>
+to <tt>srcu_read_unlock()</tt>, for example, as follows:
+
+<blockquote>
+<pre>
+ 1 int idx;
+ 2
+ 3 idx = srcu_read_lock(&amp;ss);
+ 4 do_something();
+ 5 srcu_read_unlock(&amp;ss, idx);
+</pre>
+</blockquote>
+
+<p>
+As noted above, it is legal to block within SRCU read-side critical sections,
+however, with great power comes great responsibility.
+If you block forever in one of a given domain's SRCU read-side critical
+sections, then that domain's grace periods will also be blocked forever.
+Of course, one good way to block forever is to deadlock, which can
+happen if any operation in a given domain's SRCU read-side critical
+section can block waiting, either directly or indirectly, for that domain's
+grace period to elapse.
+For example, this results in a self-deadlock:
+
+<blockquote>
+<pre>
+ 1 int idx;
+ 2
+ 3 idx = srcu_read_lock(&amp;ss);
+ 4 do_something();
+ 5 synchronize_srcu(&amp;ss);
+ 6 srcu_read_unlock(&amp;ss, idx);
+</pre>
+</blockquote>
+
+<p>
+However, if line&nbsp;5 acquired a mutex that was held across
+a <tt>synchronize_srcu()</tt> for domain <tt>ss</tt>,
+deadlock would still be possible.
+Furthermore, if line&nbsp;5 acquired a mutex that was held across
+a <tt>synchronize_srcu()</tt> for some other domain <tt>ss1</tt>,
+and if an <tt>ss1</tt>-domain SRCU read-side critical section
+acquired another mutex that was held across as <tt>ss</tt>-domain
+<tt>synchronize_srcu()</tt>,
+deadlock would again be possible.
+Such a deadlock cycle could extend across an arbitrarily large number
+of different SRCU domains.
+Again, with great power comes great responsibility.
+
+<p>
+Unlike the other RCU flavors, SRCU read-side critical sections can
+run on idle and even offline CPUs.
+This ability requires that <tt>srcu_read_lock()</tt> and
+<tt>srcu_read_unlock()</tt> contain memory barriers, which means
+that SRCU readers will run a bit slower than would RCU readers.
+It also motivates the <tt>smp_mb__after_srcu_read_unlock()</tt>
+API, which, in combination with <tt>srcu_read_unlock()</tt>,
+guarantees a full memory barrier.
+
+<p>
+The
+<a href="https://lwn.net/Articles/609973/#RCU Per-Flavor API Table">SRCU API</a>
+includes
+<tt>srcu_read_lock()</tt>,
+<tt>srcu_read_unlock()</tt>,
+<tt>srcu_dereference()</tt>,
+<tt>srcu_dereference_check()</tt>,
+<tt>synchronize_srcu()</tt>,
+<tt>synchronize_srcu_expedited()</tt>,
+<tt>call_srcu()</tt>,
+<tt>srcu_barrier()</tt>, and
+<tt>srcu_read_lock_held()</tt>.
+It also includes
+<tt>DEFINE_SRCU()</tt>,
+<tt>DEFINE_STATIC_SRCU()</tt>, and
+<tt>init_srcu_struct()</tt>
+APIs for defining and initializing <tt>srcu_struct</tt> structures.
+
+<h3><a name="Tasks RCU">Tasks RCU</a></h3>
+
+<p>
+Some forms of tracing use &ldquo;tramopolines&rdquo; to handle the
+binary rewriting required to install different types of probes.
+It would be good to be able to free old trampolines, which sounds
+like a job for some form of RCU.
+However, because it is necessary to be able to install a trace
+anywhere in the code, it is not possible to use read-side markers
+such as <tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>.
+In addition, it does not work to have these markers in the trampoline
+itself, because there would need to be instructions following
+<tt>rcu_read_unlock()</tt>.
+Although <tt>synchronize_rcu()</tt> would guarantee that execution
+reached the <tt>rcu_read_unlock()</tt>, it would not be able to
+guarantee that execution had completely left the trampoline.
+
+<p>
+The solution, in the form of
+<a href="https://lwn.net/Articles/607117/"><i>Tasks RCU</i></a>,
+is to have implicit
+read-side critical sections that are delimited by voluntary context
+switches, that is, calls to <tt>schedule()</tt>,
+<tt>cond_resched_rcu_qs()</tt>, and
+<tt>synchronize_rcu_tasks()</tt>.
+In addition, transitions to and from userspace execution also delimit
+tasks-RCU read-side critical sections.
+
+<p>
+The tasks-RCU API is quite compact, consisting only of
+<tt>call_rcu_tasks()</tt>,
+<tt>synchronize_rcu_tasks()</tt>, and
+<tt>rcu_barrier_tasks()</tt>.
+
+<h2><a name="Possible Future Changes">Possible Future Changes</a></h2>
+
+<p>
+One of the tricks that RCU uses to attain update-side scalability is
+to increase grace-period latency with increasing numbers of CPUs.
+If this becomes a serious problem, it will be necessary to rework the
+grace-period state machine so as to avoid the need for the additional
+latency.
+
+<p>
+Expedited grace periods scan the CPUs, so their latency and overhead
+increases with increasing numbers of CPUs.
+If this becomes a serious problem on large systems, it will be necessary
+to do some redesign to avoid this scalability problem.
+
+<p>
+RCU disables CPU hotplug in a few places, perhaps most notably in the
+expedited grace-period and <tt>rcu_barrier()</tt> operations.
+If there is a strong reason to use expedited grace periods in CPU-hotplug
+notifiers, it will be necessary to avoid disabling CPU hotplug.
+This would introduce some complexity, so there had better be a <i>very</i>
+good reason.
+
+<p>
+The tradeoff between grace-period latency on the one hand and interruptions
+of other CPUs on the other hand may need to be re-examined.
+The desire is of course for zero grace-period latency as well as zero
+interprocessor interrupts undertaken during an expedited grace period
+operation.
+While this ideal is unlikely to be achievable, it is quite possible that
+further improvements can be made.
+
+<p>
+The multiprocessor implementations of RCU use a combining tree that
+groups CPUs so as to reduce lock contention and increase cache locality.
+However, this combining tree does not spread its memory across NUMA
+nodes nor does it align the CPU groups with hardware features such
+as sockets or cores.
+Such spreading and alignment is currently believed to be unnecessary
+because the hotpath read-side primitives do not access the combining
+tree, nor does <tt>call_rcu()</tt> in the common case.
+If you believe that your architecture needs such spreading and alignment,
+then your architecture should also benefit from the
+<tt>rcutree.rcu_fanout_leaf</tt> boot parameter, which can be set
+to the number of CPUs in a socket, NUMA node, or whatever.
+If the number of CPUs is too large, use a fraction of the number of
+CPUs.
+If the number of CPUs is a large prime number, well, that certainly
+is an &ldquo;interesting&rdquo; architectural choice!
+More flexible arrangements might be considered, but only if
+<tt>rcutree.rcu_fanout_leaf</tt> has proven inadequate, and only
+if the inadequacy has been demonstrated by a carefully run and
+realistic system-level workload.
+
+<p>
+Please note that arrangements that require RCU to remap CPU numbers will
+require extremely good demonstration of need and full exploration of
+alternatives.
+
+<p>
+There is an embarrassingly large number of flavors of RCU, and this
+number has been increasing over time.
+Perhaps it will be possible to combine some at some future date.
+
+<p>
+RCU's various kthreads are reasonably recent additions.
+It is quite likely that adjustments will be required to more gracefully
+handle extreme loads.
+It might also be necessary to be able to relate CPU utilization by
+RCU's kthreads and softirq handlers to the code that instigated this
+CPU utilization.
+For example, RCU callback overhead might be charged back to the
+originating <tt>call_rcu()</tt> instance, though probably not
+in production kernels.
+
+<h2><a name="Summary">Summary</a></h2>
+
+<p>
+This document has presented more than two decade's worth of RCU
+requirements.
+Given that the requirements keep changing, this will not be the last
+word on this subject, but at least it serves to get an important
+subset of the requirements set forth.
+
+<h2><a name="Acknowledgments">Acknowledgments</a></h2>
+
+I am grateful to Steven Rostedt, Lai Jiangshan, Ingo Molnar,
+Oleg Nesterov, Borislav Petkov, Peter Zijlstra, Boqun Feng, and
+Andy Lutomirski for their help in rendering
+this article human readable, and to Michelle Rankin for her support
+of this effort.
+Other contributions are acknowledged in the Linux kernel's git archive.
+The cartoon is copyright (c) 2013 by Melissa Broussard,
+and is provided
+under the terms of the Creative Commons Attribution-Share Alike 3.0
+United States license.
+
+<h3><a name="Answers to Quick Quizzes">
+Answers to Quick Quizzes</a></h3>
+
+<a name="qq1answer"></a>
+<p><b>Quick Quiz 1</b>:
+Wait a minute!
+You said that updaters can make useful forward progress concurrently
+with readers, but pre-existing readers will block
+<tt>synchronize_rcu()</tt>!!!
+Just who are you trying to fool???
+
+
+</p><p><b>Answer</b>:
+First, if updaters do not wish to be blocked by readers, they can use
+<tt>call_rcu()</tt> or <tt>kfree_rcu()</tt>, which will
+be discussed later.
+Second, even when using <tt>synchronize_rcu()</tt>, the other
+update-side code does run concurrently with readers, whether pre-existing
+or not.
+
+
+</p><p><a href="#Quick%20Quiz%201"><b>Back to Quick Quiz 1</b>.</a>
+
+<a name="qq2answer"></a>
+<p><b>Quick Quiz 2</b>:
+Why is the <tt>synchronize_rcu()</tt> on line&nbsp;28 needed?
+
+
+</p><p><b>Answer</b>:
+Without that extra grace period, memory reordering could result in
+<tt>do_something_dlm()</tt> executing <tt>do_something()</tt>
+concurrently with the last bits of <tt>recovery()</tt>.
+
+
+</p><p><a href="#Quick%20Quiz%202"><b>Back to Quick Quiz 2</b>.</a>
+
+<a name="qq3answer"></a>
+<p><b>Quick Quiz 3</b>:
+But <tt>rcu_assign_pointer()</tt> does nothing to prevent the
+two assignments to <tt>p-&gt;a</tt> and <tt>p-&gt;b</tt>
+from being reordered.
+Can't that also cause problems?
+
+
+</p><p><b>Answer</b>:
+No, it cannot.
+The readers cannot see either of these two fields until
+the assignment to <tt>gp</tt>, by which time both fields are
+fully initialized.
+So reordering the assignments
+to <tt>p-&gt;a</tt> and <tt>p-&gt;b</tt> cannot possibly
+cause any problems.
+
+
+</p><p><a href="#Quick%20Quiz%203"><b>Back to Quick Quiz 3</b>.</a>
+
+<a name="qq4answer"></a>
+<p><b>Quick Quiz 4</b>:
+Without the <tt>rcu_dereference()</tt> or the
+<tt>rcu_access_pointer()</tt>, what destructive optimizations
+might the compiler make use of?
+
+
+</p><p><b>Answer</b>:
+Let's start with what happens to <tt>do_something_gp()</tt>
+if it fails to use <tt>rcu_dereference()</tt>.
+It could reuse a value formerly fetched from this same pointer.
+It could also fetch the pointer from <tt>gp</tt> in a byte-at-a-time
+manner, resulting in <i>load tearing</i>, in turn resulting a bytewise
+mash-up of two distince pointer values.
+It might even use value-speculation optimizations, where it makes a wrong
+guess, but by the time it gets around to checking the value, an update
+has changed the pointer to match the wrong guess.
+Too bad about any dereferences that returned pre-initialization garbage
+in the meantime!
+
+<p>
+For <tt>remove_gp_synchronous()</tt>, as long as all modifications
+to <tt>gp</tt> are carried out while holding <tt>gp_lock</tt>,
+the above optimizations are harmless.
+However,
+with <tt>CONFIG_SPARSE_RCU_POINTER=y</tt>,
+<tt>sparse</tt> will complain if you
+define <tt>gp</tt> with <tt>__rcu</tt> and then
+access it without using
+either <tt>rcu_access_pointer()</tt> or <tt>rcu_dereference()</tt>.
+
+
+</p><p><a href="#Quick%20Quiz%204"><b>Back to Quick Quiz 4</b>.</a>
+
+<a name="qq5answer"></a>
+<p><b>Quick Quiz 5</b>:
+Given that multiple CPUs can start RCU read-side critical sections
+at any time without any ordering whatsoever, how can RCU possibly tell whether
+or not a given RCU read-side critical section starts before a
+given instance of <tt>synchronize_rcu()</tt>?
+
+
+</p><p><b>Answer</b>:
+If RCU cannot tell whether or not a given
+RCU read-side critical section starts before a
+given instance of <tt>synchronize_rcu()</tt>,
+then it must assume that the RCU read-side critical section
+started first.
+In other words, a given instance of <tt>synchronize_rcu()</tt>
+can avoid waiting on a given RCU read-side critical section only
+if it can prove that <tt>synchronize_rcu()</tt> started first.
+
+
+</p><p><a href="#Quick%20Quiz%205"><b>Back to Quick Quiz 5</b>.</a>
+
+<a name="qq6answer"></a>
+<p><b>Quick Quiz 6</b>:
+The first and second guarantees require unbelievably strict ordering!
+Are all these memory barriers <i> really</i> required?
+
+
+</p><p><b>Answer</b>:
+Yes, they really are required.
+To see why the first guarantee is required, consider the following
+sequence of events:
+
+<ol>
+<li>	CPU 1: <tt>rcu_read_lock()</tt>
+<li>	CPU 1: <tt>q = rcu_dereference(gp);
+	/* Very likely to return p. */</tt>
+<li>	CPU 0: <tt>list_del_rcu(p);</tt>
+<li>	CPU 0: <tt>synchronize_rcu()</tt> starts.
+<li>	CPU 1: <tt>do_something_with(q-&gt;a);
+	/* No smp_mb(), so might happen after kfree(). */</tt>
+<li>	CPU 1: <tt>rcu_read_unlock()</tt>
+<li>	CPU 0: <tt>synchronize_rcu()</tt> returns.
+<li>	CPU 0: <tt>kfree(p);</tt>
+</ol>
+
+<p>
+Therefore, there absolutely must be a full memory barrier between the
+end of the RCU read-side critical section and the end of the
+grace period.
+
+<p>
+The sequence of events demonstrating the necessity of the second rule
+is roughly similar:
+
+<ol>
+<li>	CPU 0: <tt>list_del_rcu(p);</tt>
+<li>	CPU 0: <tt>synchronize_rcu()</tt> starts.
+<li>	CPU 1: <tt>rcu_read_lock()</tt>
+<li>	CPU 1: <tt>q = rcu_dereference(gp);
+	/* Might return p if no memory barrier. */</tt>
+<li>	CPU 0: <tt>synchronize_rcu()</tt> returns.
+<li>	CPU 0: <tt>kfree(p);</tt>
+<li>	CPU 1: <tt>do_something_with(q-&gt;a); /* Boom!!! */</tt>
+<li>	CPU 1: <tt>rcu_read_unlock()</tt>
+</ol>
+
+<p>
+And similarly, without a memory barrier between the beginning of the
+grace period and the beginning of the RCU read-side critical section,
+CPU&nbsp;1 might end up accessing the freelist.
+
+<p>
+The &ldquo;as if&rdquo; rule of course applies, so that any implementation
+that acts as if the appropriate memory barriers were in place is a
+correct implementation.
+That said, it is much easier to fool yourself into believing that you have
+adhered to the as-if rule than it is to actually adhere to it!
+
+
+</p><p><a href="#Quick%20Quiz%206"><b>Back to Quick Quiz 6</b>.</a>
+
+<a name="qq7answer"></a>
+<p><b>Quick Quiz 7</b>:
+But how does the upgrade-to-write operation exclude other readers?
+
+
+</p><p><b>Answer</b>:
+It doesn't, just like normal RCU updates, which also do not exclude
+RCU readers.
+
+
+</p><p><a href="#Quick%20Quiz%207"><b>Back to Quick Quiz 7</b>.</a>
+
+<a name="qq8answer"></a>
+<p><b>Quick Quiz 8</b>:
+Can't the compiler also reorder this code?
+
+
+</p><p><b>Answer</b>:
+No, the volatile casts in <tt>READ_ONCE()</tt> and
+<tt>WRITE_ONCE()</tt> prevent the compiler from reordering in
+this particular case.
+
+
+</p><p><a href="#Quick%20Quiz%208"><b>Back to Quick Quiz 8</b>.</a>
+
+<a name="qq9answer"></a>
+<p><b>Quick Quiz 9</b>:
+Suppose that synchronize_rcu() did wait until all readers had completed.
+Would the updater be able to rely on this?
+
+
+</p><p><b>Answer</b>:
+No.
+Even if <tt>synchronize_rcu()</tt> were to wait until
+all readers had completed, a new reader might start immediately after
+<tt>synchronize_rcu()</tt> completed.
+Therefore, the code following
+<tt>synchronize_rcu()</tt> cannot rely on there being no readers
+in any case.
+
+
+</p><p><a href="#Quick%20Quiz%209"><b>Back to Quick Quiz 9</b>.</a>
+
+<a name="qq10answer"></a>
+<p><b>Quick Quiz 10</b>:
+How long a sequence of grace periods, each separated by an RCU read-side
+critical section, would be required to partition the RCU read-side
+critical sections at the beginning and end of the chain?
+
+
+</p><p><b>Answer</b>:
+In theory, an infinite number.
+In practice, an unknown number that is sensitive to both implementation
+details and timing considerations.
+Therefore, even in practice, RCU users must abide by the theoretical rather
+than the practical answer.
+
+
+</p><p><a href="#Quick%20Quiz%2010"><b>Back to Quick Quiz 10</b>.</a>
+
+<a name="qq11answer"></a>
+<p><b>Quick Quiz 11</b>:
+What about sleeping locks?
+
+
+</p><p><b>Answer</b>:
+These are forbidden within Linux-kernel RCU read-side critical sections
+because it is not legal to place a quiescent state (in this case,
+voluntary context switch) within an RCU read-side critical section.
+However, sleeping locks may be used within userspace RCU read-side critical
+sections, and also within Linux-kernel sleepable RCU
+<a href="#Sleepable RCU">(SRCU)</a>
+read-side critical sections.
+In addition, the -rt patchset turns spinlocks into a sleeping locks so
+that the corresponding critical sections can be preempted, which
+also means that these sleeplockified spinlocks (but not other sleeping locks!)
+may be acquire within -rt-Linux-kernel RCU read-side critical sections.
+
+<p>
+Note that it <i>is</i> legal for a normal RCU read-side critical section
+to conditionally acquire a sleeping locks (as in <tt>mutex_trylock()</tt>),
+but only as long as it does not loop indefinitely attempting to
+conditionally acquire that sleeping locks.
+The key point is that things like <tt>mutex_trylock()</tt>
+either return with the mutex held, or return an error indication if
+the mutex was not immediately available.
+Either way, <tt>mutex_trylock()</tt> returns immediately without sleeping.
+
+
+</p><p><a href="#Quick%20Quiz%2011"><b>Back to Quick Quiz 11</b>.</a>
+
+<a name="qq12answer"></a>
+<p><b>Quick Quiz 12</b>:
+Why does line&nbsp;19 use <tt>rcu_access_pointer()</tt>?
+After all, <tt>call_rcu()</tt> on line&nbsp;25 stores into the
+structure, which would interact badly with concurrent insertions.
+Doesn't this mean that <tt>rcu_dereference()</tt> is required?
+
+
+</p><p><b>Answer</b>:
+Presumably the <tt>-&gt;gp_lock</tt> acquired on line&nbsp;18 excludes
+any changes, including any insertions that <tt>rcu_dereference()</tt>
+would protect against.
+Therefore, any insertions will be delayed until after <tt>-&gt;gp_lock</tt>
+is released on line&nbsp;25, which in turn means that
+<tt>rcu_access_pointer()</tt> suffices.
+
+
+</p><p><a href="#Quick%20Quiz%2012"><b>Back to Quick Quiz 12</b>.</a>
+
+<a name="qq13answer"></a>
+<p><b>Quick Quiz 13</b>:
+Earlier it was claimed that <tt>call_rcu()</tt> and
+<tt>kfree_rcu()</tt> allowed updaters to avoid being blocked
+by readers.
+But how can that be correct, given that the invocation of the callback
+and the freeing of the memory (respectively) must still wait for
+a grace period to elapse?
+
+
+</p><p><b>Answer</b>:
+We could define things this way, but keep in mind that this sort of
+definition would say that updates in garbage-collected languages
+cannot complete until the next time the garbage collector runs,
+which does not seem at all reasonable.
+The key point is that in most cases, an updater using either
+<tt>call_rcu()</tt> or <tt>kfree_rcu()</tt> can proceed to the
+next update as soon as it has invoked <tt>call_rcu()</tt> or
+<tt>kfree_rcu()</tt>, without having to wait for a subsequent
+grace period.
+
+
+</p><p><a href="#Quick%20Quiz%2013"><b>Back to Quick Quiz 13</b>.</a>
+
+<a name="qq14answer"></a>
+<p><b>Quick Quiz 14</b>:
+So what happens with <tt>synchronize_rcu()</tt> during
+scheduler initialization for <tt>CONFIG_PREEMPT=n</tt>
+kernels?
+
+
+</p><p><b>Answer</b>:
+In <tt>CONFIG_PREEMPT=n</tt> kernel, <tt>synchronize_rcu()</tt>
+maps directly to <tt>synchronize_sched()</tt>.
+Therefore, <tt>synchronize_rcu()</tt> works normally throughout
+boot in <tt>CONFIG_PREEMPT=n</tt> kernels.
+However, your code must also work in <tt>CONFIG_PREEMPT=y</tt> kernels,
+so it is still necessary to avoid invoking <tt>synchronize_rcu()</tt>
+during scheduler initialization.
+
+
+</p><p><a href="#Quick%20Quiz%2014"><b>Back to Quick Quiz 14</b>.</a>
+
+
+</body></html>
diff --git a/Documentation/RCU/Design/Requirements/Requirements.htmlx b/Documentation/RCU/Design/Requirements/Requirements.htmlx
new file mode 100644
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@@ -0,0 +1,2741 @@
+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
+        "http://www.w3.org/TR/html4/loose.dtd">
+        <html>
+        <head><title>A Tour Through RCU's Requirements [LWN.net]</title>
+        <meta HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=utf-8">
+
+<h1>A Tour Through RCU's Requirements</h1>
+
+<p>Copyright IBM Corporation, 2015</p>
+<p>Author: Paul E.&nbsp;McKenney</p>
+<p><i>The initial version of this document appeared in the
+<a href="https://lwn.net/">LWN</a> articles
+<a href="https://lwn.net/Articles/652156/">here</a>,
+<a href="https://lwn.net/Articles/652677/">here</a>, and
+<a href="https://lwn.net/Articles/653326/">here</a>.</i></p>
+
+<h2>Introduction</h2>
+
+<p>
+Read-copy update (RCU) is a synchronization mechanism that is often
+used as a replacement for reader-writer locking.
+RCU is unusual in that updaters do not block readers,
+which means that RCU's read-side primitives can be exceedingly fast
+and scalable.
+In addition, updaters can make useful forward progress concurrently
+with readers.
+However, all this concurrency between RCU readers and updaters does raise
+the question of exactly what RCU readers are doing, which in turn
+raises the question of exactly what RCU's requirements are.
+
+<p>
+This document therefore summarizes RCU's requirements, and can be thought
+of as an informal, high-level specification for RCU.
+It is important to understand that RCU's specification is primarily
+empirical in nature;
+in fact, I learned about many of these requirements the hard way.
+This situation might cause some consternation, however, not only
+has this learning process been a lot of fun, but it has also been
+a great privilege to work with so many people willing to apply
+technologies in interesting new ways.
+
+<p>
+All that aside, here are the categories of currently known RCU requirements:
+</p>
+
+<ol>
+<li>	<a href="#Fundamental Requirements">
+	Fundamental Requirements</a>
+<li>	<a href="#Fundamental Non-Requirements">Fundamental Non-Requirements</a>
+<li>	<a href="#Parallelism Facts of Life">
+	Parallelism Facts of Life</a>
+<li>	<a href="#Quality-of-Implementation Requirements">
+	Quality-of-Implementation Requirements</a>
+<li>	<a href="#Linux Kernel Complications">
+	Linux Kernel Complications</a>
+<li>	<a href="#Software-Engineering Requirements">
+	Software-Engineering Requirements</a>
+<li>	<a href="#Other RCU Flavors">
+	Other RCU Flavors</a>
+<li>	<a href="#Possible Future Changes">
+	Possible Future Changes</a>
+</ol>
+
+<p>
+This is followed by a <a href="#Summary">summary</a>,
+which is in turn followed by the inevitable
+<a href="#Answers to Quick Quizzes">answers to the quick quizzes</a>.
+
+<h2><a name="Fundamental Requirements">Fundamental Requirements</a></h2>
+
+<p>
+RCU's fundamental requirements are the closest thing RCU has to hard
+mathematical requirements.
+These are:
+
+<ol>
+<li>	<a href="#Grace-Period Guarantee">
+	Grace-Period Guarantee</a>
+<li>	<a href="#Publish-Subscribe Guarantee">
+	Publish-Subscribe Guarantee</a>
+<li>	<a href="#Memory-Barrier Guarantees">
+	Memory-Barrier Guarantees</a>
+<li>	<a href="#RCU Primitives Guaranteed to Execute Unconditionally">
+	RCU Primitives Guaranteed to Execute Unconditionally</a>
+<li>	<a href="#Guaranteed Read-to-Write Upgrade">
+	Guaranteed Read-to-Write Upgrade</a>
+</ol>
+
+<h3><a name="Grace-Period Guarantee">Grace-Period Guarantee</a></h3>
+
+<p>
+RCU's grace-period guarantee is unusual in being premeditated:
+Jack Slingwine and I had this guarantee firmly in mind when we started
+work on RCU (then called &ldquo;rclock&rdquo;) in the early 1990s.
+That said, the past two decades of experience with RCU have produced
+a much more detailed understanding of this guarantee.
+
+<p>
+RCU's grace-period guarantee allows updaters to wait for the completion
+of all pre-existing RCU read-side critical sections.
+An RCU read-side critical section
+begins with the marker <tt>rcu_read_lock()</tt> and ends with
+the marker <tt>rcu_read_unlock()</tt>.
+These markers may be nested, and RCU treats a nested set as one
+big RCU read-side critical section.
+Production-quality implementations of <tt>rcu_read_lock()</tt> and
+<tt>rcu_read_unlock()</tt> are extremely lightweight, and in
+fact have exactly zero overhead in Linux kernels built for production
+use with <tt>CONFIG_PREEMPT=n</tt>.
+
+<p>
+This guarantee allows ordering to be enforced with extremely low
+overhead to readers, for example:
+
+<blockquote>
+<pre>
+ 1 int x, y;
+ 2
+ 3 void thread0(void)
+ 4 {
+ 5   rcu_read_lock();
+ 6   r1 = READ_ONCE(x);
+ 7   r2 = READ_ONCE(y);
+ 8   rcu_read_unlock();
+ 9 }
+10
+11 void thread1(void)
+12 {
+13   WRITE_ONCE(x, 1);
+14   synchronize_rcu();
+15   WRITE_ONCE(y, 1);
+16 }
+</pre>
+</blockquote>
+
+<p>
+Because the <tt>synchronize_rcu()</tt> on line&nbsp;14 waits for
+all pre-existing readers, any instance of <tt>thread0()</tt> that
+loads a value of zero from <tt>x</tt> must complete before
+<tt>thread1()</tt> stores to <tt>y</tt>, so that instance must
+also load a value of zero from <tt>y</tt>.
+Similarly, any instance of <tt>thread0()</tt> that loads a value of
+one from <tt>y</tt> must have started after the
+<tt>synchronize_rcu()</tt> started, and must therefore also load
+a value of one from <tt>x</tt>.
+Therefore, the outcome:
+<blockquote>
+<pre>
+(r1 == 0 &amp;&amp; r2 == 1)
+</pre>
+</blockquote>
+cannot happen.
+
+<p>@@QQ@@
+Wait a minute!
+You said that updaters can make useful forward progress concurrently
+with readers, but pre-existing readers will block
+<tt>synchronize_rcu()</tt>!!!
+Just who are you trying to fool???
+<p>@@QQA@@
+First, if updaters do not wish to be blocked by readers, they can use
+<tt>call_rcu()</tt> or <tt>kfree_rcu()</tt>, which will
+be discussed later.
+Second, even when using <tt>synchronize_rcu()</tt>, the other
+update-side code does run concurrently with readers, whether pre-existing
+or not.
+<p>@@QQE@@
+
+<p>
+This scenario resembles one of the first uses of RCU in
+<a href="https://en.wikipedia.org/wiki/DYNIX">DYNIX/ptx</a>,
+which managed a distributed lock manager's transition into
+a state suitable for handling recovery from node failure,
+more or less as follows:
+
+<blockquote>
+<pre>
+ 1 #define STATE_NORMAL        0
+ 2 #define STATE_WANT_RECOVERY 1
+ 3 #define STATE_RECOVERING    2
+ 4 #define STATE_WANT_NORMAL   3
+ 5
+ 6 int state = STATE_NORMAL;
+ 7
+ 8 void do_something_dlm(void)
+ 9 {
+10   int state_snap;
+11
+12   rcu_read_lock();
+13   state_snap = READ_ONCE(state);
+14   if (state_snap == STATE_NORMAL)
+15     do_something();
+16   else
+17     do_something_carefully();
+18   rcu_read_unlock();
+19 }
+20
+21 void start_recovery(void)
+22 {
+23   WRITE_ONCE(state, STATE_WANT_RECOVERY);
+24   synchronize_rcu();
+25   WRITE_ONCE(state, STATE_RECOVERING);
+26   recovery();
+27   WRITE_ONCE(state, STATE_WANT_NORMAL);
+28   synchronize_rcu();
+29   WRITE_ONCE(state, STATE_NORMAL);
+30 }
+</pre>
+</blockquote>
+
+<p>
+The RCU read-side critical section in <tt>do_something_dlm()</tt>
+works with the <tt>synchronize_rcu()</tt> in <tt>start_recovery()</tt>
+to guarantee that <tt>do_something()</tt> never runs concurrently
+with <tt>recovery()</tt>, but with little or no synchronization
+overhead in <tt>do_something_dlm()</tt>.
+
+<p>@@QQ@@
+Why is the <tt>synchronize_rcu()</tt> on line&nbsp;28 needed?
+<p>@@QQA@@
+Without that extra grace period, memory reordering could result in
+<tt>do_something_dlm()</tt> executing <tt>do_something()</tt>
+concurrently with the last bits of <tt>recovery()</tt>.
+<p>@@QQE@@
+
+<p>
+In order to avoid fatal problems such as deadlocks,
+an RCU read-side critical section must not contain calls to
+<tt>synchronize_rcu()</tt>.
+Similarly, an RCU read-side critical section must not
+contain anything that waits, directly or indirectly, on completion of
+an invocation of <tt>synchronize_rcu()</tt>.
+
+<p>
+Although RCU's grace-period guarantee is useful in and of itself, with
+<a href="https://lwn.net/Articles/573497/">quite a few use cases</a>,
+it would be good to be able to use RCU to coordinate read-side
+access to linked data structures.
+For this, the grace-period guarantee is not sufficient, as can
+be seen in function <tt>add_gp_buggy()</tt> below.
+We will look at the reader's code later, but in the meantime, just think of
+the reader as locklessly picking up the <tt>gp</tt> pointer,
+and, if the value loaded is non-<tt>NULL</tt>, locklessly accessing the
+<tt>-&gt;a</tt> and <tt>-&gt;b</tt> fields.
+
+<blockquote>
+<pre>
+ 1 bool add_gp_buggy(int a, int b)
+ 2 {
+ 3   p = kmalloc(sizeof(*p), GFP_KERNEL);
+ 4   if (!p)
+ 5     return -ENOMEM;
+ 6   spin_lock(&amp;gp_lock);
+ 7   if (rcu_access_pointer(gp)) {
+ 8     spin_unlock(&amp;gp_lock);
+ 9     return false;
+10   }
+11   p-&gt;a = a;
+12   p-&gt;b = a;
+13   gp = p; /* ORDERING BUG */
+14   spin_unlock(&amp;gp_lock);
+15   return true;
+16 }
+</pre>
+</blockquote>
+
+<p>
+The problem is that both the compiler and weakly ordered CPUs are within
+their rights to reorder this code as follows:
+
+<blockquote>
+<pre>
+ 1 bool add_gp_buggy_optimized(int a, int b)
+ 2 {
+ 3   p = kmalloc(sizeof(*p), GFP_KERNEL);
+ 4   if (!p)
+ 5     return -ENOMEM;
+ 6   spin_lock(&amp;gp_lock);
+ 7   if (rcu_access_pointer(gp)) {
+ 8     spin_unlock(&amp;gp_lock);
+ 9     return false;
+10   }
+<b>11   gp = p; /* ORDERING BUG */
+12   p-&gt;a = a;
+13   p-&gt;b = a;</b>
+14   spin_unlock(&amp;gp_lock);
+15   return true;
+16 }
+</pre>
+</blockquote>
+
+<p>
+If an RCU reader fetches <tt>gp</tt> just after
+<tt>add_gp_buggy_optimized</tt> executes line&nbsp;11,
+it will see garbage in the <tt>-&gt;a</tt> and <tt>-&gt;b</tt>
+fields.
+And this is but one of many ways in which compiler and hardware optimizations
+could cause trouble.
+Therefore, we clearly need some way to prevent the compiler and the CPU from
+reordering in this manner, which brings us to the publish-subscribe
+guarantee discussed in the next section.
+
+<h3><a name="Publish-Subscribe Guarantee">Publish/Subscribe Guarantee</a></h3>
+
+<p>
+RCU's publish-subscribe guarantee allows data to be inserted
+into a linked data structure without disrupting RCU readers.
+The updater uses <tt>rcu_assign_pointer()</tt> to insert the
+new data, and readers use <tt>rcu_dereference()</tt> to
+access data, whether new or old.
+The following shows an example of insertion:
+
+<blockquote>
+<pre>
+ 1 bool add_gp(int a, int b)
+ 2 {
+ 3   p = kmalloc(sizeof(*p), GFP_KERNEL);
+ 4   if (!p)
+ 5     return -ENOMEM;
+ 6   spin_lock(&amp;gp_lock);
+ 7   if (rcu_access_pointer(gp)) {
+ 8     spin_unlock(&amp;gp_lock);
+ 9     return false;
+10   }
+11   p-&gt;a = a;
+12   p-&gt;b = a;
+13   rcu_assign_pointer(gp, p);
+14   spin_unlock(&amp;gp_lock);
+15   return true;
+16 }
+</pre>
+</blockquote>
+
+<p>
+The <tt>rcu_assign_pointer()</tt> on line&nbsp;13 is conceptually
+equivalent to a simple assignment statement, but also guarantees
+that its assignment will
+happen after the two assignments in lines&nbsp;11 and&nbsp;12,
+similar to the C11 <tt>memory_order_release</tt> store operation.
+It also prevents any number of &ldquo;interesting&rdquo; compiler
+optimizations, for example, the use of <tt>gp</tt> as a scratch
+location immediately preceding the assignment.
+
+<p>@@QQ@@
+But <tt>rcu_assign_pointer()</tt> does nothing to prevent the
+two assignments to <tt>p-&gt;a</tt> and <tt>p-&gt;b</tt>
+from being reordered.
+Can't that also cause problems?
+<p>@@QQA@@
+No, it cannot.
+The readers cannot see either of these two fields until
+the assignment to <tt>gp</tt>, by which time both fields are
+fully initialized.
+So reordering the assignments
+to <tt>p-&gt;a</tt> and <tt>p-&gt;b</tt> cannot possibly
+cause any problems.
+<p>@@QQE@@
+
+<p>
+It is tempting to assume that the reader need not do anything special
+to control its accesses to the RCU-protected data,
+as shown in <tt>do_something_gp_buggy()</tt> below:
+
+<blockquote>
+<pre>
+ 1 bool do_something_gp_buggy(void)
+ 2 {
+ 3   rcu_read_lock();
+ 4   p = gp;  /* OPTIMIZATIONS GALORE!!! */
+ 5   if (p) {
+ 6     do_something(p-&gt;a, p-&gt;b);
+ 7     rcu_read_unlock();
+ 8     return true;
+ 9   }
+10   rcu_read_unlock();
+11   return false;
+12 }
+</pre>
+</blockquote>
+
+<p>
+However, this temptation must be resisted because there are a
+surprisingly large number of ways that the compiler
+(to say nothing of
+<a href="https://h71000.www7.hp.com/wizard/wiz_2637.html">DEC Alpha CPUs</a>)
+can trip this code up.
+For but one example, if the compiler were short of registers, it
+might choose to refetch from <tt>gp</tt> rather than keeping
+a separate copy in <tt>p</tt> as follows:
+
+<blockquote>
+<pre>
+ 1 bool do_something_gp_buggy_optimized(void)
+ 2 {
+ 3   rcu_read_lock();
+ 4   if (gp) { /* OPTIMIZATIONS GALORE!!! */
+<b> 5     do_something(gp-&gt;a, gp-&gt;b);</b>
+ 6     rcu_read_unlock();
+ 7     return true;
+ 8   }
+ 9   rcu_read_unlock();
+10   return false;
+11 }
+</pre>
+</blockquote>
+
+<p>
+If this function ran concurrently with a series of updates that
+replaced the current structure with a new one,
+the fetches of <tt>gp-&gt;a</tt>
+and <tt>gp-&gt;b</tt> might well come from two different structures,
+which could cause serious confusion.
+To prevent this (and much else besides), <tt>do_something_gp()</tt> uses
+<tt>rcu_dereference()</tt> to fetch from <tt>gp</tt>:
+
+<blockquote>
+<pre>
+ 1 bool do_something_gp(void)
+ 2 {
+ 3   rcu_read_lock();
+ 4   p = rcu_dereference(gp);
+ 5   if (p) {
+ 6     do_something(p-&gt;a, p-&gt;b);
+ 7     rcu_read_unlock();
+ 8     return true;
+ 9   }
+10   rcu_read_unlock();
+11   return false;
+12 }
+</pre>
+</blockquote>
+
+<p>
+The <tt>rcu_dereference()</tt> uses volatile casts and (for DEC Alpha)
+memory barriers in the Linux kernel.
+Should a
+<a href="http://www.rdrop.com/users/paulmck/RCU/consume.2015.07.13a.pdf">high-quality implementation of C11 <tt>memory_order_consume</tt> [PDF]</a>
+ever appear, then <tt>rcu_dereference()</tt> could be implemented
+as a <tt>memory_order_consume</tt> load.
+Regardless of the exact implementation, a pointer fetched by
+<tt>rcu_dereference()</tt> may not be used outside of the
+outermost RCU read-side critical section containing that
+<tt>rcu_dereference()</tt>, unless protection of
+the corresponding data element has been passed from RCU to some
+other synchronization mechanism, most commonly locking or
+<a href="https://www.kernel.org/doc/Documentation/RCU/rcuref.txt">reference counting</a>.
+
+<p>
+In short, updaters use <tt>rcu_assign_pointer()</tt> and readers
+use <tt>rcu_dereference()</tt>, and these two RCU API elements
+work together to ensure that readers have a consistent view of
+newly added data elements.
+
+<p>
+Of course, it is also necessary to remove elements from RCU-protected
+data structures, for example, using the following process:
+
+<ol>
+<li>	Remove the data element from the enclosing structure.
+<li>	Wait for all pre-existing RCU read-side critical sections
+	to complete (because only pre-existing readers can possibly have
+	a reference to the newly removed data element).
+<li>	At this point, only the updater has a reference to the
+	newly removed data element, so it can safely reclaim
+	the data element, for example, by passing it to <tt>kfree()</tt>.
+</ol>
+
+This process is implemented by <tt>remove_gp_synchronous()</tt>:
+
+<blockquote>
+<pre>
+ 1 bool remove_gp_synchronous(void)
+ 2 {
+ 3   struct foo *p;
+ 4
+ 5   spin_lock(&amp;gp_lock);
+ 6   p = rcu_access_pointer(gp);
+ 7   if (!p) {
+ 8     spin_unlock(&amp;gp_lock);
+ 9     return false;
+10   }
+11   rcu_assign_pointer(gp, NULL);
+12   spin_unlock(&amp;gp_lock);
+13   synchronize_rcu();
+14   kfree(p);
+15   return true;
+16 }
+</pre>
+</blockquote>
+
+<p>
+This function is straightforward, with line&nbsp;13 waiting for a grace
+period before line&nbsp;14 frees the old data element.
+This waiting ensures that readers will reach line&nbsp;7 of
+<tt>do_something_gp()</tt> before the data element referenced by
+<tt>p</tt> is freed.
+The <tt>rcu_access_pointer()</tt> on line&nbsp;6 is similar to
+<tt>rcu_dereference()</tt>, except that:
+
+<ol>
+<li>	The value returned by <tt>rcu_access_pointer()</tt>
+	cannot be dereferenced.
+	If you want to access the value pointed to as well as
+	the pointer itself, use <tt>rcu_dereference()</tt>
+	instead of <tt>rcu_access_pointer()</tt>.
+<li>	The call to <tt>rcu_access_pointer()</tt> need not be
+	protected.
+	In contrast, <tt>rcu_dereference()</tt> must either be
+	within an RCU read-side critical section or in a code
+	segment where the pointer cannot change, for example, in
+	code protected by the corresponding update-side lock.
+</ol>
+
+<p>@@QQ@@
+Without the <tt>rcu_dereference()</tt> or the
+<tt>rcu_access_pointer()</tt>, what destructive optimizations
+might the compiler make use of?
+<p>@@QQA@@
+Let's start with what happens to <tt>do_something_gp()</tt>
+if it fails to use <tt>rcu_dereference()</tt>.
+It could reuse a value formerly fetched from this same pointer.
+It could also fetch the pointer from <tt>gp</tt> in a byte-at-a-time
+manner, resulting in <i>load tearing</i>, in turn resulting a bytewise
+mash-up of two distince pointer values.
+It might even use value-speculation optimizations, where it makes a wrong
+guess, but by the time it gets around to checking the value, an update
+has changed the pointer to match the wrong guess.
+Too bad about any dereferences that returned pre-initialization garbage
+in the meantime!
+
+<p>
+For <tt>remove_gp_synchronous()</tt>, as long as all modifications
+to <tt>gp</tt> are carried out while holding <tt>gp_lock</tt>,
+the above optimizations are harmless.
+However,
+with <tt>CONFIG_SPARSE_RCU_POINTER=y</tt>,
+<tt>sparse</tt> will complain if you
+define <tt>gp</tt> with <tt>__rcu</tt> and then
+access it without using
+either <tt>rcu_access_pointer()</tt> or <tt>rcu_dereference()</tt>.
+<p>@@QQE@@
+
+<p>
+In short, RCU's publish-subscribe guarantee is provided by the combination
+of <tt>rcu_assign_pointer()</tt> and <tt>rcu_dereference()</tt>.
+This guarantee allows data elements to be safely added to RCU-protected
+linked data structures without disrupting RCU readers.
+This guarantee can be used in combination with the grace-period
+guarantee to also allow data elements to be removed from RCU-protected
+linked data structures, again without disrupting RCU readers.
+
+<p>
+This guarantee was only partially premeditated.
+DYNIX/ptx used an explicit memory barrier for publication, but had nothing
+resembling <tt>rcu_dereference()</tt> for subscription, nor did it
+have anything resembling the <tt>smp_read_barrier_depends()</tt>
+that was later subsumed into <tt>rcu_dereference()</tt>.
+The need for these operations made itself known quite suddenly at a
+late-1990s meeting with the DEC Alpha architects, back in the days when
+DEC was still a free-standing company.
+It took the Alpha architects a good hour to convince me that any sort
+of barrier would ever be needed, and it then took me a good <i>two</i> hours
+to convince them that their documentation did not make this point clear.
+More recent work with the C and C++ standards committees have provided
+much education on tricks and traps from the compiler.
+In short, compilers were much less tricky in the early 1990s, but in
+2015, don't even think about omitting <tt>rcu_dereference()</tt>!
+
+<h3><a name="Memory-Barrier Guarantees">Memory-Barrier Guarantees</a></h3>
+
+<p>
+The previous section's simple linked-data-structure scenario clearly
+demonstrates the need for RCU's stringent memory-ordering guarantees on
+systems with more than one CPU:
+
+<ol>
+<li>	Each CPU that has an RCU read-side critical section that
+	begins before <tt>synchronize_rcu()</tt> starts is
+	guaranteed to execute a full memory barrier between the time
+	that the RCU read-side critical section ends and the time that
+	<tt>synchronize_rcu()</tt> returns.
+	Without this guarantee, a pre-existing RCU read-side critical section
+	might hold a reference to the newly removed <tt>struct foo</tt>
+	after the <tt>kfree()</tt> on line&nbsp;14 of
+	<tt>remove_gp_synchronous()</tt>.
+<li>	Each CPU that has an RCU read-side critical section that ends
+	after <tt>synchronize_rcu()</tt> returns is guaranteed
+	to execute a full memory barrier between the time that
+	<tt>synchronize_rcu()</tt> begins and the time that the RCU
+	read-side critical section begins.
+	Without this guarantee, a later RCU read-side critical section
+	running after the <tt>kfree()</tt> on line&nbsp;14 of
+	<tt>remove_gp_synchronous()</tt> might
+	later run <tt>do_something_gp()</tt> and find the
+	newly deleted <tt>struct foo</tt>.
+<li>	If the task invoking <tt>synchronize_rcu()</tt> remains
+	on a given CPU, then that CPU is guaranteed to execute a full
+	memory barrier sometime during the execution of
+	<tt>synchronize_rcu()</tt>.
+	This guarantee ensures that the <tt>kfree()</tt> on
+	line&nbsp;14 of <tt>remove_gp_synchronous()</tt> really does
+	execute after the removal on line&nbsp;11.
+<li>	If the task invoking <tt>synchronize_rcu()</tt> migrates
+	among a group of CPUs during that invocation, then each of the
+	CPUs in that group is guaranteed to execute a full memory barrier
+	sometime during the execution of <tt>synchronize_rcu()</tt>.
+	This guarantee also ensures that the <tt>kfree()</tt> on
+	line&nbsp;14 of <tt>remove_gp_synchronous()</tt> really does
+	execute after the removal on
+	line&nbsp;11, but also in the case where the thread executing the
+	<tt>synchronize_rcu()</tt> migrates in the meantime.
+</ol>
+
+<p>@@QQ@@
+Given that multiple CPUs can start RCU read-side critical sections
+at any time without any ordering whatsoever, how can RCU possibly tell whether
+or not a given RCU read-side critical section starts before a
+given instance of <tt>synchronize_rcu()</tt>?
+<p>@@QQA@@
+If RCU cannot tell whether or not a given
+RCU read-side critical section starts before a
+given instance of <tt>synchronize_rcu()</tt>,
+then it must assume that the RCU read-side critical section
+started first.
+In other words, a given instance of <tt>synchronize_rcu()</tt>
+can avoid waiting on a given RCU read-side critical section only
+if it can prove that <tt>synchronize_rcu()</tt> started first.
+<p>@@QQE@@
+
+<p>@@QQ@@
+The first and second guarantees require unbelievably strict ordering!
+Are all these memory barriers <i> really</i> required?
+<p>@@QQA@@
+Yes, they really are required.
+To see why the first guarantee is required, consider the following
+sequence of events:
+
+<ol>
+<li>	CPU 1: <tt>rcu_read_lock()</tt>
+<li>	CPU 1: <tt>q = rcu_dereference(gp);
+	/* Very likely to return p. */</tt>
+<li>	CPU 0: <tt>list_del_rcu(p);</tt>
+<li>	CPU 0: <tt>synchronize_rcu()</tt> starts.
+<li>	CPU 1: <tt>do_something_with(q-&gt;a);
+	/* No smp_mb(), so might happen after kfree(). */</tt>
+<li>	CPU 1: <tt>rcu_read_unlock()</tt>
+<li>	CPU 0: <tt>synchronize_rcu()</tt> returns.
+<li>	CPU 0: <tt>kfree(p);</tt>
+</ol>
+
+<p>
+Therefore, there absolutely must be a full memory barrier between the
+end of the RCU read-side critical section and the end of the
+grace period.
+
+<p>
+The sequence of events demonstrating the necessity of the second rule
+is roughly similar:
+
+<ol>
+<li>	CPU 0: <tt>list_del_rcu(p);</tt>
+<li>	CPU 0: <tt>synchronize_rcu()</tt> starts.
+<li>	CPU 1: <tt>rcu_read_lock()</tt>
+<li>	CPU 1: <tt>q = rcu_dereference(gp);
+	/* Might return p if no memory barrier. */</tt>
+<li>	CPU 0: <tt>synchronize_rcu()</tt> returns.
+<li>	CPU 0: <tt>kfree(p);</tt>
+<li>	CPU 1: <tt>do_something_with(q-&gt;a); /* Boom!!! */</tt>
+<li>	CPU 1: <tt>rcu_read_unlock()</tt>
+</ol>
+
+<p>
+And similarly, without a memory barrier between the beginning of the
+grace period and the beginning of the RCU read-side critical section,
+CPU&nbsp;1 might end up accessing the freelist.
+
+<p>
+The &ldquo;as if&rdquo; rule of course applies, so that any implementation
+that acts as if the appropriate memory barriers were in place is a
+correct implementation.
+That said, it is much easier to fool yourself into believing that you have
+adhered to the as-if rule than it is to actually adhere to it!
+<p>@@QQE@@
+
+<p>
+Note that these memory-barrier requirements do not replace the fundamental
+RCU requirement that a grace period wait for all pre-existing readers.
+On the contrary, the memory barriers called out in this section must operate in
+such a way as to <i>enforce</i> this fundamental requirement.
+Of course, different implementations enforce this requirement in different
+ways, but enforce it they must.
+
+<h3><a name="RCU Primitives Guaranteed to Execute Unconditionally">RCU Primitives Guaranteed to Execute Unconditionally</a></h3>
+
+<p>
+The common-case RCU primitives are unconditional.
+They are invoked, they do their job, and they return, with no possibility
+of error, and no need to retry.
+This is a key RCU design philosophy.
+
+<p>
+However, this philosophy is pragmatic rather than pigheaded.
+If someone comes up with a good justification for a particular conditional
+RCU primitive, it might well be implemented and added.
+After all, this guarantee was reverse-engineered, not premeditated.
+The unconditional nature of the RCU primitives was initially an
+accident of implementation, and later experience with synchronization
+primitives with conditional primitives caused me to elevate this
+accident to a guarantee.
+Therefore, the justification for adding a conditional primitive to
+RCU would need to be based on detailed and compelling use cases.
+
+<h3><a name="Guaranteed Read-to-Write Upgrade">Guaranteed Read-to-Write Upgrade</a></h3>
+
+<p>
+As far as RCU is concerned, it is always possible to carry out an
+update within an RCU read-side critical section.
+For example, that RCU read-side critical section might search for
+a given data element, and then might acquire the update-side
+spinlock in order to update that element, all while remaining
+in that RCU read-side critical section.
+Of course, it is necessary to exit the RCU read-side critical section
+before invoking <tt>synchronize_rcu()</tt>, however, this
+inconvenience can be avoided through use of the
+<tt>call_rcu()</tt> and <tt>kfree_rcu()</tt> API members
+described later in this document.
+
+<p>@@QQ@@
+But how does the upgrade-to-write operation exclude other readers?
+<p>@@QQA@@
+It doesn't, just like normal RCU updates, which also do not exclude
+RCU readers.
+<p>@@QQE@@
+
+<p>
+This guarantee allows lookup code to be shared between read-side
+and update-side code, and was premeditated, appearing in the earliest
+DYNIX/ptx RCU documentation.
+
+<h2><a name="Fundamental Non-Requirements">Fundamental Non-Requirements</a></h2>
+
+<p>
+RCU provides extremely lightweight readers, and its read-side guarantees,
+though quite useful, are correspondingly lightweight.
+It is therefore all too easy to assume that RCU is guaranteeing more
+than it really is.
+Of course, the list of things that RCU does not guarantee is infinitely
+long, however, the following sections list a few non-guarantees that
+have caused confusion.
+Except where otherwise noted, these non-guarantees were premeditated.
+
+<ol>
+<li>	<a href="#Readers Impose Minimal Ordering">
+	Readers Impose Minimal Ordering</a>
+<li>	<a href="#Readers Do Not Exclude Updaters">
+	Readers Do Not Exclude Updaters</a>
+<li>	<a href="#Updaters Only Wait For Old Readers">
+	Updaters Only Wait For Old Readers</a>
+<li>	<a href="#Grace Periods Don't Partition Read-Side Critical Sections">
+	Grace Periods Don't Partition Read-Side Critical Sections</a>
+<li>	<a href="#Read-Side Critical Sections Don't Partition Grace Periods">
+	Read-Side Critical Sections Don't Partition Grace Periods</a>
+<li>	<a href="#Disabling Preemption Does Not Block Grace Periods">
+	Disabling Preemption Does Not Block Grace Periods</a>
+</ol>
+
+<h3><a name="Readers Impose Minimal Ordering">Readers Impose Minimal Ordering</a></h3>
+
+<p>
+Reader-side markers such as <tt>rcu_read_lock()</tt> and
+<tt>rcu_read_unlock()</tt> provide absolutely no ordering guarantees
+except through their interaction with the grace-period APIs such as
+<tt>synchronize_rcu()</tt>.
+To see this, consider the following pair of threads:
+
+<blockquote>
+<pre>
+ 1 void thread0(void)
+ 2 {
+ 3   rcu_read_lock();
+ 4   WRITE_ONCE(x, 1);
+ 5   rcu_read_unlock();
+ 6   rcu_read_lock();
+ 7   WRITE_ONCE(y, 1);
+ 8   rcu_read_unlock();
+ 9 }
+10
+11 void thread1(void)
+12 {
+13   rcu_read_lock();
+14   r1 = READ_ONCE(y);
+15   rcu_read_unlock();
+16   rcu_read_lock();
+17   r2 = READ_ONCE(x);
+18   rcu_read_unlock();
+19 }
+</pre>
+</blockquote>
+
+<p>
+After <tt>thread0()</tt> and <tt>thread1()</tt> execute
+concurrently, it is quite possible to have
+
+<blockquote>
+<pre>
+(r1 == 1 &amp;&amp; r2 == 0)
+</pre>
+</blockquote>
+
+(that is, <tt>y</tt> appears to have been assigned before <tt>x</tt>),
+which would not be possible if <tt>rcu_read_lock()</tt> and
+<tt>rcu_read_unlock()</tt> had much in the way of ordering
+properties.
+But they do not, so the CPU is within its rights
+to do significant reordering.
+This is by design:  Any significant ordering constraints would slow down
+these fast-path APIs.
+
+<p>@@QQ@@
+Can't the compiler also reorder this code?
+<p>@@QQA@@
+No, the volatile casts in <tt>READ_ONCE()</tt> and
+<tt>WRITE_ONCE()</tt> prevent the compiler from reordering in
+this particular case.
+<p>@@QQE@@
+
+<h3><a name="Readers Do Not Exclude Updaters">Readers Do Not Exclude Updaters</a></h3>
+
+<p>
+Neither <tt>rcu_read_lock()</tt> nor <tt>rcu_read_unlock()</tt>
+exclude updates.
+All they do is to prevent grace periods from ending.
+The following example illustrates this:
+
+<blockquote>
+<pre>
+ 1 void thread0(void)
+ 2 {
+ 3   rcu_read_lock();
+ 4   r1 = READ_ONCE(y);
+ 5   if (r1) {
+ 6     do_something_with_nonzero_x();
+ 7     r2 = READ_ONCE(x);
+ 8     WARN_ON(!r2); /* BUG!!! */
+ 9   }
+10   rcu_read_unlock();
+11 }
+12
+13 void thread1(void)
+14 {
+15   spin_lock(&amp;my_lock);
+16   WRITE_ONCE(x, 1);
+17   WRITE_ONCE(y, 1);
+18   spin_unlock(&amp;my_lock);
+19 }
+</pre>
+</blockquote>
+
+<p>
+If the <tt>thread0()</tt> function's <tt>rcu_read_lock()</tt>
+excluded the <tt>thread1()</tt> function's update,
+the <tt>WARN_ON()</tt> could never fire.
+But the fact is that <tt>rcu_read_lock()</tt> does not exclude
+much of anything aside from subsequent grace periods, of which
+<tt>thread1()</tt> has none, so the
+<tt>WARN_ON()</tt> can and does fire.
+
+<h3><a name="Updaters Only Wait For Old Readers">Updaters Only Wait For Old Readers</a></h3>
+
+<p>
+It might be tempting to assume that after <tt>synchronize_rcu()</tt>
+completes, there are no readers executing.
+This temptation must be avoided because
+new readers can start immediately after <tt>synchronize_rcu()</tt>
+starts, and <tt>synchronize_rcu()</tt> is under no
+obligation to wait for these new readers.
+
+<p>@@QQ@@
+Suppose that synchronize_rcu() did wait until all readers had completed.
+Would the updater be able to rely on this?
+<p>@@QQA@@
+No.
+Even if <tt>synchronize_rcu()</tt> were to wait until
+all readers had completed, a new reader might start immediately after
+<tt>synchronize_rcu()</tt> completed.
+Therefore, the code following
+<tt>synchronize_rcu()</tt> cannot rely on there being no readers
+in any case.
+<p>@@QQE@@
+
+<h3><a name="Grace Periods Don't Partition Read-Side Critical Sections">
+Grace Periods Don't Partition Read-Side Critical Sections</a></h3>
+
+<p>
+It is tempting to assume that if any part of one RCU read-side critical
+section precedes a given grace period, and if any part of another RCU
+read-side critical section follows that same grace period, then all of
+the first RCU read-side critical section must precede all of the second.
+However, this just isn't the case: A single grace period does not
+partition the set of RCU read-side critical sections.
+An example of this situation can be illustrated as follows, where
+<tt>x</tt>, <tt>y</tt>, and <tt>z</tt> are initially all zero:
+
+<blockquote>
+<pre>
+ 1 void thread0(void)
+ 2 {
+ 3   rcu_read_lock();
+ 4   WRITE_ONCE(a, 1);
+ 5   WRITE_ONCE(b, 1);
+ 6   rcu_read_unlock();
+ 7 }
+ 8
+ 9 void thread1(void)
+10 {
+11   r1 = READ_ONCE(a);
+12   synchronize_rcu();
+13   WRITE_ONCE(c, 1);
+14 }
+15
+16 void thread2(void)
+17 {
+18   rcu_read_lock();
+19   r2 = READ_ONCE(b);
+20   r3 = READ_ONCE(c);
+21   rcu_read_unlock();
+22 }
+</pre>
+</blockquote>
+
+<p>
+It turns out that the outcome:
+
+<blockquote>
+<pre>
+(r1 == 1 &amp;&amp; r2 == 0 &amp;&amp; r3 == 1)
+</pre>
+</blockquote>
+
+is entirely possible.
+The following figure show how this can happen, with each circled
+<tt>QS</tt> indicating the point at which RCU recorded a
+<i>quiescent state</i> for each thread, that is, a state in which
+RCU knows that the thread cannot be in the midst of an RCU read-side
+critical section that started before the current grace period:
+
+<p><img src="GPpartitionReaders1.svg" alt="GPpartitionReaders1.svg" width="60%"></p>
+
+<p>
+If it is necessary to partition RCU read-side critical sections in this
+manner, it is necessary to use two grace periods, where the first
+grace period is known to end before the second grace period starts:
+
+<blockquote>
+<pre>
+ 1 void thread0(void)
+ 2 {
+ 3   rcu_read_lock();
+ 4   WRITE_ONCE(a, 1);
+ 5   WRITE_ONCE(b, 1);
+ 6   rcu_read_unlock();
+ 7 }
+ 8
+ 9 void thread1(void)
+10 {
+11   r1 = READ_ONCE(a);
+12   synchronize_rcu();
+13   WRITE_ONCE(c, 1);
+14 }
+15
+16 void thread2(void)
+17 {
+18   r2 = READ_ONCE(c);
+19   synchronize_rcu();
+20   WRITE_ONCE(d, 1);
+21 }
+22
+23 void thread3(void)
+24 {
+25   rcu_read_lock();
+26   r3 = READ_ONCE(b);
+27   r4 = READ_ONCE(d);
+28   rcu_read_unlock();
+29 }
+</pre>
+</blockquote>
+
+<p>
+Here, if <tt>(r1 == 1)</tt>, then
+<tt>thread0()</tt>'s write to <tt>b</tt> must happen
+before the end of <tt>thread1()</tt>'s grace period.
+If in addition <tt>(r4 == 1)</tt>, then
+<tt>thread3()</tt>'s read from <tt>b</tt> must happen
+after the beginning of <tt>thread2()</tt>'s grace period.
+If it is also the case that <tt>(r2 == 1)</tt>, then the
+end of <tt>thread1()</tt>'s grace period must precede the
+beginning of <tt>thread2()</tt>'s grace period.
+This mean that the two RCU read-side critical sections cannot overlap,
+guaranteeing that <tt>(r3 == 1)</tt>.
+As a result, the outcome:
+
+<blockquote>
+<pre>
+(r1 == 1 &amp;&amp; r2 == 1 &amp;&amp; r3 == 0 &amp;&amp; r4 == 1)
+</pre>
+</blockquote>
+
+cannot happen.
+
+<p>
+This non-requirement was also non-premeditated, but became apparent
+when studying RCU's interaction with memory ordering.
+
+<h3><a name="Read-Side Critical Sections Don't Partition Grace Periods">
+Read-Side Critical Sections Don't Partition Grace Periods</a></h3>
+
+<p>
+It is also tempting to assume that if an RCU read-side critical section
+happens between a pair of grace periods, then those grace periods cannot
+overlap.
+However, this temptation leads nowhere good, as can be illustrated by
+the following, with all variables initially zero:
+
+<blockquote>
+<pre>
+ 1 void thread0(void)
+ 2 {
+ 3   rcu_read_lock();
+ 4   WRITE_ONCE(a, 1);
+ 5   WRITE_ONCE(b, 1);
+ 6   rcu_read_unlock();
+ 7 }
+ 8
+ 9 void thread1(void)
+10 {
+11   r1 = READ_ONCE(a);
+12   synchronize_rcu();
+13   WRITE_ONCE(c, 1);
+14 }
+15
+16 void thread2(void)
+17 {
+18   rcu_read_lock();
+19   WRITE_ONCE(d, 1);
+20   r2 = READ_ONCE(c);
+21   rcu_read_unlock();
+22 }
+23
+24 void thread3(void)
+25 {
+26   r3 = READ_ONCE(d);
+27   synchronize_rcu();
+28   WRITE_ONCE(e, 1);
+29 }
+30
+31 void thread4(void)
+32 {
+33   rcu_read_lock();
+34   r4 = READ_ONCE(b);
+35   r5 = READ_ONCE(e);
+36   rcu_read_unlock();
+37 }
+</pre>
+</blockquote>
+
+<p>
+In this case, the outcome:
+
+<blockquote>
+<pre>
+(r1 == 1 &amp;&amp; r2 == 1 &amp;&amp; r3 == 1 &amp;&amp; r4 == 0 &amp&amp; r5 == 1)
+</pre>
+</blockquote>
+
+is entirely possible, as illustrated below:
+
+<p><img src="ReadersPartitionGP1.svg" alt="ReadersPartitionGP1.svg" width="100%"></p>
+
+<p>
+Again, an RCU read-side critical section can overlap almost all of a
+given grace period, just so long as it does not overlap the entire
+grace period.
+As a result, an RCU read-side critical section cannot partition a pair
+of RCU grace periods.
+
+<p>@@QQ@@
+How long a sequence of grace periods, each separated by an RCU read-side
+critical section, would be required to partition the RCU read-side
+critical sections at the beginning and end of the chain?
+<p>@@QQA@@
+In theory, an infinite number.
+In practice, an unknown number that is sensitive to both implementation
+details and timing considerations.
+Therefore, even in practice, RCU users must abide by the theoretical rather
+than the practical answer.
+<p>@@QQE@@
+
+<h3><a name="Disabling Preemption Does Not Block Grace Periods">
+Disabling Preemption Does Not Block Grace Periods</a></h3>
+
+<p>
+There was a time when disabling preemption on any given CPU would block
+subsequent grace periods.
+However, this was an accident of implementation and is not a requirement.
+And in the current Linux-kernel implementation, disabling preemption
+on a given CPU in fact does not block grace periods, as Oleg Nesterov
+<a href="https://lkml.kernel.org/g/20150614193825.GA19582@redhat.com">demonstrated</a>.
+
+<p>
+If you need a preempt-disable region to block grace periods, you need to add
+<tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>, for example
+as follows:
+
+<blockquote>
+<pre>
+ 1 preempt_disable();
+ 2 rcu_read_lock();
+ 3 do_something();
+ 4 rcu_read_unlock();
+ 5 preempt_enable();
+ 6
+ 7 /* Spinlocks implicitly disable preemption. */
+ 8 spin_lock(&amp;mylock);
+ 9 rcu_read_lock();
+10 do_something();
+11 rcu_read_unlock();
+12 spin_unlock(&amp;mylock);
+</pre>
+</blockquote>
+
+<p>
+In theory, you could enter the RCU read-side critical section first,
+but it is more efficient to keep the entire RCU read-side critical
+section contained in the preempt-disable region as shown above.
+Of course, RCU read-side critical sections that extend outside of
+preempt-disable regions will work correctly, but such critical sections
+can be preempted, which forces <tt>rcu_read_unlock()</tt> to do
+more work.
+And no, this is <i>not</i> an invitation to enclose all of your RCU
+read-side critical sections within preempt-disable regions, because
+doing so would degrade real-time response.
+
+<p>
+This non-requirement appeared with preemptible RCU.
+If you need a grace period that waits on non-preemptible code regions, use
+<a href="#Sched Flavor">RCU-sched</a>.
+
+<h2><a name="Parallelism Facts of Life">Parallelism Facts of Life</a></h2>
+
+<p>
+These parallelism facts of life are by no means specific to RCU, but
+the RCU implementation must abide by them.
+They therefore bear repeating:
+
+<ol>
+<li>	Any CPU or task may be delayed at any time,
+	and any attempts to avoid these delays by disabling
+	preemption, interrupts, or whatever are completely futile.
+	This is most obvious in preemptible user-level
+	environments and in virtualized environments (where
+	a given guest OS's VCPUs can be preempted at any time by
+	the underlying hypervisor), but can also happen in bare-metal
+	environments due to ECC errors, NMIs, and other hardware
+	events.
+	Although a delay of more than about 20 seconds can result
+	in splats, the RCU implementation is obligated to use
+	algorithms that can tolerate extremely long delays, but where
+	&ldquo;extremely long&rdquo; is not long enough to allow
+	wrap-around when incrementing a 64-bit counter.
+<li>	Both the compiler and the CPU can reorder memory accesses.
+	Where it matters, RCU must use compiler directives and
+	memory-barrier instructions to preserve ordering.
+<li>	Conflicting writes to memory locations in any given cache line
+	will result in expensive cache misses.
+	Greater numbers of concurrent writes and more-frequent
+	concurrent writes will result in more dramatic slowdowns.
+	RCU is therefore obligated to use algorithms that have
+	sufficient locality to avoid significant performance and
+	scalability problems.
+<li>	As a rough rule of thumb, only one CPU's worth of processing
+	may be carried out under the protection of any given exclusive
+	lock.
+	RCU must therefore use scalable locking designs.
+<li>	Counters are finite, especially on 32-bit systems.
+	RCU's use of counters must therefore tolerate counter wrap,
+	or be designed such that counter wrap would take way more
+	time than a single system is likely to run.
+	An uptime of ten years is quite possible, a runtime
+	of a century much less so.
+	As an example of the latter, RCU's dyntick-idle nesting counter
+	allows 54 bits for interrupt nesting level (this counter
+	is 64 bits even on a 32-bit system).
+	Overflowing this counter requires 2<sup>54</sup>
+	half-interrupts on a given CPU without that CPU ever going idle.
+	If a half-interrupt happened every microsecond, it would take
+	570 years of runtime to overflow this counter, which is currently
+	believed to be an acceptably long time.
+<li>	Linux systems can have thousands of CPUs running a single
+	Linux kernel in a single shared-memory environment.
+	RCU must therefore pay close attention to high-end scalability.
+</ol>
+
+<p>
+This last parallelism fact of life means that RCU must pay special
+attention to the preceding facts of life.
+The idea that Linux might scale to systems with thousands of CPUs would
+have been met with some skepticism in the 1990s, but these requirements
+would have otherwise have been unsurprising, even in the early 1990s.
+
+<h2><a name="Quality-of-Implementation Requirements">Quality-of-Implementation Requirements</a></h2>
+
+<p>
+These sections list quality-of-implementation requirements.
+Although an RCU implementation that ignores these requirements could
+still be used, it would likely be subject to limitations that would
+make it inappropriate for industrial-strength production use.
+Classes of quality-of-implementation requirements are as follows:
+
+<ol>
+<li>	<a href="#Specialization">Specialization</a>
+<li>	<a href="#Performance and Scalability">Performance and Scalability</a>
+<li>	<a href="#Composability">Composability</a>
+<li>	<a href="#Corner Cases">Corner Cases</a>
+</ol>
+
+<p>
+These classes is covered in the following sections.
+
+<h3><a name="Specialization">Specialization</a></h3>
+
+<p>
+RCU is and always has been intended primarily for read-mostly situations, as
+illustrated by the following figure.
+This means that RCU's read-side primitives are optimized, often at the
+expense of its update-side primitives.
+
+<p><img src="RCUApplicability.svg" alt="RCUApplicability.svg" width="70%"></p>
+
+<p>
+This focus on read-mostly situations means that RCU must interoperate
+with other synchronization primitives.
+For example, the <tt>add_gp()</tt> and <tt>remove_gp_synchronous()</tt>
+examples discussed earlier use RCU to protect readers and locking to
+coordinate updaters.
+However, the need extends much farther, requiring that a variety of
+synchronization primitives be legal within RCU read-side critical sections,
+including spinlocks, sequence locks, atomic operations, reference
+counters, and memory barriers.
+
+<p>@@QQ@@
+What about sleeping locks?
+<p>@@QQA@@
+These are forbidden within Linux-kernel RCU read-side critical sections
+because it is not legal to place a quiescent state (in this case,
+voluntary context switch) within an RCU read-side critical section.
+However, sleeping locks may be used within userspace RCU read-side critical
+sections, and also within Linux-kernel sleepable RCU
+<a href="#Sleepable RCU">(SRCU)</a>
+read-side critical sections.
+In addition, the -rt patchset turns spinlocks into a sleeping locks so
+that the corresponding critical sections can be preempted, which
+also means that these sleeplockified spinlocks (but not other sleeping locks!)
+may be acquire within -rt-Linux-kernel RCU read-side critical sections.
+
+<p>
+Note that it <i>is</i> legal for a normal RCU read-side critical section
+to conditionally acquire a sleeping locks (as in <tt>mutex_trylock()</tt>),
+but only as long as it does not loop indefinitely attempting to
+conditionally acquire that sleeping locks.
+The key point is that things like <tt>mutex_trylock()</tt>
+either return with the mutex held, or return an error indication if
+the mutex was not immediately available.
+Either way, <tt>mutex_trylock()</tt> returns immediately without sleeping.
+<p>@@QQE@@
+
+<p>
+It often comes as a surprise that many algorithms do not require a
+consistent view of data, but many can function in that mode,
+with network routing being the poster child.
+Internet routing algorithms take significant time to propagate
+updates, so that by the time an update arrives at a given system,
+that system has been sending network traffic the wrong way for
+a considerable length of time.
+Having a few threads continue to send traffic the wrong way for a
+few more milliseconds is clearly not a problem:  In the worst case,
+TCP retransmissions will eventually get the data where it needs to go.
+In general, when tracking the state of the universe outside of the
+computer, some level of inconsistency must be tolerated due to
+speed-of-light delays if nothing else.
+
+<p>
+Furthermore, uncertainty about external state is inherent in many cases.
+For example, a pair of veternarians might use heartbeat to determine
+whether or not a given cat was alive.
+But how long should they wait after the last heartbeat to decide that
+the cat is in fact dead?
+Waiting less than 400 milliseconds makes no sense because this would
+mean that a relaxed cat would be considered to cycle between death
+and life more than 100 times per minute.
+Moreover, just as with human beings, a cat's heart might stop for
+some period of time, so the exact wait period is a judgment call.
+One of our pair of veternarians might wait 30 seconds before pronouncing
+the cat dead, while the other might insist on waiting a full minute.
+The two veternarians would then disagree on the state of the cat during
+the final 30 seconds of the minute following the last heartbeat, as
+fancifully illustrated below:
+
+<p><img src="2013-08-is-it-dead.png" alt="2013-08-is-it-dead.png" width="431"></p>
+
+<p>
+Interestingly enough, this same situation applies to hardware.
+When push comes to shove, how do we tell whether or not some
+external server has failed?
+We send messages to it periodically, and declare it failed if we
+don't receive a response within a given period of time.
+Policy decisions can usually tolerate short
+periods of inconsistency.
+The policy was decided some time ago, and is only now being put into
+effect, so a few milliseconds of delay is normally inconsequential.
+
+<p>
+However, there are algorithms that absolutely must see consistent data.
+For example, the translation between a user-level SystemV semaphore
+ID to the corresponding in-kernel data structure is protected by RCU,
+but it is absolutely forbidden to update a semaphore that has just been
+removed.
+In the Linux kernel, this need for consistency is accommodated by acquiring
+spinlocks located in the in-kernel data structure from within
+the RCU read-side critical section, and this is indicated by the
+green box in the figure above.
+Many other techniques may be used, and are in fact used within the
+Linux kernel.
+
+<p>
+In short, RCU is not required to maintain consistency, and other
+mechanisms may be used in concert with RCU when consistency is required.
+RCU's specialization allows it to do its job extremely well, and its
+ability to interoperate with other synchronization mechanisms allows
+the right mix of synchronization tools to be used for a given job.
+
+<h3><a name="Performance and Scalability">Performance and Scalability</a></h3>
+
+<p>
+Energy efficiency is a critical component of performance today,
+and Linux-kernel RCU implementations must therefore avoid unnecessarily
+awakening idle CPUs.
+I cannot claim that this requirement was premeditated.
+In fact, I learned of it during a telephone conversation in which I
+was given &ldquo;frank and open&rdquo; feedback on the importance
+of energy efficiency in battery-powered systems and on specific
+energy-efficiency shortcomings of the Linux-kernel RCU implementation.
+In my experience, the battery-powered embedded community will consider
+any unnecessary wakeups to be extremely unfriendly acts.
+So much so that mere Linux-kernel-mailing-list posts are
+insufficient to vent their ire.
+
+<p>
+Memory consumption is not particularly important for in most
+situations, and has become decreasingly
+so as memory sizes have expanded and memory
+costs have plummeted.
+However, as I learned from Matt Mackall's
+<a href="http://elinux.org/Linux_Tiny-FAQ">bloatwatch</a>
+efforts, memory footprint is critically important on single-CPU systems with
+non-preemptible (<tt>CONFIG_PREEMPT=n</tt>) kernels, and thus
+<a href="https://lkml.kernel.org/g/20090113221724.GA15307@linux.vnet.ibm.com">tiny RCU</a>
+was born.
+Josh Triplett has since taken over the small-memory banner with his
+<a href="https://tiny.wiki.kernel.org/">Linux kernel tinification</a>
+project, which resulted in
+<a href="#Sleepable RCU">SRCU</a>
+becoming optional for those kernels not needing it.
+
+<p>
+The remaining performance requirements are, for the most part,
+unsurprising.
+For example, in keeping with RCU's read-side specialization,
+<tt>rcu_dereference()</tt> should have negligible overhead (for
+example, suppression of a few minor compiler optimizations).
+Similarly, in non-preemptible environments, <tt>rcu_read_lock()</tt> and
+<tt>rcu_read_unlock()</tt> should have exactly zero overhead.
+
+<p>
+In preemptible environments, in the case where the RCU read-side
+critical section was not preempted (as will be the case for the
+highest-priority real-time process), <tt>rcu_read_lock()</tt> and
+<tt>rcu_read_unlock()</tt> should have minimal overhead.
+In particular, they should not contain atomic read-modify-write
+operations, memory-barrier instructions, preemption disabling,
+interrupt disabling, or backwards branches.
+However, in the case where the RCU read-side critical section was preempted,
+<tt>rcu_read_unlock()</tt> may acquire spinlocks and disable interrupts.
+This is why it is better to nest an RCU read-side critical section
+within a preempt-disable region than vice versa, at least in cases
+where that critical section is short enough to avoid unduly degrading
+real-time latencies.
+
+<p>
+The <tt>synchronize_rcu()</tt> grace-period-wait primitive is
+optimized for throughput.
+It may therefore incur several milliseconds of latency in addition to
+the duration of the longest RCU read-side critical section.
+On the other hand, multiple concurrent invocations of
+<tt>synchronize_rcu()</tt> are required to use batching optimizations
+so that they can be satisfied by a single underlying grace-period-wait
+operation.
+For example, in the Linux kernel, it is not unusual for a single
+grace-period-wait operation to serve more than
+<a href="https://www.usenix.org/conference/2004-usenix-annual-technical-conference/making-rcu-safe-deep-sub-millisecond-response">1,000 separate invocations</a>
+of <tt>synchronize_rcu()</tt>, thus amortizing the per-invocation
+overhead down to nearly zero.
+However, the grace-period optimization is also required to avoid
+measurable degradation of real-time scheduling and interrupt latencies.
+
+<p>
+In some cases, the multi-millisecond <tt>synchronize_rcu()</tt>
+latencies are unacceptable.
+In these cases, <tt>synchronize_rcu_expedited()</tt> may be used
+instead, reducing the grace-period latency down to a few tens of
+microseconds on small systems, at least in cases where the RCU read-side
+critical sections are short.
+There are currently no special latency requirements for
+<tt>synchronize_rcu_expedited()</tt> on large systems, but,
+consistent with the empirical nature of the RCU specification,
+that is subject to change.
+However, there most definitely are scalability requirements:
+A storm of <tt>synchronize_rcu_expedited()</tt> invocations on 4096
+CPUs should at least make reasonable forward progress.
+In return for its shorter latencies, <tt>synchronize_rcu_expedited()</tt>
+is permitted to impose modest degradation of real-time latency
+on non-idle online CPUs.
+That said, it will likely be necessary to take further steps to reduce this
+degradation, hopefully to roughly that of a scheduling-clock interrupt.
+
+<p>
+There are a number of situations where even
+<tt>synchronize_rcu_expedited()</tt>'s reduced grace-period
+latency is unacceptable.
+In these situations, the asynchronous <tt>call_rcu()</tt> can be
+used in place of <tt>synchronize_rcu()</tt> as follows:
+
+<blockquote>
+<pre>
+ 1 struct foo {
+ 2   int a;
+ 3   int b;
+ 4   struct rcu_head rh;
+ 5 };
+ 6
+ 7 static void remove_gp_cb(struct rcu_head *rhp)
+ 8 {
+ 9   struct foo *p = container_of(rhp, struct foo, rh);
+10
+11   kfree(p);
+12 }
+13
+14 bool remove_gp_asynchronous(void)
+15 {
+16   struct foo *p;
+17
+18   spin_lock(&amp;gp_lock);
+19   p = rcu_dereference(gp);
+20   if (!p) {
+21     spin_unlock(&amp;gp_lock);
+22     return false;
+23   }
+24   rcu_assign_pointer(gp, NULL);
+25   call_rcu(&amp;p-&gt;rh, remove_gp_cb);
+26   spin_unlock(&amp;gp_lock);
+27   return true;
+28 }
+</pre>
+</blockquote>
+
+<p>
+A definition of <tt>struct foo</tt> is finally needed, and appears
+on lines&nbsp;1-5.
+The function <tt>remove_gp_cb()</tt> is passed to <tt>call_rcu()</tt>
+on line&nbsp;25, and will be invoked after the end of a subsequent
+grace period.
+This gets the same effect as <tt>remove_gp_synchronous()</tt>,
+but without forcing the updater to wait for a grace period to elapse.
+The <tt>call_rcu()</tt> function may be used in a number of
+situations where neither <tt>synchronize_rcu()</tt> nor
+<tt>synchronize_rcu_expedited()</tt> would be legal,
+including within preempt-disable code, <tt>local_bh_disable()</tt> code,
+interrupt-disable code, and interrupt handlers.
+However, even <tt>call_rcu()</tt> is illegal within NMI handlers.
+The callback function (<tt>remove_gp_cb()</tt> in this case) will be
+executed within softirq (software interrupt) environment within the
+Linux kernel,
+either within a real softirq handler or under the protection
+of <tt>local_bh_disable()</tt>.
+In both the Linux kernel and in userspace, it is bad practice to
+write an RCU callback function that takes too long.
+Long-running operations should be relegated to separate threads or
+(in the Linux kernel) workqueues.
+
+<p>@@QQ@@
+Why does line&nbsp;19 use <tt>rcu_access_pointer()</tt>?
+After all, <tt>call_rcu()</tt> on line&nbsp;25 stores into the
+structure, which would interact badly with concurrent insertions.
+Doesn't this mean that <tt>rcu_dereference()</tt> is required?
+<p>@@QQA@@
+Presumably the <tt>-&gt;gp_lock</tt> acquired on line&nbsp;18 excludes
+any changes, including any insertions that <tt>rcu_dereference()</tt>
+would protect against.
+Therefore, any insertions will be delayed until after <tt>-&gt;gp_lock</tt>
+is released on line&nbsp;25, which in turn means that
+<tt>rcu_access_pointer()</tt> suffices.
+<p>@@QQE@@
+
+<p>
+However, all that <tt>remove_gp_cb()</tt> is doing is
+invoking <tt>kfree()</tt> on the data element.
+This is a common idiom, and is supported by <tt>kfree_rcu()</tt>,
+which allows &ldquo;fire and forget&rdquo; operation as shown below:
+
+<blockquote>
+<pre>
+ 1 struct foo {
+ 2   int a;
+ 3   int b;
+ 4   struct rcu_head rh;
+ 5 };
+ 6
+ 7 bool remove_gp_faf(void)
+ 8 {
+ 9   struct foo *p;
+10
+11   spin_lock(&amp;gp_lock);
+12   p = rcu_dereference(gp);
+13   if (!p) {
+14     spin_unlock(&amp;gp_lock);
+15     return false;
+16   }
+17   rcu_assign_pointer(gp, NULL);
+18   kfree_rcu(p, rh);
+19   spin_unlock(&amp;gp_lock);
+20   return true;
+21 }
+</pre>
+</blockquote>
+
+<p>
+Note that <tt>remove_gp_faf()</tt> simply invokes
+<tt>kfree_rcu()</tt> and proceeds, without any need to pay any
+further attention to the subsequent grace period and <tt>kfree()</tt>.
+It is permissible to invoke <tt>kfree_rcu()</tt> from the same
+environments as for <tt>call_rcu()</tt>.
+Interestingly enough, DYNIX/ptx had the equivalents of
+<tt>call_rcu()</tt> and <tt>kfree_rcu()</tt>, but not
+<tt>synchronize_rcu()</tt>.
+This was due to the fact that RCU was not heavily used within DYNIX/ptx,
+so the very few places that needed something like
+<tt>synchronize_rcu()</tt> simply open-coded it.
+
+<p>@@QQ@@
+Earlier it was claimed that <tt>call_rcu()</tt> and
+<tt>kfree_rcu()</tt> allowed updaters to avoid being blocked
+by readers.
+But how can that be correct, given that the invocation of the callback
+and the freeing of the memory (respectively) must still wait for
+a grace period to elapse?
+<p>@@QQA@@
+We could define things this way, but keep in mind that this sort of
+definition would say that updates in garbage-collected languages
+cannot complete until the next time the garbage collector runs,
+which does not seem at all reasonable.
+The key point is that in most cases, an updater using either
+<tt>call_rcu()</tt> or <tt>kfree_rcu()</tt> can proceed to the
+next update as soon as it has invoked <tt>call_rcu()</tt> or
+<tt>kfree_rcu()</tt>, without having to wait for a subsequent
+grace period.
+<p>@@QQE@@
+
+<p>
+But what if the updater must wait for the completion of code to be
+executed after the end of the grace period, but has other tasks
+that can be carried out in the meantime?
+The polling-style <tt>get_state_synchronize_rcu()</tt> and
+<tt>cond_synchronize_rcu()</tt> functions may be used for this
+purpose, as shown below:
+
+<blockquote>
+<pre>
+ 1 bool remove_gp_poll(void)
+ 2 {
+ 3   struct foo *p;
+ 4   unsigned long s;
+ 5
+ 6   spin_lock(&amp;gp_lock);
+ 7   p = rcu_access_pointer(gp);
+ 8   if (!p) {
+ 9     spin_unlock(&amp;gp_lock);
+10     return false;
+11   }
+12   rcu_assign_pointer(gp, NULL);
+13   spin_unlock(&amp;gp_lock);
+14   s = get_state_synchronize_rcu();
+15   do_something_while_waiting();
+16   cond_synchronize_rcu(s);
+17   kfree(p);
+18   return true;
+19 }
+</pre>
+</blockquote>
+
+<p>
+On line&nbsp;14, <tt>get_state_synchronize_rcu()</tt> obtains a
+&ldquo;cookie&rdquo; from RCU,
+then line&nbsp;15 carries out other tasks,
+and finally, line&nbsp;16 returns immediately if a grace period has
+elapsed in the meantime, but otherwise waits as required.
+The need for <tt>get_state_synchronize_rcu</tt> and
+<tt>cond_synchronize_rcu()</tt> has appeared quite recently,
+so it is too early to tell whether they will stand the test of time.
+
+<p>
+RCU thus provides a range of tools to allow updaters to strike the
+required tradeoff between latency, flexibility and CPU overhead.
+
+<h3><a name="Composability">Composability</a></h3>
+
+<p>
+Composability has received much attention in recent years, perhaps in part
+due to the collision of multicore hardware with object-oriented techniques
+designed in single-threaded environments for single-threaded use.
+And in theory, RCU read-side critical sections may be composed, and in
+fact may be nested arbitrarily deeply.
+In practice, as with all real-world implementations of composable
+constructs, there are limitations.
+
+<p>
+Implementations of RCU for which <tt>rcu_read_lock()</tt>
+and <tt>rcu_read_unlock()</tt> generate no code, such as
+Linux-kernel RCU when <tt>CONFIG_PREEMPT=n</tt>, can be
+nested arbitrarily deeply.
+After all, there is no overhead.
+Except that if all these instances of <tt>rcu_read_lock()</tt>
+and <tt>rcu_read_unlock()</tt> are visible to the compiler,
+compilation will eventually fail due to exhausting memory,
+mass storage, or user patience, whichever comes first.
+If the nesting is not visible to the compiler, as is the case with
+mutually recursive functions each in its own translation unit,
+stack overflow will result.
+If the nesting takes the form of loops, either the control variable
+will overflow or (in the Linux kernel) you will get an RCU CPU stall warning.
+Nevertheless, this class of RCU implementations is one
+of the most composable constructs in existence.
+
+<p>
+RCU implementations that explicitly track nesting depth
+are limited by the nesting-depth counter.
+For example, the Linux kernel's preemptible RCU limits nesting to
+<tt>INT_MAX</tt>.
+This should suffice for almost all practical purposes.
+That said, a consecutive pair of RCU read-side critical sections
+between which there is an operation that waits for a grace period
+cannot be enclosed in another RCU read-side critical section.
+This is because it is not legal to wait for a grace period within
+an RCU read-side critical section:  To do so would result either
+in deadlock or
+in RCU implicitly splitting the enclosing RCU read-side critical
+section, neither of which is conducive to a long-lived and prosperous
+kernel.
+
+<p>
+It is worth noting that RCU is not alone in limiting composability.
+For example, many transactional-memory implementations prohibit
+composing a pair of transactions separated by an irrevocable
+operation (for example, a network receive operation).
+For another example, lock-based critical sections can be composed
+surprisingly freely, but only if deadlock is avoided.
+
+<p>
+In short, although RCU read-side critical sections are highly composable,
+care is required in some situations, just as is the case for any other
+composable synchronization mechanism.
+
+<h3><a name="Corner Cases">Corner Cases</a></h3>
+
+<p>
+A given RCU workload might have an endless and intense stream of
+RCU read-side critical sections, perhaps even so intense that there
+was never a point in time during which there was not at least one
+RCU read-side critical section in flight.
+RCU cannot allow this situation to block grace periods:  As long as
+all the RCU read-side critical sections are finite, grace periods
+must also be finite.
+
+<p>
+That said, preemptible RCU implementations could potentially result
+in RCU read-side critical sections being preempted for long durations,
+which has the effect of creating a long-duration RCU read-side
+critical section.
+This situation can arise only in heavily loaded systems, but systems using
+real-time priorities are of course more vulnerable.
+Therefore, RCU priority boosting is provided to help deal with this
+case.
+That said, the exact requirements on RCU priority boosting will likely
+evolve as more experience accumulates.
+
+<p>
+Other workloads might have very high update rates.
+Although one can argue that such workloads should instead use
+something other than RCU, the fact remains that RCU must
+handle such workloads gracefully.
+This requirement is another factor driving batching of grace periods,
+but it is also the driving force behind the checks for large numbers
+of queued RCU callbacks in the <tt>call_rcu()</tt> code path.
+Finally, high update rates should not delay RCU read-side critical
+sections, although some read-side delays can occur when using
+<tt>synchronize_rcu_expedited()</tt>, courtesy of this function's use
+of <tt>try_stop_cpus()</tt>.
+(In the future, <tt>synchronize_rcu_expedited()</tt> will be
+converted to use lighter-weight inter-processor interrupts (IPIs),
+but this will still disturb readers, though to a much smaller degree.)
+
+<p>
+Although all three of these corner cases were understood in the early
+1990s, a simple user-level test consisting of <tt>close(open(path))</tt>
+in a tight loop
+in the early 2000s suddenly provided a much deeper appreciation of the
+high-update-rate corner case.
+This test also motivated addition of some RCU code to react to high update
+rates, for example, if a given CPU finds itself with more than 10,000
+RCU callbacks queued, it will cause RCU to take evasive action by
+more aggressively starting grace periods and more aggressively forcing
+completion of grace-period processing.
+This evasive action causes the grace period to complete more quickly,
+but at the cost of restricting RCU's batching optimizations, thus
+increasing the CPU overhead incurred by that grace period.
+
+<h2><a name="Software-Engineering Requirements">
+Software-Engineering Requirements</a></h2>
+
+<p>
+Between Murphy's Law and &ldquo;To err is human&rdquo;, it is necessary to
+guard against mishaps and misuse:
+
+<ol>
+<li>	It is all too easy to forget to use <tt>rcu_read_lock()</tt>
+	everywhere that it is needed, so kernels built with
+	<tt>CONFIG_PROVE_RCU=y</tt> will spat if
+	<tt>rcu_dereference()</tt> is used outside of an
+	RCU read-side critical section.
+	Update-side code can use <tt>rcu_dereference_protected()</tt>,
+	which takes a
+	<a href="https://lwn.net/Articles/371986/">lockdep expression</a>
+	to indicate what is providing the protection.
+	If the indicated protection is not provided, a lockdep splat
+	is emitted.
+
+	<p>
+	Code shared between readers and updaters can use
+	<tt>rcu_dereference_check()</tt>, which also takes a
+	lockdep expression, and emits a lockdep splat if neither
+	<tt>rcu_read_lock()</tt> nor the indicated protection
+	is in place.
+	In addition, <tt>rcu_dereference_raw()</tt> is used in those
+	(hopefully rare) cases where the required protection cannot
+	be easily described.
+	Finally, <tt>rcu_read_lock_held()</tt> is provided to
+	allow a function to verify that it has been invoked within
+	an RCU read-side critical section.
+	I was made aware of this set of requirements shortly after Thomas
+	Gleixner audited a number of RCU uses.
+<li>	A given function might wish to check for RCU-related preconditions
+	upon entry, before using any other RCU API.
+	The <tt>rcu_lockdep_assert()</tt> does this job,
+	asserting the expression in kernels having lockdep enabled
+	and doing nothing otherwise.
+<li>	It is also easy to forget to use <tt>rcu_assign_pointer()</tt>
+	and <tt>rcu_dereference()</tt>, perhaps (incorrectly)
+	substituting a simple assignment.
+	To catch this sort of error, a given RCU-protected pointer may be
+	tagged with <tt>__rcu</tt>, after which running sparse
+	with <tt>CONFIG_SPARSE_RCU_POINTER=y</tt> will complain
+	about simple-assignment accesses to that pointer.
+	Arnd Bergmann made me aware of this requirement, and also
+	supplied the needed
+	<a href="https://lwn.net/Articles/376011/">patch series</a>.
+<li>	Kernels built with <tt>CONFIG_DEBUG_OBJECTS_RCU_HEAD=y</tt>
+	will splat if a data element is passed to <tt>call_rcu()</tt>
+	twice in a row, without a grace period in between.
+	(This error is similar to a double free.)
+	The corresponding <tt>rcu_head</tt> structures that are
+	dynamically allocated are automatically tracked, but
+	<tt>rcu_head</tt> structures allocated on the stack
+	must be initialized with <tt>init_rcu_head_on_stack()</tt>
+	and cleaned up with <tt>destroy_rcu_head_on_stack()</tt>.
+	Similarly, statically allocated non-stack <tt>rcu_head</tt>
+	structures must be initialized with <tt>init_rcu_head()</tt>
+	and cleaned up with <tt>destroy_rcu_head()</tt>.
+	Mathieu Desnoyers made me aware of this requirement, and also
+	supplied the needed
+	<a href="https://lkml.kernel.org/g/20100319013024.GA28456@Krystal">patch</a>.
+<li>	An infinite loop in an RCU read-side critical section will
+	eventually trigger an RCU CPU stall warning splat, with
+	the duration of &ldquo;eventually&rdquo; being controlled by the
+	<tt>RCU_CPU_STALL_TIMEOUT</tt> <tt>Kconfig</tt> option, or,
+	alternatively, by the
+	<tt>rcupdate.rcu_cpu_stall_timeout</tt> boot/sysfs
+	parameter.
+	However, RCU is not obligated to produce this splat
+	unless there is a grace period waiting on that particular
+	RCU read-side critical section.
+	<p>
+	Some extreme workloads might intentionally delay
+	RCU grace periods, and systems running those workloads can
+	be booted with <tt>rcupdate.rcu_cpu_stall_suppress</tt>
+	to suppress the splats.
+	This kernel parameter may also be set via <tt>sysfs</tt>.
+	Furthermore, RCU CPU stall warnings are counter-productive
+	during sysrq dumps and during panics.
+	RCU therefore supplies the <tt>rcu_sysrq_start()</tt> and
+	<tt>rcu_sysrq_end()</tt> API members to be called before
+	and after long sysrq dumps.
+	RCU also supplies the <tt>rcu_panic()</tt> notifier that is
+	automatically invoked at the beginning of a panic to suppress
+	further RCU CPU stall warnings.
+
+	<p>
+	This requirement made itself known in the early 1990s, pretty
+	much the first time that it was necessary to debug a CPU stall.
+	That said, the initial implementation in DYNIX/ptx was quite
+	generic in comparison with that of Linux.
+<li>	Although it would be very good to detect pointers leaking out
+	of RCU read-side critical sections, there is currently no
+	good way of doing this.
+	One complication is the need to distinguish between pointers
+	leaking and pointers that have been handed off from RCU to
+	some other synchronization mechanism, for example, reference
+	counting.
+<li>	In kernels built with <tt>CONFIG_RCU_TRACE=y</tt>, RCU-related
+	information is provided via both debugfs and event tracing.
+<li>	Open-coded use of <tt>rcu_assign_pointer()</tt> and
+	<tt>rcu_dereference()</tt> to create typical linked
+	data structures can be surprisingly error-prone.
+	Therefore, RCU-protected
+	<a href="https://lwn.net/Articles/609973/#RCU List APIs">linked lists</a>
+	and, more recently, RCU-protected
+	<a href="https://lwn.net/Articles/612100/">hash tables</a>
+	are available.
+	Many other special-purpose RCU-protected data structures are
+	available in the Linux kernel and the userspace RCU library.
+<li>	Some linked structures are created at compile time, but still
+	require <tt>__rcu</tt> checking.
+	The <tt>RCU_POINTER_INITIALIZER()</tt> macro serves this
+	purpose.
+<li>	It is not necessary to use <tt>rcu_assign_pointer()</tt>
+	when creating linked structures that are to be published via
+	a single external pointer.
+	The <tt>RCU_INIT_POINTER()</tt> macro is provided for
+	this task and also for assigning <tt>NULL</tt> pointers
+	at runtime.
+</ol>
+
+<p>
+This not a hard-and-fast list:  RCU's diagnostic capabilities will
+continue to be guided by the number and type of usage bugs found
+in real-world RCU usage.
+
+<h2><a name="Linux Kernel Complications">Linux Kernel Complications</a></h2>
+
+<p>
+The Linux kernel provides an interesting environment for all kinds of
+software, including RCU.
+Some of the relevant points of interest are as follows:
+
+<ol>
+<li>	<a href="#Configuration">Configuration</a>.
+<li>	<a href="#Firmware Interface">Firmware Interface</a>.
+<li>	<a href="#Early Boot">Early Boot</a>.
+<li>	<a href="#Interrupts and NMIs">
+	Interrupts and non-maskable interrupts (NMIs)</a>.
+<li>	<a href="#Loadable Modules">Loadable Modules</a>.
+<li>	<a href="#Hotplug CPU">Hotplug CPU</a>.
+<li>	<a href="#Scheduler and RCU">Scheduler and RCU</a>.
+<li>	<a href="#Tracing and RCU">Tracing and RCU</a>.
+<li>	<a href="#Energy Efficiency">Energy Efficiency</a>.
+<li>	<a href="#Memory Efficiency">Memory Efficiency</a>.
+<li>	<a href="#Performance, Scalability, Response Time, and Reliability">
+	Performance, Scalability, Response Time, and Reliability</a>.
+</ol>
+
+<p>
+This list is probably incomplete, but it does give a feel for the
+most notable Linux-kernel complications.
+Each of the following sections covers one of the above topics.
+
+<h3><a name="Configuration">Configuration</a></h3>
+
+<p>
+RCU's goal is automatic configuration, so that almost nobody
+needs to worry about RCU's <tt>Kconfig</tt> options.
+And for almost all users, RCU does in fact work well
+&ldquo;out of the box.&rdquo;
+
+<p>
+However, there are specialized use cases that are handled by
+kernel boot parameters and <tt>Kconfig</tt> options.
+Unfortunately, the <tt>Kconfig</tt> system will explicitly ask users
+about new <tt>Kconfig</tt> options, which requires almost all of them
+be hidden behind a <tt>CONFIG_RCU_EXPERT</tt> <tt>Kconfig</tt> option.
+
+<p>
+This all should be quite obvious, but the fact remains that
+Linus Torvalds recently had to
+<a href="https://lkml.kernel.org/g/CA+55aFy4wcCwaL4okTs8wXhGZ5h-ibecy_Meg9C4MNQrUnwMcg@mail.gmail.com">remind</a>
+me of this requirement.
+
+<h3><a name="Firmware Interface">Firmware Interface</a></h3>
+
+<p>
+In many cases, kernel obtains information about the system from the
+firmware, and sometimes things are lost in translation.
+Or the translation is accurate, but the original message is bogus.
+
+<p>
+For example, some systems' firmware overreports the number of CPUs,
+sometimes by a large factor.
+If RCU naively believed the firmware, as it used to do,
+it would create too many per-CPU kthreads.
+Although the resulting system will still run correctly, the extra
+kthreads needlessly consume memory and can cause confusion
+when they show up in <tt>ps</tt> listings.
+
+<p>
+RCU must therefore wait for a given CPU to actually come online before
+it can allow itself to believe that the CPU actually exists.
+The resulting &ldquo;ghost CPUs&rdquo; (which are never going to
+come online) cause a number of
+<a href="https://paulmck.livejournal.com/37494.html">interesting complications</a>.
+
+<h3><a name="Early Boot">Early Boot</a></h3>
+
+<p>
+The Linux kernel's boot sequence is an interesting process,
+and RCU is used early, even before <tt>rcu_init()</tt>
+is invoked.
+In fact, a number of RCU's primitives can be used as soon as the
+initial task's <tt>task_struct</tt> is available and the
+boot CPU's per-CPU variables are set up.
+The read-side primitives (<tt>rcu_read_lock()</tt>,
+<tt>rcu_read_unlock()</tt>, <tt>rcu_dereference()</tt>,
+and <tt>rcu_access_pointer()</tt>) will operate normally very early on,
+as will <tt>rcu_assign_pointer()</tt>.
+
+<p>
+Although <tt>call_rcu()</tt> may be invoked at any
+time during boot, callbacks are not guaranteed to be invoked until after
+the scheduler is fully up and running.
+This delay in callback invocation is due to the fact that RCU does not
+invoke callbacks until it is fully initialized, and this full initialization
+cannot occur until after the scheduler has initialized itself to the
+point where RCU can spawn and run its kthreads.
+In theory, it would be possible to invoke callbacks earlier,
+however, this is not a panacea because there would be severe restrictions
+on what operations those callbacks could invoke.
+
+<p>
+Perhaps surprisingly, <tt>synchronize_rcu()</tt>,
+<a href="#Bottom-Half Flavor"><tt>synchronize_rcu_bh()</tt></a>
+(<a href="#Bottom-Half Flavor">discussed below</a>),
+and
+<a href="#Sched Flavor"><tt>synchronize_sched()</tt></a>
+will all operate normally
+during very early boot, the reason being that there is only one CPU
+and preemption is disabled.
+This means that the call <tt>synchronize_rcu()</tt> (or friends)
+itself is a quiescent
+state and thus a grace period, so the early-boot implementation can
+be a no-op.
+
+<p>
+Both <tt>synchronize_rcu_bh()</tt> and <tt>synchronize_sched()</tt>
+continue to operate normally through the remainder of boot, courtesy
+of the fact that preemption is disabled across their RCU read-side
+critical sections and also courtesy of the fact that there is still
+only one CPU.
+However, once the scheduler starts initializing, preemption is enabled.
+There is still only a single CPU, but the fact that preemption is enabled
+means that the no-op implementation of <tt>synchronize_rcu()</tt> no
+longer works in <tt>CONFIG_PREEMPT=y</tt> kernels.
+Therefore, as soon as the scheduler starts initializing, the early-boot
+fastpath is disabled.
+This means that <tt>synchronize_rcu()</tt> switches to its runtime
+mode of operation where it posts callbacks, which in turn means that
+any call to <tt>synchronize_rcu()</tt> will block until the corresponding
+callback is invoked.
+Unfortunately, the callback cannot be invoked until RCU's runtime
+grace-period machinery is up and running, which cannot happen until
+the scheduler has initialized itself sufficiently to allow RCU's
+kthreads to be spawned.
+Therefore, invoking <tt>synchronize_rcu()</tt> during scheduler
+initialization can result in deadlock.
+
+<p>@@QQ@@
+So what happens with <tt>synchronize_rcu()</tt> during
+scheduler initialization for <tt>CONFIG_PREEMPT=n</tt>
+kernels?
+<p>@@QQA@@
+In <tt>CONFIG_PREEMPT=n</tt> kernel, <tt>synchronize_rcu()</tt>
+maps directly to <tt>synchronize_sched()</tt>.
+Therefore, <tt>synchronize_rcu()</tt> works normally throughout
+boot in <tt>CONFIG_PREEMPT=n</tt> kernels.
+However, your code must also work in <tt>CONFIG_PREEMPT=y</tt> kernels,
+so it is still necessary to avoid invoking <tt>synchronize_rcu()</tt>
+during scheduler initialization.
+<p>@@QQE@@
+
+<p>
+I learned of these boot-time requirements as a result of a series of
+system hangs.
+
+<h3><a name="Interrupts and NMIs">Interrupts and NMIs</a></h3>
+
+<p>
+The Linux kernel has interrupts, and RCU read-side critical sections are
+legal within interrupt handlers and within interrupt-disabled regions
+of code, as are invocations of <tt>call_rcu()</tt>.
+
+<p>
+Some Linux-kernel architectures can enter an interrupt handler from
+non-idle process context, and then just never leave it, instead stealthily
+transitioning back to process context.
+This trick is sometimes used to invoke system calls from inside the kernel.
+These &ldquo;half-interrupts&rdquo; mean that RCU has to be very careful
+about how it counts interrupt nesting levels.
+I learned of this requirement the hard way during a rewrite
+of RCU's dyntick-idle code.
+
+<p>
+The Linux kernel has non-maskable interrupts (NMIs), and
+RCU read-side critical sections are legal within NMI handlers.
+Thankfully, RCU update-side primitives, including
+<tt>call_rcu()</tt>, are prohibited within NMI handlers.
+
+<p>
+The name notwithstanding, some Linux-kernel architectures
+can have nested NMIs, which RCU must handle correctly.
+Andy Lutomirski
+<a href="https://lkml.kernel.org/g/CALCETrXLq1y7e_dKFPgou-FKHB6Pu-r8+t-6Ds+8=va7anBWDA@mail.gmail.com">surprised me</a>
+with this requirement;
+he also kindly surprised me with
+<a href="https://lkml.kernel.org/g/CALCETrXSY9JpW3uE6H8WYk81sg56qasA2aqmjMPsq5dOtzso=g@mail.gmail.com">an algorithm</a>
+that meets this requirement.
+
+<h3><a name="Loadable Modules">Loadable Modules</a></h3>
+
+<p>
+The Linux kernel has loadable modules, and these modules can
+also be unloaded.
+After a given module has been unloaded, any attempt to call
+one of its functions results in a segmentation fault.
+The module-unload functions must therefore cancel any
+delayed calls to loadable-module functions, for example,
+any outstanding <tt>mod_timer()</tt> must be dealt with
+via <tt>del_timer_sync()</tt> or similar.
+
+<p>
+Unfortunately, there is no way to cancel an RCU callback;
+once you invoke <tt>call_rcu()</tt>, the callback function is
+going to eventually be invoked, unless the system goes down first.
+Because it is normally considered socially irresponsible to crash the system
+in response to a module unload request, we need some other way
+to deal with in-flight RCU callbacks.
+
+<p>
+RCU therefore provides
+<tt><a href="https://lwn.net/Articles/217484/">rcu_barrier()</a></tt>,
+which waits until all in-flight RCU callbacks have been invoked.
+If a module uses <tt>call_rcu()</tt>, its exit function should therefore
+prevent any future invocation of <tt>call_rcu()</tt>, then invoke
+<tt>rcu_barrier()</tt>.
+In theory, the underlying module-unload code could invoke
+<tt>rcu_barrier()</tt> unconditionally, but in practice this would
+incur unacceptable latencies.
+
+<p>
+Nikita Danilov noted this requirement for an analogous filesystem-unmount
+situation, and Dipankar Sarma incorporated <tt>rcu_barrier()</tt> into RCU.
+The need for <tt>rcu_barrier()</tt> for module unloading became
+apparent later.
+
+<h3><a name="Hotplug CPU">Hotplug CPU</a></h3>
+
+<p>
+The Linux kernel supports CPU hotplug, which means that CPUs
+can come and go.
+It is of course illegal to use any RCU API member from an offline CPU.
+This requirement was present from day one in DYNIX/ptx, but
+on the other hand, the Linux kernel's CPU-hotplug implementation
+is &ldquo;interesting.&rdquo;
+
+<p>
+The Linux-kernel CPU-hotplug implementation has notifiers that
+are used to allow the various kernel subsystems (including RCU)
+to respond appropriately to a given CPU-hotplug operation.
+Most RCU operations may be invoked from CPU-hotplug notifiers,
+including even normal synchronous grace-period operations
+such as <tt>synchronize_rcu()</tt>.
+However, expedited grace-period operations such as
+<tt>synchronize_rcu_expedited()</tt> are not supported,
+due to the fact that current implementations block CPU-hotplug
+operations, which could result in deadlock.
+
+<p>
+In addition, all-callback-wait operations such as
+<tt>rcu_barrier()</tt> are also not supported, due to the
+fact that there are phases of CPU-hotplug operations where
+the outgoing CPU's callbacks will not be invoked until after
+the CPU-hotplug operation ends, which could also result in deadlock.
+
+<h3><a name="Scheduler and RCU">Scheduler and RCU</a></h3>
+
+<p>
+RCU depends on the scheduler, and the scheduler uses RCU to
+protect some of its data structures.
+This means the scheduler is forbidden from acquiring
+the runqueue locks and the priority-inheritance locks
+in the middle of an outermost RCU read-side critical section unless either
+(1)&nbsp;it releases them before exiting that same
+RCU read-side critical section, or
+(2)&nbsp;interrupts are disabled across
+that entire RCU read-side critical section.
+This same prohibition also applies (recursively!) to any lock that is acquired
+while holding any lock to which this prohibition applies.
+Adhering to this rule prevents preemptible RCU from invoking
+<tt>rcu_read_unlock_special()</tt> while either runqueue or
+priority-inheritance locks are held, thus avoiding deadlock.
+
+<p>
+Prior to v4.4, it was only necessary to disable preemption across
+RCU read-side critical sections that acquired scheduler locks.
+In v4.4, expedited grace periods started using IPIs, and these
+IPIs could force a <tt>rcu_read_unlock()</tt> to take the slowpath.
+Therefore, this expedited-grace-period change required disabling of
+interrupts, not just preemption.
+
+<p>
+For RCU's part, the preemptible-RCU <tt>rcu_read_unlock()</tt>
+implementation must be written carefully to avoid similar deadlocks.
+In particular, <tt>rcu_read_unlock()</tt> must tolerate an
+interrupt where the interrupt handler invokes both
+<tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>.
+This possibility requires <tt>rcu_read_unlock()</tt> to use
+negative nesting levels to avoid destructive recursion via
+interrupt handler's use of RCU.
+
+<p>
+This pair of mutual scheduler-RCU requirements came as a
+<a href="https://lwn.net/Articles/453002/">complete surprise</a>.
+
+<p>
+As noted above, RCU makes use of kthreads, and it is necessary to
+avoid excessive CPU-time accumulation by these kthreads.
+This requirement was no surprise, but RCU's violation of it
+when running context-switch-heavy workloads when built with
+<tt>CONFIG_NO_HZ_FULL=y</tt>
+<a href="http://www.rdrop.com/users/paulmck/scalability/paper/BareMetal.2015.01.15b.pdf">did come as a surprise [PDF]</a>.
+RCU has made good progress towards meeting this requirement, even
+for context-switch-have <tt>CONFIG_NO_HZ_FULL=y</tt> workloads,
+but there is room for further improvement.
+
+<h3><a name="Tracing and RCU">Tracing and RCU</a></h3>
+
+<p>
+It is possible to use tracing on RCU code, but tracing itself
+uses RCU.
+For this reason, <tt>rcu_dereference_raw_notrace()</tt>
+is provided for use by tracing, which avoids the destructive
+recursion that could otherwise ensue.
+This API is also used by virtualization in some architectures,
+where RCU readers execute in environments in which tracing
+cannot be used.
+The tracing folks both located the requirement and provided the
+needed fix, so this surprise requirement was relatively painless.
+
+<h3><a name="Energy Efficiency">Energy Efficiency</a></h3>
+
+<p>
+Interrupting idle CPUs is considered socially unacceptable,
+especially by people with battery-powered embedded systems.
+RCU therefore conserves energy by detecting which CPUs are
+idle, including tracking CPUs that have been interrupted from idle.
+This is a large part of the energy-efficiency requirement,
+so I learned of this via an irate phone call.
+
+<p>
+Because RCU avoids interrupting idle CPUs, it is illegal to
+execute an RCU read-side critical section on an idle CPU.
+(Kernels built with <tt>CONFIG_PROVE_RCU=y</tt> will splat
+if you try it.)
+The <tt>RCU_NONIDLE()</tt> macro and <tt>_rcuidle</tt>
+event tracing is provided to work around this restriction.
+In addition, <tt>rcu_is_watching()</tt> may be used to
+test whether or not it is currently legal to run RCU read-side
+critical sections on this CPU.
+I learned of the need for diagnostics on the one hand
+and <tt>RCU_NONIDLE()</tt> on the other while inspecting
+idle-loop code.
+Steven Rostedt supplied <tt>_rcuidle</tt> event tracing,
+which is used quite heavily in the idle loop.
+
+<p>
+It is similarly socially unacceptable to interrupt an
+<tt>nohz_full</tt> CPU running in userspace.
+RCU must therefore track <tt>nohz_full</tt> userspace
+execution.
+And in
+<a href="https://lwn.net/Articles/558284/"><tt>CONFIG_NO_HZ_FULL_SYSIDLE=y</tt></a>
+kernels, RCU must separately track idle CPUs on the one hand and
+CPUs that are either idle or executing in userspace on the other.
+In both cases, RCU must be able to sample state at two points in
+time, and be able to determine whether or not some other CPU spent
+any time idle and/or executing in userspace.
+
+<p>
+These energy-efficiency requirements have proven quite difficult to
+understand and to meet, for example, there have been more than five
+clean-sheet rewrites of RCU's energy-efficiency code, the last of
+which was finally able to demonstrate
+<a href="http://www.rdrop.com/users/paulmck/realtime/paper/AMPenergy.2013.04.19a.pdf">real energy savings running on real hardware [PDF]</a>.
+As noted earlier,
+I learned of many of these requirements via angry phone calls:
+Flaming me on the Linux-kernel mailing list was apparently not
+sufficient to fully vent their ire at RCU's energy-efficiency bugs!
+
+<h3><a name="Memory Efficiency">Memory Efficiency</a></h3>
+
+<p>
+Although small-memory non-realtime systems can simply use Tiny RCU,
+code size is only one aspect of memory efficiency.
+Another aspect is the size of the <tt>rcu_head</tt> structure
+used by <tt>call_rcu()</tt> and <tt>kfree_rcu()</tt>.
+Although this structure contains nothing more than a pair of pointers,
+it does appear in many RCU-protected data structures, including
+some that are size critical.
+The <tt>page</tt> structure is a case in point, as evidenced by
+the many occurrences of the <tt>union</tt> keyword within that structure.
+
+<p>
+This need for memory efficiency is one reason that RCU uses hand-crafted
+singly linked lists to track the <tt>rcu_head</tt> structures that
+are waiting for a grace period to elapse.
+It is also the reason why <tt>rcu_head</tt> structures do not contain
+debug information, such as fields tracking the file and line of the
+<tt>call_rcu()</tt> or <tt>kfree_rcu()</tt> that posted them.
+Although this information might appear in debug-only kernel builds at some
+point, in the meantime, the <tt>-&gt;func</tt> field will often provide
+the needed debug information.
+
+<p>
+However, in some cases, the need for memory efficiency leads to even
+more extreme measures.
+Returning to the <tt>page</tt> structure, the <tt>rcu_head</tt> field
+shares storage with a great many other structures that are used at
+various points in the corresponding page's lifetime.
+In order to correctly resolve certain
+<a href="https://lkml.kernel.org/g/1439976106-137226-1-git-send-email-kirill.shutemov@linux.intel.com">race conditions</a>,
+the Linux kernel's memory-management subsystem needs a particular bit
+to remain zero during all phases of grace-period processing,
+and that bit happens to map to the bottom bit of the
+<tt>rcu_head</tt> structure's <tt>-&gt;next</tt> field.
+RCU makes this guarantee as long as <tt>call_rcu()</tt>
+is used to post the callback, as opposed to <tt>kfree_rcu()</tt>
+or some future &ldquo;lazy&rdquo;
+variant of <tt>call_rcu()</tt> that might one day be created for
+energy-efficiency purposes.
+
+<h3><a name="Performance, Scalability, Response Time, and Reliability">
+Performance, Scalability, Response Time, and Reliability</a></h3>
+
+<p>
+Expanding on the
+<a href="#Performance and Scalability">earlier discussion</a>,
+RCU is used heavily by hot code paths in performance-critical
+portions of the Linux kernel's networking, security, virtualization,
+and scheduling code paths.
+RCU must therefore use efficient implementations, especially in its
+read-side primitives.
+To that end, it would be good if preemptible RCU's implementation
+of <tt>rcu_read_lock()</tt> could be inlined, however, doing
+this requires resolving <tt>#include</tt> issues with the
+<tt>task_struct</tt> structure.
+
+<p>
+The Linux kernel supports hardware configurations with up to
+4096 CPUs, which means that RCU must be extremely scalable.
+Algorithms that involve frequent acquisitions of global locks or
+frequent atomic operations on global variables simply cannot be
+tolerated within the RCU implementation.
+RCU therefore makes heavy use of a combining tree based on the
+<tt>rcu_node</tt> structure.
+RCU is required to tolerate all CPUs continuously invoking any
+combination of RCU's runtime primitives with minimal per-operation
+overhead.
+In fact, in many cases, increasing load must <i>decrease</i> the
+per-operation overhead, witness the batching optimizations for
+<tt>synchronize_rcu()</tt>, <tt>call_rcu()</tt>,
+<tt>synchronize_rcu_expedited()</tt>, and <tt>rcu_barrier()</tt>.
+As a general rule, RCU must cheerfully accept whatever the
+rest of the Linux kernel decides to throw at it.
+
+<p>
+The Linux kernel is used for real-time workloads, especially
+in conjunction with the
+<a href="https://rt.wiki.kernel.org/index.php/Main_Page">-rt patchset</a>.
+The real-time-latency response requirements are such that the
+traditional approach of disabling preemption across RCU
+read-side critical sections is inappropriate.
+Kernels built with <tt>CONFIG_PREEMPT=y</tt> therefore
+use an RCU implementation that allows RCU read-side critical
+sections to be preempted.
+This requirement made its presence known after users made it
+clear that an earlier
+<a href="https://lwn.net/Articles/107930/">real-time patch</a>
+did not meet their needs, in conjunction with some
+<a href="https://lkml.kernel.org/g/20050318002026.GA2693@us.ibm.com">RCU issues</a>
+encountered by a very early version of the -rt patchset.
+
+<p>
+In addition, RCU must make do with a sub-100-microsecond real-time latency
+budget.
+In fact, on smaller systems with the -rt patchset, the Linux kernel
+provides sub-20-microsecond real-time latencies for the whole kernel,
+including RCU.
+RCU's scalability and latency must therefore be sufficient for
+these sorts of configurations.
+To my surprise, the sub-100-microsecond real-time latency budget
+<a href="http://www.rdrop.com/users/paulmck/realtime/paper/bigrt.2013.01.31a.LCA.pdf">
+applies to even the largest systems [PDF]</a>,
+up to and including systems with 4096 CPUs.
+This real-time requirement motivated the grace-period kthread, which
+also simplified handling of a number of race conditions.
+
+<p>
+Finally, RCU's status as a synchronization primitive means that
+any RCU failure can result in arbitrary memory corruption that can be
+extremely difficult to debug.
+This means that RCU must be extremely reliable, which in
+practice also means that RCU must have an aggressive stress-test
+suite.
+This stress-test suite is called <tt>rcutorture</tt>.
+
+<p>
+Although the need for <tt>rcutorture</tt> was no surprise,
+the current immense popularity of the Linux kernel is posing
+interesting&mdash;and perhaps unprecedented&mdash;validation
+challenges.
+To see this, keep in mind that there are well over one billion
+instances of the Linux kernel running today, given Android
+smartphones, Linux-powered televisions, and servers.
+This number can be expected to increase sharply with the advent of
+the celebrated Internet of Things.
+
+<p>
+Suppose that RCU contains a race condition that manifests on average
+once per million years of runtime.
+This bug will be occurring about three times per <i>day</i> across
+the installed base.
+RCU could simply hide behind hardware error rates, given that no one
+should really expect their smartphone to last for a million years.
+However, anyone taking too much comfort from this thought should
+consider the fact that in most jurisdictions, a successful multi-year
+test of a given mechanism, which might include a Linux kernel,
+suffices for a number of types of safety-critical certifications.
+In fact, rumor has it that the Linux kernel is already being used
+in production for safety-critical applications.
+I don't know about you, but I would feel quite bad if a bug in RCU
+killed someone.
+Which might explain my recent focus on validation and verification.
+
+<h2><a name="Other RCU Flavors">Other RCU Flavors</a></h2>
+
+<p>
+One of the more surprising things about RCU is that there are now
+no fewer than five <i>flavors</i>, or API families.
+In addition, the primary flavor that has been the sole focus up to
+this point has two different implementations, non-preemptible and
+preemptible.
+The other four flavors are listed below, with requirements for each
+described in a separate section.
+
+<ol>
+<li>	<a href="#Bottom-Half Flavor">Bottom-Half Flavor</a>
+<li>	<a href="#Sched Flavor">Sched Flavor</a>
+<li>	<a href="#Sleepable RCU">Sleepable RCU</a>
+<li>	<a href="#Tasks RCU">Tasks RCU</a>
+</ol>
+
+<h3><a name="Bottom-Half Flavor">Bottom-Half Flavor</a></h3>
+
+<p>
+The softirq-disable (AKA &ldquo;bottom-half&rdquo;,
+hence the &ldquo;_bh&rdquo; abbreviations)
+flavor of RCU, or <i>RCU-bh</i>, was developed by
+Dipankar Sarma to provide a flavor of RCU that could withstand the
+network-based denial-of-service attacks researched by Robert
+Olsson.
+These attacks placed so much networking load on the system
+that some of the CPUs never exited softirq execution,
+which in turn prevented those CPUs from ever executing a context switch,
+which, in the RCU implementation of that time, prevented grace periods
+from ever ending.
+The result was an out-of-memory condition and a system hang.
+
+<p>
+The solution was the creation of RCU-bh, which does
+<tt>local_bh_disable()</tt>
+across its read-side critical sections, and which uses the transition
+from one type of softirq processing to another as a quiescent state
+in addition to context switch, idle, user mode, and offline.
+This means that RCU-bh grace periods can complete even when some of
+the CPUs execute in softirq indefinitely, thus allowing algorithms
+based on RCU-bh to withstand network-based denial-of-service attacks.
+
+<p>
+Because
+<tt>rcu_read_lock_bh()</tt> and <tt>rcu_read_unlock_bh()</tt>
+disable and re-enable softirq handlers, any attempt to start a softirq
+handlers during the
+RCU-bh read-side critical section will be deferred.
+In this case, <tt>rcu_read_unlock_bh()</tt>
+will invoke softirq processing, which can take considerable time.
+One can of course argue that this softirq overhead should be associated
+with the code following the RCU-bh read-side critical section rather
+than <tt>rcu_read_unlock_bh()</tt>, but the fact
+is that most profiling tools cannot be expected to make this sort
+of fine distinction.
+For example, suppose that a three-millisecond-long RCU-bh read-side
+critical section executes during a time of heavy networking load.
+There will very likely be an attempt to invoke at least one softirq
+handler during that three milliseconds, but any such invocation will
+be delayed until the time of the <tt>rcu_read_unlock_bh()</tt>.
+This can of course make it appear at first glance as if
+<tt>rcu_read_unlock_bh()</tt> was executing very slowly.
+
+<p>
+The
+<a href="https://lwn.net/Articles/609973/#RCU Per-Flavor API Table">RCU-bh API</a>
+includes
+<tt>rcu_read_lock_bh()</tt>,
+<tt>rcu_read_unlock_bh()</tt>,
+<tt>rcu_dereference_bh()</tt>,
+<tt>rcu_dereference_bh_check()</tt>,
+<tt>synchronize_rcu_bh()</tt>,
+<tt>synchronize_rcu_bh_expedited()</tt>,
+<tt>call_rcu_bh()</tt>,
+<tt>rcu_barrier_bh()</tt>, and
+<tt>rcu_read_lock_bh_held()</tt>.
+
+<h3><a name="Sched Flavor">Sched Flavor</a></h3>
+
+<p>
+Before preemptible RCU, waiting for an RCU grace period had the
+side effect of also waiting for all pre-existing interrupt
+and NMI handlers.
+However, there are legitimate preemptible-RCU implementations that
+do not have this property, given that any point in the code outside
+of an RCU read-side critical section can be a quiescent state.
+Therefore, <i>RCU-sched</i> was created, which follows &ldquo;classic&rdquo;
+RCU in that an RCU-sched grace period waits for for pre-existing
+interrupt and NMI handlers.
+In kernels built with <tt>CONFIG_PREEMPT=n</tt>, the RCU and RCU-sched
+APIs have identical implementations, while kernels built with
+<tt>CONFIG_PREEMPT=y</tt> provide a separate implementation for each.
+
+<p>
+Note well that in <tt>CONFIG_PREEMPT=y</tt> kernels,
+<tt>rcu_read_lock_sched()</tt> and <tt>rcu_read_unlock_sched()</tt>
+disable and re-enable preemption, respectively.
+This means that if there was a preemption attempt during the
+RCU-sched read-side critical section, <tt>rcu_read_unlock_sched()</tt>
+will enter the scheduler, with all the latency and overhead entailed.
+Just as with <tt>rcu_read_unlock_bh()</tt>, this can make it look
+as if <tt>rcu_read_unlock_sched()</tt> was executing very slowly.
+However, the highest-priority task won't be preempted, so that task
+will enjoy low-overhead <tt>rcu_read_unlock_sched()</tt> invocations.
+
+<p>
+The
+<a href="https://lwn.net/Articles/609973/#RCU Per-Flavor API Table">RCU-sched API</a>
+includes
+<tt>rcu_read_lock_sched()</tt>,
+<tt>rcu_read_unlock_sched()</tt>,
+<tt>rcu_read_lock_sched_notrace()</tt>,
+<tt>rcu_read_unlock_sched_notrace()</tt>,
+<tt>rcu_dereference_sched()</tt>,
+<tt>rcu_dereference_sched_check()</tt>,
+<tt>synchronize_sched()</tt>,
+<tt>synchronize_rcu_sched_expedited()</tt>,
+<tt>call_rcu_sched()</tt>,
+<tt>rcu_barrier_sched()</tt>, and
+<tt>rcu_read_lock_sched_held()</tt>.
+However, anything that disables preemption also marks an RCU-sched
+read-side critical section, including
+<tt>preempt_disable()</tt> and <tt>preempt_enable()</tt>,
+<tt>local_irq_save()</tt> and <tt>local_irq_restore()</tt>,
+and so on.
+
+<h3><a name="Sleepable RCU">Sleepable RCU</a></h3>
+
+<p>
+For well over a decade, someone saying &ldquo;I need to block within
+an RCU read-side critical section&rdquo; was a reliable indication
+that this someone did not understand RCU.
+After all, if you are always blocking in an RCU read-side critical
+section, you can probably afford to use a higher-overhead synchronization
+mechanism.
+However, that changed with the advent of the Linux kernel's notifiers,
+whose RCU read-side critical
+sections almost never sleep, but sometimes need to.
+This resulted in the introduction of
+<a href="https://lwn.net/Articles/202847/">sleepable RCU</a>,
+or <i>SRCU</i>.
+
+<p>
+SRCU allows different domains to be defined, with each such domain
+defined by an instance of an <tt>srcu_struct</tt> structure.
+A pointer to this structure must be passed in to each SRCU function,
+for example, <tt>synchronize_srcu(&amp;ss)</tt>, where
+<tt>ss</tt> is the <tt>srcu_struct</tt> structure.
+The key benefit of these domains is that a slow SRCU reader in one
+domain does not delay an SRCU grace period in some other domain.
+That said, one consequence of these domains is that read-side code
+must pass a &ldquo;cookie&rdquo; from <tt>srcu_read_lock()</tt>
+to <tt>srcu_read_unlock()</tt>, for example, as follows:
+
+<blockquote>
+<pre>
+ 1 int idx;
+ 2
+ 3 idx = srcu_read_lock(&amp;ss);
+ 4 do_something();
+ 5 srcu_read_unlock(&amp;ss, idx);
+</pre>
+</blockquote>
+
+<p>
+As noted above, it is legal to block within SRCU read-side critical sections,
+however, with great power comes great responsibility.
+If you block forever in one of a given domain's SRCU read-side critical
+sections, then that domain's grace periods will also be blocked forever.
+Of course, one good way to block forever is to deadlock, which can
+happen if any operation in a given domain's SRCU read-side critical
+section can block waiting, either directly or indirectly, for that domain's
+grace period to elapse.
+For example, this results in a self-deadlock:
+
+<blockquote>
+<pre>
+ 1 int idx;
+ 2
+ 3 idx = srcu_read_lock(&amp;ss);
+ 4 do_something();
+ 5 synchronize_srcu(&amp;ss);
+ 6 srcu_read_unlock(&amp;ss, idx);
+</pre>
+</blockquote>
+
+<p>
+However, if line&nbsp;5 acquired a mutex that was held across
+a <tt>synchronize_srcu()</tt> for domain <tt>ss</tt>,
+deadlock would still be possible.
+Furthermore, if line&nbsp;5 acquired a mutex that was held across
+a <tt>synchronize_srcu()</tt> for some other domain <tt>ss1</tt>,
+and if an <tt>ss1</tt>-domain SRCU read-side critical section
+acquired another mutex that was held across as <tt>ss</tt>-domain
+<tt>synchronize_srcu()</tt>,
+deadlock would again be possible.
+Such a deadlock cycle could extend across an arbitrarily large number
+of different SRCU domains.
+Again, with great power comes great responsibility.
+
+<p>
+Unlike the other RCU flavors, SRCU read-side critical sections can
+run on idle and even offline CPUs.
+This ability requires that <tt>srcu_read_lock()</tt> and
+<tt>srcu_read_unlock()</tt> contain memory barriers, which means
+that SRCU readers will run a bit slower than would RCU readers.
+It also motivates the <tt>smp_mb__after_srcu_read_unlock()</tt>
+API, which, in combination with <tt>srcu_read_unlock()</tt>,
+guarantees a full memory barrier.
+
+<p>
+The
+<a href="https://lwn.net/Articles/609973/#RCU Per-Flavor API Table">SRCU API</a>
+includes
+<tt>srcu_read_lock()</tt>,
+<tt>srcu_read_unlock()</tt>,
+<tt>srcu_dereference()</tt>,
+<tt>srcu_dereference_check()</tt>,
+<tt>synchronize_srcu()</tt>,
+<tt>synchronize_srcu_expedited()</tt>,
+<tt>call_srcu()</tt>,
+<tt>srcu_barrier()</tt>, and
+<tt>srcu_read_lock_held()</tt>.
+It also includes
+<tt>DEFINE_SRCU()</tt>,
+<tt>DEFINE_STATIC_SRCU()</tt>, and
+<tt>init_srcu_struct()</tt>
+APIs for defining and initializing <tt>srcu_struct</tt> structures.
+
+<h3><a name="Tasks RCU">Tasks RCU</a></h3>
+
+<p>
+Some forms of tracing use &ldquo;tramopolines&rdquo; to handle the
+binary rewriting required to install different types of probes.
+It would be good to be able to free old trampolines, which sounds
+like a job for some form of RCU.
+However, because it is necessary to be able to install a trace
+anywhere in the code, it is not possible to use read-side markers
+such as <tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>.
+In addition, it does not work to have these markers in the trampoline
+itself, because there would need to be instructions following
+<tt>rcu_read_unlock()</tt>.
+Although <tt>synchronize_rcu()</tt> would guarantee that execution
+reached the <tt>rcu_read_unlock()</tt>, it would not be able to
+guarantee that execution had completely left the trampoline.
+
+<p>
+The solution, in the form of
+<a href="https://lwn.net/Articles/607117/"><i>Tasks RCU</i></a>,
+is to have implicit
+read-side critical sections that are delimited by voluntary context
+switches, that is, calls to <tt>schedule()</tt>,
+<tt>cond_resched_rcu_qs()</tt>, and
+<tt>synchronize_rcu_tasks()</tt>.
+In addition, transitions to and from userspace execution also delimit
+tasks-RCU read-side critical sections.
+
+<p>
+The tasks-RCU API is quite compact, consisting only of
+<tt>call_rcu_tasks()</tt>,
+<tt>synchronize_rcu_tasks()</tt>, and
+<tt>rcu_barrier_tasks()</tt>.
+
+<h2><a name="Possible Future Changes">Possible Future Changes</a></h2>
+
+<p>
+One of the tricks that RCU uses to attain update-side scalability is
+to increase grace-period latency with increasing numbers of CPUs.
+If this becomes a serious problem, it will be necessary to rework the
+grace-period state machine so as to avoid the need for the additional
+latency.
+
+<p>
+Expedited grace periods scan the CPUs, so their latency and overhead
+increases with increasing numbers of CPUs.
+If this becomes a serious problem on large systems, it will be necessary
+to do some redesign to avoid this scalability problem.
+
+<p>
+RCU disables CPU hotplug in a few places, perhaps most notably in the
+expedited grace-period and <tt>rcu_barrier()</tt> operations.
+If there is a strong reason to use expedited grace periods in CPU-hotplug
+notifiers, it will be necessary to avoid disabling CPU hotplug.
+This would introduce some complexity, so there had better be a <i>very</i>
+good reason.
+
+<p>
+The tradeoff between grace-period latency on the one hand and interruptions
+of other CPUs on the other hand may need to be re-examined.
+The desire is of course for zero grace-period latency as well as zero
+interprocessor interrupts undertaken during an expedited grace period
+operation.
+While this ideal is unlikely to be achievable, it is quite possible that
+further improvements can be made.
+
+<p>
+The multiprocessor implementations of RCU use a combining tree that
+groups CPUs so as to reduce lock contention and increase cache locality.
+However, this combining tree does not spread its memory across NUMA
+nodes nor does it align the CPU groups with hardware features such
+as sockets or cores.
+Such spreading and alignment is currently believed to be unnecessary
+because the hotpath read-side primitives do not access the combining
+tree, nor does <tt>call_rcu()</tt> in the common case.
+If you believe that your architecture needs such spreading and alignment,
+then your architecture should also benefit from the
+<tt>rcutree.rcu_fanout_leaf</tt> boot parameter, which can be set
+to the number of CPUs in a socket, NUMA node, or whatever.
+If the number of CPUs is too large, use a fraction of the number of
+CPUs.
+If the number of CPUs is a large prime number, well, that certainly
+is an &ldquo;interesting&rdquo; architectural choice!
+More flexible arrangements might be considered, but only if
+<tt>rcutree.rcu_fanout_leaf</tt> has proven inadequate, and only
+if the inadequacy has been demonstrated by a carefully run and
+realistic system-level workload.
+
+<p>
+Please note that arrangements that require RCU to remap CPU numbers will
+require extremely good demonstration of need and full exploration of
+alternatives.
+
+<p>
+There is an embarrassingly large number of flavors of RCU, and this
+number has been increasing over time.
+Perhaps it will be possible to combine some at some future date.
+
+<p>
+RCU's various kthreads are reasonably recent additions.
+It is quite likely that adjustments will be required to more gracefully
+handle extreme loads.
+It might also be necessary to be able to relate CPU utilization by
+RCU's kthreads and softirq handlers to the code that instigated this
+CPU utilization.
+For example, RCU callback overhead might be charged back to the
+originating <tt>call_rcu()</tt> instance, though probably not
+in production kernels.
+
+<h2><a name="Summary">Summary</a></h2>
+
+<p>
+This document has presented more than two decade's worth of RCU
+requirements.
+Given that the requirements keep changing, this will not be the last
+word on this subject, but at least it serves to get an important
+subset of the requirements set forth.
+
+<h2><a name="Acknowledgments">Acknowledgments</a></h2>
+
+I am grateful to Steven Rostedt, Lai Jiangshan, Ingo Molnar,
+Oleg Nesterov, Borislav Petkov, Peter Zijlstra, Boqun Feng, and
+Andy Lutomirski for their help in rendering
+this article human readable, and to Michelle Rankin for her support
+of this effort.
+Other contributions are acknowledged in the Linux kernel's git archive.
+The cartoon is copyright (c) 2013 by Melissa Broussard,
+and is provided
+under the terms of the Creative Commons Attribution-Share Alike 3.0
+United States license.
+
+<p>@@QQAL@@
+
+</body></html>
diff --git a/Documentation/RCU/Design/htmlqqz.sh b/Documentation/RCU/Design/htmlqqz.sh
new file mode 100755
index 0000000..d354f06
--- /dev/null
+++ b/Documentation/RCU/Design/htmlqqz.sh
@@ -0,0 +1,108 @@
+#!/bin/sh
+#
+# Usage: sh htmlqqz.sh file
+#
+# Extracts and converts quick quizzes in a proto-HTML document file.htmlx.
+# Commands, all of which must be on a line by themselves:
+#
+#	"<p>@@QQ@@": Start of a quick quiz.
+#	"<p>@@QQA@@": Start of a quick-quiz answer.
+#	"<p>@@QQE@@": End of a quick-quiz answer, and thus of the quick quiz.
+#	"<p>@@QQAL@@": Place to put quick-quiz answer list.
+#
+# Places the result in file.html.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, you can access it online at
+# http://www.gnu.org/licenses/gpl-2.0.html.
+#
+# Copyright (c) 2013 Paul E. McKenney, IBM Corporation.
+
+fn=$1
+if test ! -r $fn.htmlx
+then
+	echo "Error: $fn.htmlx unreadable."
+	exit 1
+fi
+
+echo "<!-- DO NOT HAND EDIT. -->" > $fn.html
+echo "<!-- Instead, edit $fn.htmlx and run 'sh htmlqqz.sh $fn' -->" >> $fn.html
+awk < $fn.htmlx >> $fn.html '
+
+state == "" && $1 != "<p>@@QQ@@" && $1 != "<p>@@QQAL@@" {
+	print $0;
+	if ($0 ~ /^<p>@@QQ/)
+		print "Bad Quick Quiz command: " NR " (expected <p>@@QQ@@ or <p>@@QQAL@@)." > "/dev/stderr"
+	next;
+}
+
+state == "" && $1 == "<p>@@QQ@@" {
+	qqn++;
+	qqlineno = NR;
+	haveqq = 1;
+	state = "qq";
+	print "<p><a name=\"Quick Quiz " qqn "\"><b>Quick Quiz " qqn "</b>:</a>"
+	next;
+}
+
+state == "qq" && $1 != "<p>@@QQA@@" {
+	qq[qqn] = qq[qqn] $0 "\n";
+	print $0
+	if ($0 ~ /^<p>@@QQ/)
+		print "Bad Quick Quiz command: " NR ". (expected <p>@@QQA@@)" > "/dev/stderr"
+	next;
+}
+
+state == "qq" && $1 == "<p>@@QQA@@" {
+	state = "qqa";
+	print "<br><a href=\"#qq" qqn "answer\">Answer</a>"
+	next;
+}
+
+state == "qqa" && $1 != "<p>@@QQE@@" {
+	qqa[qqn] = qqa[qqn] $0 "\n";
+	if ($0 ~ /^<p>@@QQ/)
+		print "Bad Quick Quiz command: " NR " (expected <p>@@QQE@@)." > "/dev/stderr"
+	next;
+}
+
+state == "qqa" && $1 == "<p>@@QQE@@" {
+	state = "";
+	next;
+}
+
+state == "" && $1 == "<p>@@QQAL@@" {
+	haveqq = "";
+	print "<h3><a name=\"Answers to Quick Quizzes\">"
+	print "Answers to Quick Quizzes</a></h3>"
+	print "";
+	for (i = 1; i <= qqn; i++) {
+		print "<a name=\"qq" i "answer\"></a>"
+		print "<p><b>Quick Quiz " i "</b>:"
+		print qq[i];
+		print "";
+		print "</p><p><b>Answer</b>:"
+		print qqa[i];
+		print "";
+		print "</p><p><a href=\"#Quick%20Quiz%20" i "\"><b>Back to Quick Quiz " i "</b>.</a>"
+		print "";
+	}
+	next;
+}
+
+END {
+	if (state != "")
+		print "Unterminated Quick Quiz: " qqlineno "." > "/dev/stderr"
+	else if (haveqq)
+		print "Missing \"<p>@@QQAL@@\", no Quick Quiz." > "/dev/stderr"
+}'
diff --git a/Documentation/accounting/getdelays.c b/Documentation/accounting/getdelays.c
index f405780..7785fb5 100644
--- a/Documentation/accounting/getdelays.c
+++ b/Documentation/accounting/getdelays.c
@@ -375,7 +375,8 @@
 		}
 	}
 
-	if ((nl_sd = create_nl_socket(NETLINK_GENERIC)) < 0)
+	nl_sd = create_nl_socket(NETLINK_GENERIC);
+	if (nl_sd < 0)
 		err(1, "error creating Netlink socket\n");
 
 
diff --git a/Documentation/arm/Marvell/README b/Documentation/arm/Marvell/README
index 18a775d..ae89b67 100644
--- a/Documentation/arm/Marvell/README
+++ b/Documentation/arm/Marvell/README
@@ -233,29 +233,30 @@
    Linux kernel mach directory: arch/arm/mach-mmp
    Linux kernel plat directory: arch/arm/plat-pxa
 
-Berlin family (Digital Entertainment)
+Berlin family (Multimedia Solutions)
 -------------------------------------
 
   Flavors:
-	88DE3005, Armada 1500-mini
+	88DE3005, Armada 1500 Mini
 		Design name:	BG2CD
 		Core:		ARM Cortex-A9, PL310 L2CC
-		Homepage:	http://www.marvell.com/digital-entertainment/armada-1500-mini/
+		Homepage:	http://www.marvell.com/multimedia-solutions/armada-1500-mini/
+        88DE3006, Armada 1500 Mini Plus
+                Design name:    BG2CDP
+                Core:           Dual Core ARM Cortex-A7
+                Homepage:       http://www.marvell.com/multimedia-solutions/armada-1500-mini-plus/
 	88DE3100, Armada 1500
 		Design name:	BG2
 		Core:		Marvell PJ4B (ARMv7), Tauros3 L2CC
-		Homepage:	http://www.marvell.com/digital-entertainment/armada-1500/
-		Product Brief:	http://www.marvell.com/digital-entertainment/armada-1500/assets/Marvell-ARMADA-1500-Product-Brief.pdf
+		Product Brief:	http://www.marvell.com/multimedia-solutions/armada-1500/assets/Marvell-ARMADA-1500-Product-Brief.pdf
 	88DE3114, Armada 1500 Pro
-		Design name:	BG2-Q
+		Design name:	BG2Q
 		Core:		Quad Core ARM Cortex-A9, PL310 L2CC
-		Homepage:	http://www.marvell.com/digital-entertainment/armada-1500-pro/
-		Product Brief:	http://www.marvell.com/digital-entertainment/armada-1500-pro/assets/Marvell_ARMADA_1500_PRO-01_product_brief.pdf
 	88DE????
 		Design name:	BG3
 		Core:		ARM Cortex-A15, CA15 integrated L2CC
 
-  Homepage: http://www.marvell.com/digital-entertainment/
+  Homepage: http://www.marvell.com/multimedia-solutions/
   Directory: arch/arm/mach-berlin
 
   Comments:
diff --git a/Documentation/arm/keystone/Overview.txt b/Documentation/arm/keystone/Overview.txt
index f17bc4c..400c0c2 100644
--- a/Documentation/arm/keystone/Overview.txt
+++ b/Documentation/arm/keystone/Overview.txt
@@ -49,24 +49,6 @@
 The device tree documentation for the keystone machines are located at
         Documentation/devicetree/bindings/arm/keystone/keystone.txt
 
-Known issues & workaround
--------------------------
-
-Some of the device drivers used on keystone are re-used from that from
-DaVinci and other TI SoCs. These device drivers may use clock APIs directly.
-Some of the keystone specific drivers such as netcp uses run time power
-management API instead to enable clock. As this API has limitations on
-keystone, following workaround is needed to boot Linux.
-
-   Add 'clk_ignore_unused' to the bootargs env variable in u-boot. Otherwise
-   clock frameworks will try to disable clocks that are unused and disable
-   the hardware. This is because netcp related power domain and clock
-   domains are enabled in u-boot as run time power management API currently
-   doesn't enable clocks for netcp due to a limitation. This workaround is
-   expected to be removed in the future when proper API support becomes
-   available. Until then, this work around is needed.
-
-
 Document Author
 ---------------
 Murali Karicheri <m-karicheri2@ti.com>
diff --git a/Documentation/arm/pxa/mfp.txt b/Documentation/arm/pxa/mfp.txt
index a179e5b..0b7cab9 100644
--- a/Documentation/arm/pxa/mfp.txt
+++ b/Documentation/arm/pxa/mfp.txt
@@ -49,7 +49,7 @@
      internal controllers like PWM, SSP and UART, with 128 internal signals
      which can be routed to external through one or more MFPs (e.g. GPIO<0>
      can be routed through either MFP_PIN_GPIO0 as well as MFP_PIN_GPIO0_2,
-     see arch/arm/mach-pxa/mach/include/mfp-pxa300.h)
+     see arch/arm/mach-pxa/mfp-pxa300.h)
 
   2. Alternate function configuration is removed from this GPIO controller,
      the remaining functions are pure GPIO-specific, i.e.
@@ -76,11 +76,11 @@
 
 1. include ONE of the following header files in your <board>.c:
 
-   - #include <mach/mfp-pxa25x.h>
-   - #include <mach/mfp-pxa27x.h>
-   - #include <mach/mfp-pxa300.h>
-   - #include <mach/mfp-pxa320.h>
-   - #include <mach/mfp-pxa930.h>
+   - #include "mfp-pxa25x.h"
+   - #include "mfp-pxa27x.h"
+   - #include "mfp-pxa300.h"
+   - #include "mfp-pxa320.h"
+   - #include "mfp-pxa930.h"
 
    NOTE: only one file in your <board>.c, depending on the processors used,
    because pin configuration definitions may conflict in these file (i.e.
@@ -203,20 +203,20 @@
     1. Unified pin definitions - enum constants for all configurable pins
     2. processor-neutral bit definitions for a possible MFP configuration
 
-  - arch/arm/mach-pxa/include/mach/mfp-pxa3xx.h
+  - arch/arm/mach-pxa/mfp-pxa3xx.h
 
   for PXA3xx specific MFPR register bit definitions and PXA3xx common pin
   configurations
 
-  - arch/arm/mach-pxa/include/mach/mfp-pxa2xx.h
+  - arch/arm/mach-pxa/mfp-pxa2xx.h
 
   for PXA2xx specific definitions and PXA25x/PXA27x common pin configurations
 
-  - arch/arm/mach-pxa/include/mach/mfp-pxa25x.h
-    arch/arm/mach-pxa/include/mach/mfp-pxa27x.h
-    arch/arm/mach-pxa/include/mach/mfp-pxa300.h
-    arch/arm/mach-pxa/include/mach/mfp-pxa320.h
-    arch/arm/mach-pxa/include/mach/mfp-pxa930.h
+  - arch/arm/mach-pxa/mfp-pxa25x.h
+    arch/arm/mach-pxa/mfp-pxa27x.h
+    arch/arm/mach-pxa/mfp-pxa300.h
+    arch/arm/mach-pxa/mfp-pxa320.h
+    arch/arm/mach-pxa/mfp-pxa930.h
 
   for processor specific definitions
 
diff --git a/Documentation/arm64/silicon-errata.txt b/Documentation/arm64/silicon-errata.txt
new file mode 100644
index 0000000..58b71dd
--- /dev/null
+++ b/Documentation/arm64/silicon-errata.txt
@@ -0,0 +1,58 @@
+                Silicon Errata and Software Workarounds
+                =======================================
+
+Author: Will Deacon <will.deacon@arm.com>
+Date  : 27 November 2015
+
+It is an unfortunate fact of life that hardware is often produced with
+so-called "errata", which can cause it to deviate from the architecture
+under specific circumstances.  For hardware produced by ARM, these
+errata are broadly classified into the following categories:
+
+  Category A: A critical error without a viable workaround.
+  Category B: A significant or critical error with an acceptable
+              workaround.
+  Category C: A minor error that is not expected to occur under normal
+              operation.
+
+For more information, consult one of the "Software Developers Errata
+Notice" documents available on infocenter.arm.com (registration
+required).
+
+As far as Linux is concerned, Category B errata may require some special
+treatment in the operating system. For example, avoiding a particular
+sequence of code, or configuring the processor in a particular way. A
+less common situation may require similar actions in order to declassify
+a Category A erratum into a Category C erratum. These are collectively
+known as "software workarounds" and are only required in the minority of
+cases (e.g. those cases that both require a non-secure workaround *and*
+can be triggered by Linux).
+
+For software workarounds that may adversely impact systems unaffected by
+the erratum in question, a Kconfig entry is added under "Kernel
+Features" -> "ARM errata workarounds via the alternatives framework".
+These are enabled by default and patched in at runtime when an affected
+CPU is detected. For less-intrusive workarounds, a Kconfig option is not
+available and the code is structured (preferably with a comment) in such
+a way that the erratum will not be hit.
+
+This approach can make it slightly onerous to determine exactly which
+errata are worked around in an arbitrary kernel source tree, so this
+file acts as a registry of software workarounds in the Linux Kernel and
+will be updated when new workarounds are committed and backported to
+stable kernels.
+
+| Implementor    | Component       | Erratum ID      | Kconfig                 |
++----------------+-----------------+-----------------+-------------------------+
+| ARM            | Cortex-A53      | #826319         | ARM64_ERRATUM_826319    |
+| ARM            | Cortex-A53      | #827319         | ARM64_ERRATUM_827319    |
+| ARM            | Cortex-A53      | #824069         | ARM64_ERRATUM_824069    |
+| ARM            | Cortex-A53      | #819472         | ARM64_ERRATUM_819472    |
+| ARM            | Cortex-A53      | #845719         | ARM64_ERRATUM_845719    |
+| ARM            | Cortex-A53      | #843419         | ARM64_ERRATUM_843419    |
+| ARM            | Cortex-A57      | #832075         | ARM64_ERRATUM_832075    |
+| ARM            | Cortex-A57      | #852523         | N/A                     |
+| ARM            | Cortex-A57      | #834220         | ARM64_ERRATUM_834220    |
+|                |                 |                 |                         |
+| Cavium         | ThunderX ITS    | #22375, #24313  | CAVIUM_ERRATUM_22375    |
+| Cavium         | ThunderX GICv3  | #23154          | CAVIUM_ERRATUM_23154    |
diff --git a/Documentation/block/cfq-iosched.txt b/Documentation/block/cfq-iosched.txt
index f3bc729..1e4f835 100644
--- a/Documentation/block/cfq-iosched.txt
+++ b/Documentation/block/cfq-iosched.txt
@@ -81,14 +81,13 @@
 
 Default value for this parameter is 8ms.
 
-latency
--------
-This parameter is used to enable/disable the latency mode of the CFQ
-scheduler. If latency mode (called low_latency) is enabled, CFQ tries
-to recompute the slice time for each process based on the target_latency set
-for the system. This favors fairness over throughput. Disabling low
-latency (setting it to 0) ignores target latency, allowing each process in the
-system to get a full time slice.
+low_latency
+-----------
+This parameter is used to enable/disable the low latency mode of the CFQ
+scheduler. If enabled, CFQ tries to recompute the slice time for each process
+based on the target_latency set for the system. This favors fairness over
+throughput. Disabling low latency (setting it to 0) ignores target latency,
+allowing each process in the system to get a full time slice.
 
 By default low latency mode is enabled.
 
diff --git a/Documentation/block/null_blk.txt b/Documentation/block/null_blk.txt
index 2f6c6ff..d8880ca 100644
--- a/Documentation/block/null_blk.txt
+++ b/Documentation/block/null_blk.txt
@@ -70,3 +70,6 @@
      parameter.
   1: The multi-queue block layer is instantiated with a hardware dispatch
      queue for each CPU node in the system.
+
+use_lightnvm=[0/1]: Default: 0
+  Register device with LightNVM. Requires blk-mq to be used.
diff --git a/Documentation/cgroup-v1/00-INDEX b/Documentation/cgroup-v1/00-INDEX
new file mode 100644
index 0000000..6ad425f
--- /dev/null
+++ b/Documentation/cgroup-v1/00-INDEX
@@ -0,0 +1,28 @@
+00-INDEX
+	- this file
+blkio-controller.txt
+	- Description for Block IO Controller, implementation and usage details.
+cgroups.txt
+	- Control Groups definition, implementation details, examples and API.
+cpuacct.txt
+	- CPU Accounting Controller; account CPU usage for groups of tasks.
+cpusets.txt
+	- documents the cpusets feature; assign CPUs and Mem to a set of tasks.
+devices.txt
+	- Device Whitelist Controller; description, interface and security.
+freezer-subsystem.txt
+	- checkpointing; rationale to not use signals, interface.
+hugetlb.txt
+	- HugeTLB Controller implementation and usage details.
+memcg_test.txt
+	- Memory Resource Controller; implementation details.
+memory.txt
+	- Memory Resource Controller; design, accounting, interface, testing.
+net_cls.txt
+	- Network classifier cgroups details and usages.
+net_prio.txt
+	- Network priority cgroups details and usages.
+pids.txt
+	- Process number cgroups details and usages.
+unified-hierarchy.txt
+	- Description the new/next cgroup interface.
diff --git a/Documentation/cgroup-v1/blkio-controller.txt b/Documentation/cgroup-v1/blkio-controller.txt
new file mode 100644
index 0000000..673dc34
--- /dev/null
+++ b/Documentation/cgroup-v1/blkio-controller.txt
@@ -0,0 +1,375 @@
+				Block IO Controller
+				===================
+Overview
+========
+cgroup subsys "blkio" implements the block io controller. There seems to be
+a need of various kinds of IO control policies (like proportional BW, max BW)
+both at leaf nodes as well as at intermediate nodes in a storage hierarchy.
+Plan is to use the same cgroup based management interface for blkio controller
+and based on user options switch IO policies in the background.
+
+Currently two IO control policies are implemented. First one is proportional
+weight time based division of disk policy. It is implemented in CFQ. Hence
+this policy takes effect only on leaf nodes when CFQ is being used. The second
+one is throttling policy which can be used to specify upper IO rate limits
+on devices. This policy is implemented in generic block layer and can be
+used on leaf nodes as well as higher level logical devices like device mapper.
+
+HOWTO
+=====
+Proportional Weight division of bandwidth
+-----------------------------------------
+You can do a very simple testing of running two dd threads in two different
+cgroups. Here is what you can do.
+
+- Enable Block IO controller
+	CONFIG_BLK_CGROUP=y
+
+- Enable group scheduling in CFQ
+	CONFIG_CFQ_GROUP_IOSCHED=y
+
+- Compile and boot into kernel and mount IO controller (blkio); see
+  cgroups.txt, Why are cgroups needed?.
+
+	mount -t tmpfs cgroup_root /sys/fs/cgroup
+	mkdir /sys/fs/cgroup/blkio
+	mount -t cgroup -o blkio none /sys/fs/cgroup/blkio
+
+- Create two cgroups
+	mkdir -p /sys/fs/cgroup/blkio/test1/ /sys/fs/cgroup/blkio/test2
+
+- Set weights of group test1 and test2
+	echo 1000 > /sys/fs/cgroup/blkio/test1/blkio.weight
+	echo 500 > /sys/fs/cgroup/blkio/test2/blkio.weight
+
+- Create two same size files (say 512MB each) on same disk (file1, file2) and
+  launch two dd threads in different cgroup to read those files.
+
+	sync
+	echo 3 > /proc/sys/vm/drop_caches
+
+	dd if=/mnt/sdb/zerofile1 of=/dev/null &
+	echo $! > /sys/fs/cgroup/blkio/test1/tasks
+	cat /sys/fs/cgroup/blkio/test1/tasks
+
+	dd if=/mnt/sdb/zerofile2 of=/dev/null &
+	echo $! > /sys/fs/cgroup/blkio/test2/tasks
+	cat /sys/fs/cgroup/blkio/test2/tasks
+
+- At macro level, first dd should finish first. To get more precise data, keep
+  on looking at (with the help of script), at blkio.disk_time and
+  blkio.disk_sectors files of both test1 and test2 groups. This will tell how
+  much disk time (in milliseconds), each group got and how many sectors each
+  group dispatched to the disk. We provide fairness in terms of disk time, so
+  ideally io.disk_time of cgroups should be in proportion to the weight.
+
+Throttling/Upper Limit policy
+-----------------------------
+- Enable Block IO controller
+	CONFIG_BLK_CGROUP=y
+
+- Enable throttling in block layer
+	CONFIG_BLK_DEV_THROTTLING=y
+
+- Mount blkio controller (see cgroups.txt, Why are cgroups needed?)
+        mount -t cgroup -o blkio none /sys/fs/cgroup/blkio
+
+- Specify a bandwidth rate on particular device for root group. The format
+  for policy is "<major>:<minor>  <bytes_per_second>".
+
+        echo "8:16  1048576" > /sys/fs/cgroup/blkio/blkio.throttle.read_bps_device
+
+  Above will put a limit of 1MB/second on reads happening for root group
+  on device having major/minor number 8:16.
+
+- Run dd to read a file and see if rate is throttled to 1MB/s or not.
+
+        # dd iflag=direct if=/mnt/common/zerofile of=/dev/null bs=4K count=1024
+        1024+0 records in
+        1024+0 records out
+        4194304 bytes (4.2 MB) copied, 4.0001 s, 1.0 MB/s
+
+ Limits for writes can be put using blkio.throttle.write_bps_device file.
+
+Hierarchical Cgroups
+====================
+
+Both CFQ and throttling implement hierarchy support; however,
+throttling's hierarchy support is enabled iff "sane_behavior" is
+enabled from cgroup side, which currently is a development option and
+not publicly available.
+
+If somebody created a hierarchy like as follows.
+
+			root
+			/  \
+		     test1 test2
+			|
+		     test3
+
+CFQ by default and throttling with "sane_behavior" will handle the
+hierarchy correctly.  For details on CFQ hierarchy support, refer to
+Documentation/block/cfq-iosched.txt.  For throttling, all limits apply
+to the whole subtree while all statistics are local to the IOs
+directly generated by tasks in that cgroup.
+
+Throttling without "sane_behavior" enabled from cgroup side will
+practically treat all groups at same level as if it looks like the
+following.
+
+				pivot
+			     /  /   \  \
+			root  test1 test2  test3
+
+Various user visible config options
+===================================
+CONFIG_BLK_CGROUP
+	- Block IO controller.
+
+CONFIG_DEBUG_BLK_CGROUP
+	- Debug help. Right now some additional stats file show up in cgroup
+	  if this option is enabled.
+
+CONFIG_CFQ_GROUP_IOSCHED
+	- Enables group scheduling in CFQ. Currently only 1 level of group
+	  creation is allowed.
+
+CONFIG_BLK_DEV_THROTTLING
+	- Enable block device throttling support in block layer.
+
+Details of cgroup files
+=======================
+Proportional weight policy files
+--------------------------------
+- blkio.weight
+	- Specifies per cgroup weight. This is default weight of the group
+	  on all the devices until and unless overridden by per device rule.
+	  (See blkio.weight_device).
+	  Currently allowed range of weights is from 10 to 1000.
+
+- blkio.weight_device
+	- One can specify per cgroup per device rules using this interface.
+	  These rules override the default value of group weight as specified
+	  by blkio.weight.
+
+	  Following is the format.
+
+	  # echo dev_maj:dev_minor weight > blkio.weight_device
+	  Configure weight=300 on /dev/sdb (8:16) in this cgroup
+	  # echo 8:16 300 > blkio.weight_device
+	  # cat blkio.weight_device
+	  dev     weight
+	  8:16    300
+
+	  Configure weight=500 on /dev/sda (8:0) in this cgroup
+	  # echo 8:0 500 > blkio.weight_device
+	  # cat blkio.weight_device
+	  dev     weight
+	  8:0     500
+	  8:16    300
+
+	  Remove specific weight for /dev/sda in this cgroup
+	  # echo 8:0 0 > blkio.weight_device
+	  # cat blkio.weight_device
+	  dev     weight
+	  8:16    300
+
+- blkio.leaf_weight[_device]
+	- Equivalents of blkio.weight[_device] for the purpose of
+          deciding how much weight tasks in the given cgroup has while
+          competing with the cgroup's child cgroups. For details,
+          please refer to Documentation/block/cfq-iosched.txt.
+
+- blkio.time
+	- disk time allocated to cgroup per device in milliseconds. First
+	  two fields specify the major and minor number of the device and
+	  third field specifies the disk time allocated to group in
+	  milliseconds.
+
+- blkio.sectors
+	- number of sectors transferred to/from disk by the group. First
+	  two fields specify the major and minor number of the device and
+	  third field specifies the number of sectors transferred by the
+	  group to/from the device.
+
+- blkio.io_service_bytes
+	- Number of bytes transferred to/from the disk by the group. These
+	  are further divided by the type of operation - read or write, sync
+	  or async. First two fields specify the major and minor number of the
+	  device, third field specifies the operation type and the fourth field
+	  specifies the number of bytes.
+
+- blkio.io_serviced
+	- Number of IOs (bio) issued to the disk by the group. These
+	  are further divided by the type of operation - read or write, sync
+	  or async. First two fields specify the major and minor number of the
+	  device, third field specifies the operation type and the fourth field
+	  specifies the number of IOs.
+
+- blkio.io_service_time
+	- Total amount of time between request dispatch and request completion
+	  for the IOs done by this cgroup. This is in nanoseconds to make it
+	  meaningful for flash devices too. For devices with queue depth of 1,
+	  this time represents the actual service time. When queue_depth > 1,
+	  that is no longer true as requests may be served out of order. This
+	  may cause the service time for a given IO to include the service time
+	  of multiple IOs when served out of order which may result in total
+	  io_service_time > actual time elapsed. This time is further divided by
+	  the type of operation - read or write, sync or async. First two fields
+	  specify the major and minor number of the device, third field
+	  specifies the operation type and the fourth field specifies the
+	  io_service_time in ns.
+
+- blkio.io_wait_time
+	- Total amount of time the IOs for this cgroup spent waiting in the
+	  scheduler queues for service. This can be greater than the total time
+	  elapsed since it is cumulative io_wait_time for all IOs. It is not a
+	  measure of total time the cgroup spent waiting but rather a measure of
+	  the wait_time for its individual IOs. For devices with queue_depth > 1
+	  this metric does not include the time spent waiting for service once
+	  the IO is dispatched to the device but till it actually gets serviced
+	  (there might be a time lag here due to re-ordering of requests by the
+	  device). This is in nanoseconds to make it meaningful for flash
+	  devices too. This time is further divided by the type of operation -
+	  read or write, sync or async. First two fields specify the major and
+	  minor number of the device, third field specifies the operation type
+	  and the fourth field specifies the io_wait_time in ns.
+
+- blkio.io_merged
+	- Total number of bios/requests merged into requests belonging to this
+	  cgroup. This is further divided by the type of operation - read or
+	  write, sync or async.
+
+- blkio.io_queued
+	- Total number of requests queued up at any given instant for this
+	  cgroup. This is further divided by the type of operation - read or
+	  write, sync or async.
+
+- blkio.avg_queue_size
+	- Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y.
+	  The average queue size for this cgroup over the entire time of this
+	  cgroup's existence. Queue size samples are taken each time one of the
+	  queues of this cgroup gets a timeslice.
+
+- blkio.group_wait_time
+	- Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y.
+	  This is the amount of time the cgroup had to wait since it became busy
+	  (i.e., went from 0 to 1 request queued) to get a timeslice for one of
+	  its queues. This is different from the io_wait_time which is the
+	  cumulative total of the amount of time spent by each IO in that cgroup
+	  waiting in the scheduler queue. This is in nanoseconds. If this is
+	  read when the cgroup is in a waiting (for timeslice) state, the stat
+	  will only report the group_wait_time accumulated till the last time it
+	  got a timeslice and will not include the current delta.
+
+- blkio.empty_time
+	- Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y.
+	  This is the amount of time a cgroup spends without any pending
+	  requests when not being served, i.e., it does not include any time
+	  spent idling for one of the queues of the cgroup. This is in
+	  nanoseconds. If this is read when the cgroup is in an empty state,
+	  the stat will only report the empty_time accumulated till the last
+	  time it had a pending request and will not include the current delta.
+
+- blkio.idle_time
+	- Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y.
+	  This is the amount of time spent by the IO scheduler idling for a
+	  given cgroup in anticipation of a better request than the existing ones
+	  from other queues/cgroups. This is in nanoseconds. If this is read
+	  when the cgroup is in an idling state, the stat will only report the
+	  idle_time accumulated till the last idle period and will not include
+	  the current delta.
+
+- blkio.dequeue
+	- Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y. This
+	  gives the statistics about how many a times a group was dequeued
+	  from service tree of the device. First two fields specify the major
+	  and minor number of the device and third field specifies the number
+	  of times a group was dequeued from a particular device.
+
+- blkio.*_recursive
+	- Recursive version of various stats. These files show the
+          same information as their non-recursive counterparts but
+          include stats from all the descendant cgroups.
+
+Throttling/Upper limit policy files
+-----------------------------------
+- blkio.throttle.read_bps_device
+	- Specifies upper limit on READ rate from the device. IO rate is
+	  specified in bytes per second. Rules are per device. Following is
+	  the format.
+
+  echo "<major>:<minor>  <rate_bytes_per_second>" > /cgrp/blkio.throttle.read_bps_device
+
+- blkio.throttle.write_bps_device
+	- Specifies upper limit on WRITE rate to the device. IO rate is
+	  specified in bytes per second. Rules are per device. Following is
+	  the format.
+
+  echo "<major>:<minor>  <rate_bytes_per_second>" > /cgrp/blkio.throttle.write_bps_device
+
+- blkio.throttle.read_iops_device
+	- Specifies upper limit on READ rate from the device. IO rate is
+	  specified in IO per second. Rules are per device. Following is
+	  the format.
+
+  echo "<major>:<minor>  <rate_io_per_second>" > /cgrp/blkio.throttle.read_iops_device
+
+- blkio.throttle.write_iops_device
+	- Specifies upper limit on WRITE rate to the device. IO rate is
+	  specified in io per second. Rules are per device. Following is
+	  the format.
+
+  echo "<major>:<minor>  <rate_io_per_second>" > /cgrp/blkio.throttle.write_iops_device
+
+Note: If both BW and IOPS rules are specified for a device, then IO is
+      subjected to both the constraints.
+
+- blkio.throttle.io_serviced
+	- Number of IOs (bio) issued to the disk by the group. These
+	  are further divided by the type of operation - read or write, sync
+	  or async. First two fields specify the major and minor number of the
+	  device, third field specifies the operation type and the fourth field
+	  specifies the number of IOs.
+
+- blkio.throttle.io_service_bytes
+	- Number of bytes transferred to/from the disk by the group. These
+	  are further divided by the type of operation - read or write, sync
+	  or async. First two fields specify the major and minor number of the
+	  device, third field specifies the operation type and the fourth field
+	  specifies the number of bytes.
+
+Common files among various policies
+-----------------------------------
+- blkio.reset_stats
+	- Writing an int to this file will result in resetting all the stats
+	  for that cgroup.
+
+CFQ sysfs tunable
+=================
+/sys/block/<disk>/queue/iosched/slice_idle
+------------------------------------------
+On a faster hardware CFQ can be slow, especially with sequential workload.
+This happens because CFQ idles on a single queue and single queue might not
+drive deeper request queue depths to keep the storage busy. In such scenarios
+one can try setting slice_idle=0 and that would switch CFQ to IOPS
+(IO operations per second) mode on NCQ supporting hardware.
+
+That means CFQ will not idle between cfq queues of a cfq group and hence be
+able to driver higher queue depth and achieve better throughput. That also
+means that cfq provides fairness among groups in terms of IOPS and not in
+terms of disk time.
+
+/sys/block/<disk>/queue/iosched/group_idle
+------------------------------------------
+If one disables idling on individual cfq queues and cfq service trees by
+setting slice_idle=0, group_idle kicks in. That means CFQ will still idle
+on the group in an attempt to provide fairness among groups.
+
+By default group_idle is same as slice_idle and does not do anything if
+slice_idle is enabled.
+
+One can experience an overall throughput drop if you have created multiple
+groups and put applications in that group which are not driving enough
+IO to keep disk busy. In that case set group_idle=0, and CFQ will not idle
+on individual groups and throughput should improve.
diff --git a/Documentation/cgroups/cgroups.txt b/Documentation/cgroup-v1/cgroups.txt
similarity index 100%
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diff --git a/Documentation/cgroups/devices.txt b/Documentation/cgroup-v1/devices.txt
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diff --git a/Documentation/cgroup-v2.txt b/Documentation/cgroup-v2.txt
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--- /dev/null
+++ b/Documentation/cgroup-v2.txt
@@ -0,0 +1,1386 @@
+
+Control Group v2
+
+October, 2015		Tejun Heo <tj@kernel.org>
+
+This is the authoritative documentation on the design, interface and
+conventions of cgroup v2.  It describes all userland-visible aspects
+of cgroup including core and specific controller behaviors.  All
+future changes must be reflected in this document.  Documentation for
+v1 is available under Documentation/cgroup-legacy/.
+
+CONTENTS
+
+1. Introduction
+  1-1. Terminology
+  1-2. What is cgroup?
+2. Basic Operations
+  2-1. Mounting
+  2-2. Organizing Processes
+  2-3. [Un]populated Notification
+  2-4. Controlling Controllers
+    2-4-1. Enabling and Disabling
+    2-4-2. Top-down Constraint
+    2-4-3. No Internal Process Constraint
+  2-5. Delegation
+    2-5-1. Model of Delegation
+    2-5-2. Delegation Containment
+  2-6. Guidelines
+    2-6-1. Organize Once and Control
+    2-6-2. Avoid Name Collisions
+3. Resource Distribution Models
+  3-1. Weights
+  3-2. Limits
+  3-3. Protections
+  3-4. Allocations
+4. Interface Files
+  4-1. Format
+  4-2. Conventions
+  4-3. Core Interface Files
+5. Controllers
+  5-1. CPU
+    5-1-1. CPU Interface Files
+  5-2. Memory
+    5-2-1. Memory Interface Files
+    5-2-2. Usage Guidelines
+    5-2-3. Memory Ownership
+  5-3. IO
+    5-3-1. IO Interface Files
+    5-3-2. Writeback
+P. Information on Kernel Programming
+  P-1. Filesystem Support for Writeback
+D. Deprecated v1 Core Features
+R. Issues with v1 and Rationales for v2
+  R-1. Multiple Hierarchies
+  R-2. Thread Granularity
+  R-3. Competition Between Inner Nodes and Threads
+  R-4. Other Interface Issues
+  R-5. Controller Issues and Remedies
+    R-5-1. Memory
+
+
+1. Introduction
+
+1-1. Terminology
+
+"cgroup" stands for "control group" and is never capitalized.  The
+singular form is used to designate the whole feature and also as a
+qualifier as in "cgroup controllers".  When explicitly referring to
+multiple individual control groups, the plural form "cgroups" is used.
+
+
+1-2. What is cgroup?
+
+cgroup is a mechanism to organize processes hierarchically and
+distribute system resources along the hierarchy in a controlled and
+configurable manner.
+
+cgroup is largely composed of two parts - the core and controllers.
+cgroup core is primarily responsible for hierarchically organizing
+processes.  A cgroup controller is usually responsible for
+distributing a specific type of system resource along the hierarchy
+although there are utility controllers which serve purposes other than
+resource distribution.
+
+cgroups form a tree structure and every process in the system belongs
+to one and only one cgroup.  All threads of a process belong to the
+same cgroup.  On creation, all processes are put in the cgroup that
+the parent process belongs to at the time.  A process can be migrated
+to another cgroup.  Migration of a process doesn't affect already
+existing descendant processes.
+
+Following certain structural constraints, controllers may be enabled or
+disabled selectively on a cgroup.  All controller behaviors are
+hierarchical - if a controller is enabled on a cgroup, it affects all
+processes which belong to the cgroups consisting the inclusive
+sub-hierarchy of the cgroup.  When a controller is enabled on a nested
+cgroup, it always restricts the resource distribution further.  The
+restrictions set closer to the root in the hierarchy can not be
+overridden from further away.
+
+
+2. Basic Operations
+
+2-1. Mounting
+
+Unlike v1, cgroup v2 has only single hierarchy.  The cgroup v2
+hierarchy can be mounted with the following mount command.
+
+  # mount -t cgroup2 none $MOUNT_POINT
+
+cgroup2 filesystem has the magic number 0x63677270 ("cgrp").  All
+controllers which support v2 and are not bound to a v1 hierarchy are
+automatically bound to the v2 hierarchy and show up at the root.
+Controllers which are not in active use in the v2 hierarchy can be
+bound to other hierarchies.  This allows mixing v2 hierarchy with the
+legacy v1 multiple hierarchies in a fully backward compatible way.
+
+A controller can be moved across hierarchies only after the controller
+is no longer referenced in its current hierarchy.  Because per-cgroup
+controller states are destroyed asynchronously and controllers may
+have lingering references, a controller may not show up immediately on
+the v2 hierarchy after the final umount of the previous hierarchy.
+Similarly, a controller should be fully disabled to be moved out of
+the unified hierarchy and it may take some time for the disabled
+controller to become available for other hierarchies; furthermore, due
+to inter-controller dependencies, other controllers may need to be
+disabled too.
+
+While useful for development and manual configurations, moving
+controllers dynamically between the v2 and other hierarchies is
+strongly discouraged for production use.  It is recommended to decide
+the hierarchies and controller associations before starting using the
+controllers after system boot.
+
+
+2-2. Organizing Processes
+
+Initially, only the root cgroup exists to which all processes belong.
+A child cgroup can be created by creating a sub-directory.
+
+  # mkdir $CGROUP_NAME
+
+A given cgroup may have multiple child cgroups forming a tree
+structure.  Each cgroup has a read-writable interface file
+"cgroup.procs".  When read, it lists the PIDs of all processes which
+belong to the cgroup one-per-line.  The PIDs are not ordered and the
+same PID may show up more than once if the process got moved to
+another cgroup and then back or the PID got recycled while reading.
+
+A process can be migrated into a cgroup by writing its PID to the
+target cgroup's "cgroup.procs" file.  Only one process can be migrated
+on a single write(2) call.  If a process is composed of multiple
+threads, writing the PID of any thread migrates all threads of the
+process.
+
+When a process forks a child process, the new process is born into the
+cgroup that the forking process belongs to at the time of the
+operation.  After exit, a process stays associated with the cgroup
+that it belonged to at the time of exit until it's reaped; however, a
+zombie process does not appear in "cgroup.procs" and thus can't be
+moved to another cgroup.
+
+A cgroup which doesn't have any children or live processes can be
+destroyed by removing the directory.  Note that a cgroup which doesn't
+have any children and is associated only with zombie processes is
+considered empty and can be removed.
+
+  # rmdir $CGROUP_NAME
+
+"/proc/$PID/cgroup" lists a process's cgroup membership.  If legacy
+cgroup is in use in the system, this file may contain multiple lines,
+one for each hierarchy.  The entry for cgroup v2 is always in the
+format "0::$PATH".
+
+  # cat /proc/842/cgroup
+  ...
+  0::/test-cgroup/test-cgroup-nested
+
+If the process becomes a zombie and the cgroup it was associated with
+is removed subsequently, " (deleted)" is appended to the path.
+
+  # cat /proc/842/cgroup
+  ...
+  0::/test-cgroup/test-cgroup-nested (deleted)
+
+
+2-3. [Un]populated Notification
+
+Each non-root cgroup has a "cgroup.events" file which contains
+"populated" field indicating whether the cgroup's sub-hierarchy has
+live processes in it.  Its value is 0 if there is no live process in
+the cgroup and its descendants; otherwise, 1.  poll and [id]notify
+events are triggered when the value changes.  This can be used, for
+example, to start a clean-up operation after all processes of a given
+sub-hierarchy have exited.  The populated state updates and
+notifications are recursive.  Consider the following sub-hierarchy
+where the numbers in the parentheses represent the numbers of processes
+in each cgroup.
+
+  A(4) - B(0) - C(1)
+              \ D(0)
+
+A, B and C's "populated" fields would be 1 while D's 0.  After the one
+process in C exits, B and C's "populated" fields would flip to "0" and
+file modified events will be generated on the "cgroup.events" files of
+both cgroups.
+
+
+2-4. Controlling Controllers
+
+2-4-1. Enabling and Disabling
+
+Each cgroup has a "cgroup.controllers" file which lists all
+controllers available for the cgroup to enable.
+
+  # cat cgroup.controllers
+  cpu io memory
+
+No controller is enabled by default.  Controllers can be enabled and
+disabled by writing to the "cgroup.subtree_control" file.
+
+  # echo "+cpu +memory -io" > cgroup.subtree_control
+
+Only controllers which are listed in "cgroup.controllers" can be
+enabled.  When multiple operations are specified as above, either they
+all succeed or fail.  If multiple operations on the same controller
+are specified, the last one is effective.
+
+Enabling a controller in a cgroup indicates that the distribution of
+the target resource across its immediate children will be controlled.
+Consider the following sub-hierarchy.  The enabled controllers are
+listed in parentheses.
+
+  A(cpu,memory) - B(memory) - C()
+                            \ D()
+
+As A has "cpu" and "memory" enabled, A will control the distribution
+of CPU cycles and memory to its children, in this case, B.  As B has
+"memory" enabled but not "CPU", C and D will compete freely on CPU
+cycles but their division of memory available to B will be controlled.
+
+As a controller regulates the distribution of the target resource to
+the cgroup's children, enabling it creates the controller's interface
+files in the child cgroups.  In the above example, enabling "cpu" on B
+would create the "cpu." prefixed controller interface files in C and
+D.  Likewise, disabling "memory" from B would remove the "memory."
+prefixed controller interface files from C and D.  This means that the
+controller interface files - anything which doesn't start with
+"cgroup." are owned by the parent rather than the cgroup itself.
+
+
+2-4-2. Top-down Constraint
+
+Resources are distributed top-down and a cgroup can further distribute
+a resource only if the resource has been distributed to it from the
+parent.  This means that all non-root "cgroup.subtree_control" files
+can only contain controllers which are enabled in the parent's
+"cgroup.subtree_control" file.  A controller can be enabled only if
+the parent has the controller enabled and a controller can't be
+disabled if one or more children have it enabled.
+
+
+2-4-3. No Internal Process Constraint
+
+Non-root cgroups can only distribute resources to their children when
+they don't have any processes of their own.  In other words, only
+cgroups which don't contain any processes can have controllers enabled
+in their "cgroup.subtree_control" files.
+
+This guarantees that, when a controller is looking at the part of the
+hierarchy which has it enabled, processes are always only on the
+leaves.  This rules out situations where child cgroups compete against
+internal processes of the parent.
+
+The root cgroup is exempt from this restriction.  Root contains
+processes and anonymous resource consumption which can't be associated
+with any other cgroups and requires special treatment from most
+controllers.  How resource consumption in the root cgroup is governed
+is up to each controller.
+
+Note that the restriction doesn't get in the way if there is no
+enabled controller in the cgroup's "cgroup.subtree_control".  This is
+important as otherwise it wouldn't be possible to create children of a
+populated cgroup.  To control resource distribution of a cgroup, the
+cgroup must create children and transfer all its processes to the
+children before enabling controllers in its "cgroup.subtree_control"
+file.
+
+
+2-5. Delegation
+
+2-5-1. Model of Delegation
+
+A cgroup can be delegated to a less privileged user by granting write
+access of the directory and its "cgroup.procs" file to the user.  Note
+that resource control interface files in a given directory control the
+distribution of the parent's resources and thus must not be delegated
+along with the directory.
+
+Once delegated, the user can build sub-hierarchy under the directory,
+organize processes as it sees fit and further distribute the resources
+it received from the parent.  The limits and other settings of all
+resource controllers are hierarchical and regardless of what happens
+in the delegated sub-hierarchy, nothing can escape the resource
+restrictions imposed by the parent.
+
+Currently, cgroup doesn't impose any restrictions on the number of
+cgroups in or nesting depth of a delegated sub-hierarchy; however,
+this may be limited explicitly in the future.
+
+
+2-5-2. Delegation Containment
+
+A delegated sub-hierarchy is contained in the sense that processes
+can't be moved into or out of the sub-hierarchy by the delegatee.  For
+a process with a non-root euid to migrate a target process into a
+cgroup by writing its PID to the "cgroup.procs" file, the following
+conditions must be met.
+
+- The writer's euid must match either uid or suid of the target process.
+
+- The writer must have write access to the "cgroup.procs" file.
+
+- The writer must have write access to the "cgroup.procs" file of the
+  common ancestor of the source and destination cgroups.
+
+The above three constraints ensure that while a delegatee may migrate
+processes around freely in the delegated sub-hierarchy it can't pull
+in from or push out to outside the sub-hierarchy.
+
+For an example, let's assume cgroups C0 and C1 have been delegated to
+user U0 who created C00, C01 under C0 and C10 under C1 as follows and
+all processes under C0 and C1 belong to U0.
+
+  ~~~~~~~~~~~~~ - C0 - C00
+  ~ cgroup    ~      \ C01
+  ~ hierarchy ~
+  ~~~~~~~~~~~~~ - C1 - C10
+
+Let's also say U0 wants to write the PID of a process which is
+currently in C10 into "C00/cgroup.procs".  U0 has write access to the
+file and uid match on the process; however, the common ancestor of the
+source cgroup C10 and the destination cgroup C00 is above the points
+of delegation and U0 would not have write access to its "cgroup.procs"
+files and thus the write will be denied with -EACCES.
+
+
+2-6. Guidelines
+
+2-6-1. Organize Once and Control
+
+Migrating a process across cgroups is a relatively expensive operation
+and stateful resources such as memory are not moved together with the
+process.  This is an explicit design decision as there often exist
+inherent trade-offs between migration and various hot paths in terms
+of synchronization cost.
+
+As such, migrating processes across cgroups frequently as a means to
+apply different resource restrictions is discouraged.  A workload
+should be assigned to a cgroup according to the system's logical and
+resource structure once on start-up.  Dynamic adjustments to resource
+distribution can be made by changing controller configuration through
+the interface files.
+
+
+2-6-2. Avoid Name Collisions
+
+Interface files for a cgroup and its children cgroups occupy the same
+directory and it is possible to create children cgroups which collide
+with interface files.
+
+All cgroup core interface files are prefixed with "cgroup." and each
+controller's interface files are prefixed with the controller name and
+a dot.  A controller's name is composed of lower case alphabets and
+'_'s but never begins with an '_' so it can be used as the prefix
+character for collision avoidance.  Also, interface file names won't
+start or end with terms which are often used in categorizing workloads
+such as job, service, slice, unit or workload.
+
+cgroup doesn't do anything to prevent name collisions and it's the
+user's responsibility to avoid them.
+
+
+3. Resource Distribution Models
+
+cgroup controllers implement several resource distribution schemes
+depending on the resource type and expected use cases.  This section
+describes major schemes in use along with their expected behaviors.
+
+
+3-1. Weights
+
+A parent's resource is distributed by adding up the weights of all
+active children and giving each the fraction matching the ratio of its
+weight against the sum.  As only children which can make use of the
+resource at the moment participate in the distribution, this is
+work-conserving.  Due to the dynamic nature, this model is usually
+used for stateless resources.
+
+All weights are in the range [1, 10000] with the default at 100.  This
+allows symmetric multiplicative biases in both directions at fine
+enough granularity while staying in the intuitive range.
+
+As long as the weight is in range, all configuration combinations are
+valid and there is no reason to reject configuration changes or
+process migrations.
+
+"cpu.weight" proportionally distributes CPU cycles to active children
+and is an example of this type.
+
+
+3-2. Limits
+
+A child can only consume upto the configured amount of the resource.
+Limits can be over-committed - the sum of the limits of children can
+exceed the amount of resource available to the parent.
+
+Limits are in the range [0, max] and defaults to "max", which is noop.
+
+As limits can be over-committed, all configuration combinations are
+valid and there is no reason to reject configuration changes or
+process migrations.
+
+"io.max" limits the maximum BPS and/or IOPS that a cgroup can consume
+on an IO device and is an example of this type.
+
+
+3-3. Protections
+
+A cgroup is protected to be allocated upto the configured amount of
+the resource if the usages of all its ancestors are under their
+protected levels.  Protections can be hard guarantees or best effort
+soft boundaries.  Protections can also be over-committed in which case
+only upto the amount available to the parent is protected among
+children.
+
+Protections are in the range [0, max] and defaults to 0, which is
+noop.
+
+As protections can be over-committed, all configuration combinations
+are valid and there is no reason to reject configuration changes or
+process migrations.
+
+"memory.low" implements best-effort memory protection and is an
+example of this type.
+
+
+3-4. Allocations
+
+A cgroup is exclusively allocated a certain amount of a finite
+resource.  Allocations can't be over-committed - the sum of the
+allocations of children can not exceed the amount of resource
+available to the parent.
+
+Allocations are in the range [0, max] and defaults to 0, which is no
+resource.
+
+As allocations can't be over-committed, some configuration
+combinations are invalid and should be rejected.  Also, if the
+resource is mandatory for execution of processes, process migrations
+may be rejected.
+
+"cpu.rt.max" hard-allocates realtime slices and is an example of this
+type.
+
+
+4. Interface Files
+
+4-1. Format
+
+All interface files should be in one of the following formats whenever
+possible.
+
+  New-line separated values
+  (when only one value can be written at once)
+
+	VAL0\n
+	VAL1\n
+	...
+
+  Space separated values
+  (when read-only or multiple values can be written at once)
+
+	VAL0 VAL1 ...\n
+
+  Flat keyed
+
+	KEY0 VAL0\n
+	KEY1 VAL1\n
+	...
+
+  Nested keyed
+
+	KEY0 SUB_KEY0=VAL00 SUB_KEY1=VAL01...
+	KEY1 SUB_KEY0=VAL10 SUB_KEY1=VAL11...
+	...
+
+For a writable file, the format for writing should generally match
+reading; however, controllers may allow omitting later fields or
+implement restricted shortcuts for most common use cases.
+
+For both flat and nested keyed files, only the values for a single key
+can be written at a time.  For nested keyed files, the sub key pairs
+may be specified in any order and not all pairs have to be specified.
+
+
+4-2. Conventions
+
+- Settings for a single feature should be contained in a single file.
+
+- The root cgroup should be exempt from resource control and thus
+  shouldn't have resource control interface files.  Also,
+  informational files on the root cgroup which end up showing global
+  information available elsewhere shouldn't exist.
+
+- If a controller implements weight based resource distribution, its
+  interface file should be named "weight" and have the range [1,
+  10000] with 100 as the default.  The values are chosen to allow
+  enough and symmetric bias in both directions while keeping it
+  intuitive (the default is 100%).
+
+- If a controller implements an absolute resource guarantee and/or
+  limit, the interface files should be named "min" and "max"
+  respectively.  If a controller implements best effort resource
+  guarantee and/or limit, the interface files should be named "low"
+  and "high" respectively.
+
+  In the above four control files, the special token "max" should be
+  used to represent upward infinity for both reading and writing.
+
+- If a setting has a configurable default value and keyed specific
+  overrides, the default entry should be keyed with "default" and
+  appear as the first entry in the file.
+
+  The default value can be updated by writing either "default $VAL" or
+  "$VAL".
+
+  When writing to update a specific override, "default" can be used as
+  the value to indicate removal of the override.  Override entries
+  with "default" as the value must not appear when read.
+
+  For example, a setting which is keyed by major:minor device numbers
+  with integer values may look like the following.
+
+    # cat cgroup-example-interface-file
+    default 150
+    8:0 300
+
+  The default value can be updated by
+
+    # echo 125 > cgroup-example-interface-file
+
+  or
+
+    # echo "default 125" > cgroup-example-interface-file
+
+  An override can be set by
+
+    # echo "8:16 170" > cgroup-example-interface-file
+
+  and cleared by
+
+    # echo "8:0 default" > cgroup-example-interface-file
+    # cat cgroup-example-interface-file
+    default 125
+    8:16 170
+
+- For events which are not very high frequency, an interface file
+  "events" should be created which lists event key value pairs.
+  Whenever a notifiable event happens, file modified event should be
+  generated on the file.
+
+
+4-3. Core Interface Files
+
+All cgroup core files are prefixed with "cgroup."
+
+  cgroup.procs
+
+	A read-write new-line separated values file which exists on
+	all cgroups.
+
+	When read, it lists the PIDs of all processes which belong to
+	the cgroup one-per-line.  The PIDs are not ordered and the
+	same PID may show up more than once if the process got moved
+	to another cgroup and then back or the PID got recycled while
+	reading.
+
+	A PID can be written to migrate the process associated with
+	the PID to the cgroup.  The writer should match all of the
+	following conditions.
+
+	- Its euid is either root or must match either uid or suid of
+          the target process.
+
+	- It must have write access to the "cgroup.procs" file.
+
+	- It must have write access to the "cgroup.procs" file of the
+	  common ancestor of the source and destination cgroups.
+
+	When delegating a sub-hierarchy, write access to this file
+	should be granted along with the containing directory.
+
+  cgroup.controllers
+
+	A read-only space separated values file which exists on all
+	cgroups.
+
+	It shows space separated list of all controllers available to
+	the cgroup.  The controllers are not ordered.
+
+  cgroup.subtree_control
+
+	A read-write space separated values file which exists on all
+	cgroups.  Starts out empty.
+
+	When read, it shows space separated list of the controllers
+	which are enabled to control resource distribution from the
+	cgroup to its children.
+
+	Space separated list of controllers prefixed with '+' or '-'
+	can be written to enable or disable controllers.  A controller
+	name prefixed with '+' enables the controller and '-'
+	disables.  If a controller appears more than once on the list,
+	the last one is effective.  When multiple enable and disable
+	operations are specified, either all succeed or all fail.
+
+  cgroup.events
+
+	A read-only flat-keyed file which exists on non-root cgroups.
+	The following entries are defined.  Unless specified
+	otherwise, a value change in this file generates a file
+	modified event.
+
+	  populated
+
+		1 if the cgroup or its descendants contains any live
+		processes; otherwise, 0.
+
+
+5. Controllers
+
+5-1. CPU
+
+[NOTE: The interface for the cpu controller hasn't been merged yet]
+
+The "cpu" controllers regulates distribution of CPU cycles.  This
+controller implements weight and absolute bandwidth limit models for
+normal scheduling policy and absolute bandwidth allocation model for
+realtime scheduling policy.
+
+
+5-1-1. CPU Interface Files
+
+All time durations are in microseconds.
+
+  cpu.stat
+
+	A read-only flat-keyed file which exists on non-root cgroups.
+
+	It reports the following six stats.
+
+	  usage_usec
+	  user_usec
+	  system_usec
+	  nr_periods
+	  nr_throttled
+	  throttled_usec
+
+  cpu.weight
+
+	A read-write single value file which exists on non-root
+	cgroups.  The default is "100".
+
+	The weight in the range [1, 10000].
+
+  cpu.max
+
+	A read-write two value file which exists on non-root cgroups.
+	The default is "max 100000".
+
+	The maximum bandwidth limit.  It's in the following format.
+
+	  $MAX $PERIOD
+
+	which indicates that the group may consume upto $MAX in each
+	$PERIOD duration.  "max" for $MAX indicates no limit.  If only
+	one number is written, $MAX is updated.
+
+  cpu.rt.max
+
+  [NOTE: The semantics of this file is still under discussion and the
+   interface hasn't been merged yet]
+
+	A read-write two value file which exists on all cgroups.
+	The default is "0 100000".
+
+	The maximum realtime runtime allocation.  Over-committing
+	configurations are disallowed and process migrations are
+	rejected if not enough bandwidth is available.  It's in the
+	following format.
+
+	  $MAX $PERIOD
+
+	which indicates that the group may consume upto $MAX in each
+	$PERIOD duration.  If only one number is written, $MAX is
+	updated.
+
+
+5-2. Memory
+
+The "memory" controller regulates distribution of memory.  Memory is
+stateful and implements both limit and protection models.  Due to the
+intertwining between memory usage and reclaim pressure and the
+stateful nature of memory, the distribution model is relatively
+complex.
+
+While not completely water-tight, all major memory usages by a given
+cgroup are tracked so that the total memory consumption can be
+accounted and controlled to a reasonable extent.  Currently, the
+following types of memory usages are tracked.
+
+- Userland memory - page cache and anonymous memory.
+
+- Kernel data structures such as dentries and inodes.
+
+- TCP socket buffers.
+
+The above list may expand in the future for better coverage.
+
+
+5-2-1. Memory Interface Files
+
+All memory amounts are in bytes.  If a value which is not aligned to
+PAGE_SIZE is written, the value may be rounded up to the closest
+PAGE_SIZE multiple when read back.
+
+  memory.current
+
+	A read-only single value file which exists on non-root
+	cgroups.
+
+	The total amount of memory currently being used by the cgroup
+	and its descendants.
+
+  memory.low
+
+	A read-write single value file which exists on non-root
+	cgroups.  The default is "0".
+
+	Best-effort memory protection.  If the memory usages of a
+	cgroup and all its ancestors are below their low boundaries,
+	the cgroup's memory won't be reclaimed unless memory can be
+	reclaimed from unprotected cgroups.
+
+	Putting more memory than generally available under this
+	protection is discouraged.
+
+  memory.high
+
+	A read-write single value file which exists on non-root
+	cgroups.  The default is "max".
+
+	Memory usage throttle limit.  This is the main mechanism to
+	control memory usage of a cgroup.  If a cgroup's usage goes
+	over the high boundary, the processes of the cgroup are
+	throttled and put under heavy reclaim pressure.
+
+	Going over the high limit never invokes the OOM killer and
+	under extreme conditions the limit may be breached.
+
+  memory.max
+
+	A read-write single value file which exists on non-root
+	cgroups.  The default is "max".
+
+	Memory usage hard limit.  This is the final protection
+	mechanism.  If a cgroup's memory usage reaches this limit and
+	can't be reduced, the OOM killer is invoked in the cgroup.
+	Under certain circumstances, the usage may go over the limit
+	temporarily.
+
+	This is the ultimate protection mechanism.  As long as the
+	high limit is used and monitored properly, this limit's
+	utility is limited to providing the final safety net.
+
+  memory.events
+
+	A read-only flat-keyed file which exists on non-root cgroups.
+	The following entries are defined.  Unless specified
+	otherwise, a value change in this file generates a file
+	modified event.
+
+	  low
+
+		The number of times the cgroup is reclaimed due to
+		high memory pressure even though its usage is under
+		the low boundary.  This usually indicates that the low
+		boundary is over-committed.
+
+	  high
+
+		The number of times processes of the cgroup are
+		throttled and routed to perform direct memory reclaim
+		because the high memory boundary was exceeded.  For a
+		cgroup whose memory usage is capped by the high limit
+		rather than global memory pressure, this event's
+		occurrences are expected.
+
+	  max
+
+		The number of times the cgroup's memory usage was
+		about to go over the max boundary.  If direct reclaim
+		fails to bring it down, the OOM killer is invoked.
+
+	  oom
+
+		The number of times the OOM killer has been invoked in
+		the cgroup.  This may not exactly match the number of
+		processes killed but should generally be close.
+
+  memory.stat
+
+	A read-only flat-keyed file which exists on non-root cgroups.
+
+	This breaks down the cgroup's memory footprint into different
+	types of memory, type-specific details, and other information
+	on the state and past events of the memory management system.
+
+	All memory amounts are in bytes.
+
+	The entries are ordered to be human readable, and new entries
+	can show up in the middle. Don't rely on items remaining in a
+	fixed position; use the keys to look up specific values!
+
+	  anon
+
+		Amount of memory used in anonymous mappings such as
+		brk(), sbrk(), and mmap(MAP_ANONYMOUS)
+
+	  file
+
+		Amount of memory used to cache filesystem data,
+		including tmpfs and shared memory.
+
+	  sock
+
+		Amount of memory used in network transmission buffers
+
+	  file_mapped
+
+		Amount of cached filesystem data mapped with mmap()
+
+	  file_dirty
+
+		Amount of cached filesystem data that was modified but
+		not yet written back to disk
+
+	  file_writeback
+
+		Amount of cached filesystem data that was modified and
+		is currently being written back to disk
+
+	  inactive_anon
+	  active_anon
+	  inactive_file
+	  active_file
+	  unevictable
+
+		Amount of memory, swap-backed and filesystem-backed,
+		on the internal memory management lists used by the
+		page reclaim algorithm
+
+	  pgfault
+
+		Total number of page faults incurred
+
+	  pgmajfault
+
+		Number of major page faults incurred
+
+  memory.swap.current
+
+	A read-only single value file which exists on non-root
+	cgroups.
+
+	The total amount of swap currently being used by the cgroup
+	and its descendants.
+
+  memory.swap.max
+
+	A read-write single value file which exists on non-root
+	cgroups.  The default is "max".
+
+	Swap usage hard limit.  If a cgroup's swap usage reaches this
+	limit, anonymous meomry of the cgroup will not be swapped out.
+
+
+5-2-2. General Usage
+
+"memory.high" is the main mechanism to control memory usage.
+Over-committing on high limit (sum of high limits > available memory)
+and letting global memory pressure to distribute memory according to
+usage is a viable strategy.
+
+Because breach of the high limit doesn't trigger the OOM killer but
+throttles the offending cgroup, a management agent has ample
+opportunities to monitor and take appropriate actions such as granting
+more memory or terminating the workload.
+
+Determining whether a cgroup has enough memory is not trivial as
+memory usage doesn't indicate whether the workload can benefit from
+more memory.  For example, a workload which writes data received from
+network to a file can use all available memory but can also operate as
+performant with a small amount of memory.  A measure of memory
+pressure - how much the workload is being impacted due to lack of
+memory - is necessary to determine whether a workload needs more
+memory; unfortunately, memory pressure monitoring mechanism isn't
+implemented yet.
+
+
+5-2-3. Memory Ownership
+
+A memory area is charged to the cgroup which instantiated it and stays
+charged to the cgroup until the area is released.  Migrating a process
+to a different cgroup doesn't move the memory usages that it
+instantiated while in the previous cgroup to the new cgroup.
+
+A memory area may be used by processes belonging to different cgroups.
+To which cgroup the area will be charged is in-deterministic; however,
+over time, the memory area is likely to end up in a cgroup which has
+enough memory allowance to avoid high reclaim pressure.
+
+If a cgroup sweeps a considerable amount of memory which is expected
+to be accessed repeatedly by other cgroups, it may make sense to use
+POSIX_FADV_DONTNEED to relinquish the ownership of memory areas
+belonging to the affected files to ensure correct memory ownership.
+
+
+5-3. IO
+
+The "io" controller regulates the distribution of IO resources.  This
+controller implements both weight based and absolute bandwidth or IOPS
+limit distribution; however, weight based distribution is available
+only if cfq-iosched is in use and neither scheme is available for
+blk-mq devices.
+
+
+5-3-1. IO Interface Files
+
+  io.stat
+
+	A read-only nested-keyed file which exists on non-root
+	cgroups.
+
+	Lines are keyed by $MAJ:$MIN device numbers and not ordered.
+	The following nested keys are defined.
+
+	  rbytes	Bytes read
+	  wbytes	Bytes written
+	  rios		Number of read IOs
+	  wios		Number of write IOs
+
+	An example read output follows.
+
+	  8:16 rbytes=1459200 wbytes=314773504 rios=192 wios=353
+	  8:0 rbytes=90430464 wbytes=299008000 rios=8950 wios=1252
+
+  io.weight
+
+	A read-write flat-keyed file which exists on non-root cgroups.
+	The default is "default 100".
+
+	The first line is the default weight applied to devices
+	without specific override.  The rest are overrides keyed by
+	$MAJ:$MIN device numbers and not ordered.  The weights are in
+	the range [1, 10000] and specifies the relative amount IO time
+	the cgroup can use in relation to its siblings.
+
+	The default weight can be updated by writing either "default
+	$WEIGHT" or simply "$WEIGHT".  Overrides can be set by writing
+	"$MAJ:$MIN $WEIGHT" and unset by writing "$MAJ:$MIN default".
+
+	An example read output follows.
+
+	  default 100
+	  8:16 200
+	  8:0 50
+
+  io.max
+
+	A read-write nested-keyed file which exists on non-root
+	cgroups.
+
+	BPS and IOPS based IO limit.  Lines are keyed by $MAJ:$MIN
+	device numbers and not ordered.  The following nested keys are
+	defined.
+
+	  rbps		Max read bytes per second
+	  wbps		Max write bytes per second
+	  riops		Max read IO operations per second
+	  wiops		Max write IO operations per second
+
+	When writing, any number of nested key-value pairs can be
+	specified in any order.  "max" can be specified as the value
+	to remove a specific limit.  If the same key is specified
+	multiple times, the outcome is undefined.
+
+	BPS and IOPS are measured in each IO direction and IOs are
+	delayed if limit is reached.  Temporary bursts are allowed.
+
+	Setting read limit at 2M BPS and write at 120 IOPS for 8:16.
+
+	  echo "8:16 rbps=2097152 wiops=120" > io.max
+
+	Reading returns the following.
+
+	  8:16 rbps=2097152 wbps=max riops=max wiops=120
+
+	Write IOPS limit can be removed by writing the following.
+
+	  echo "8:16 wiops=max" > io.max
+
+	Reading now returns the following.
+
+	  8:16 rbps=2097152 wbps=max riops=max wiops=max
+
+
+5-3-2. Writeback
+
+Page cache is dirtied through buffered writes and shared mmaps and
+written asynchronously to the backing filesystem by the writeback
+mechanism.  Writeback sits between the memory and IO domains and
+regulates the proportion of dirty memory by balancing dirtying and
+write IOs.
+
+The io controller, in conjunction with the memory controller,
+implements control of page cache writeback IOs.  The memory controller
+defines the memory domain that dirty memory ratio is calculated and
+maintained for and the io controller defines the io domain which
+writes out dirty pages for the memory domain.  Both system-wide and
+per-cgroup dirty memory states are examined and the more restrictive
+of the two is enforced.
+
+cgroup writeback requires explicit support from the underlying
+filesystem.  Currently, cgroup writeback is implemented on ext2, ext4
+and btrfs.  On other filesystems, all writeback IOs are attributed to
+the root cgroup.
+
+There are inherent differences in memory and writeback management
+which affects how cgroup ownership is tracked.  Memory is tracked per
+page while writeback per inode.  For the purpose of writeback, an
+inode is assigned to a cgroup and all IO requests to write dirty pages
+from the inode are attributed to that cgroup.
+
+As cgroup ownership for memory is tracked per page, there can be pages
+which are associated with different cgroups than the one the inode is
+associated with.  These are called foreign pages.  The writeback
+constantly keeps track of foreign pages and, if a particular foreign
+cgroup becomes the majority over a certain period of time, switches
+the ownership of the inode to that cgroup.
+
+While this model is enough for most use cases where a given inode is
+mostly dirtied by a single cgroup even when the main writing cgroup
+changes over time, use cases where multiple cgroups write to a single
+inode simultaneously are not supported well.  In such circumstances, a
+significant portion of IOs are likely to be attributed incorrectly.
+As memory controller assigns page ownership on the first use and
+doesn't update it until the page is released, even if writeback
+strictly follows page ownership, multiple cgroups dirtying overlapping
+areas wouldn't work as expected.  It's recommended to avoid such usage
+patterns.
+
+The sysctl knobs which affect writeback behavior are applied to cgroup
+writeback as follows.
+
+  vm.dirty_background_ratio
+  vm.dirty_ratio
+
+	These ratios apply the same to cgroup writeback with the
+	amount of available memory capped by limits imposed by the
+	memory controller and system-wide clean memory.
+
+  vm.dirty_background_bytes
+  vm.dirty_bytes
+
+	For cgroup writeback, this is calculated into ratio against
+	total available memory and applied the same way as
+	vm.dirty[_background]_ratio.
+
+
+P. Information on Kernel Programming
+
+This section contains kernel programming information in the areas
+where interacting with cgroup is necessary.  cgroup core and
+controllers are not covered.
+
+
+P-1. Filesystem Support for Writeback
+
+A filesystem can support cgroup writeback by updating
+address_space_operations->writepage[s]() to annotate bio's using the
+following two functions.
+
+  wbc_init_bio(@wbc, @bio)
+
+	Should be called for each bio carrying writeback data and
+	associates the bio with the inode's owner cgroup.  Can be
+	called anytime between bio allocation and submission.
+
+  wbc_account_io(@wbc, @page, @bytes)
+
+	Should be called for each data segment being written out.
+	While this function doesn't care exactly when it's called
+	during the writeback session, it's the easiest and most
+	natural to call it as data segments are added to a bio.
+
+With writeback bio's annotated, cgroup support can be enabled per
+super_block by setting SB_I_CGROUPWB in ->s_iflags.  This allows for
+selective disabling of cgroup writeback support which is helpful when
+certain filesystem features, e.g. journaled data mode, are
+incompatible.
+
+wbc_init_bio() binds the specified bio to its cgroup.  Depending on
+the configuration, the bio may be executed at a lower priority and if
+the writeback session is holding shared resources, e.g. a journal
+entry, may lead to priority inversion.  There is no one easy solution
+for the problem.  Filesystems can try to work around specific problem
+cases by skipping wbc_init_bio() or using bio_associate_blkcg()
+directly.
+
+
+D. Deprecated v1 Core Features
+
+- Multiple hierarchies including named ones are not supported.
+
+- All mount options and remounting are not supported.
+
+- The "tasks" file is removed and "cgroup.procs" is not sorted.
+
+- "cgroup.clone_children" is removed.
+
+- /proc/cgroups is meaningless for v2.  Use "cgroup.controllers" file
+  at the root instead.
+
+
+R. Issues with v1 and Rationales for v2
+
+R-1. Multiple Hierarchies
+
+cgroup v1 allowed an arbitrary number of hierarchies and each
+hierarchy could host any number of controllers.  While this seemed to
+provide a high level of flexibility, it wasn't useful in practice.
+
+For example, as there is only one instance of each controller, utility
+type controllers such as freezer which can be useful in all
+hierarchies could only be used in one.  The issue is exacerbated by
+the fact that controllers couldn't be moved to another hierarchy once
+hierarchies were populated.  Another issue was that all controllers
+bound to a hierarchy were forced to have exactly the same view of the
+hierarchy.  It wasn't possible to vary the granularity depending on
+the specific controller.
+
+In practice, these issues heavily limited which controllers could be
+put on the same hierarchy and most configurations resorted to putting
+each controller on its own hierarchy.  Only closely related ones, such
+as the cpu and cpuacct controllers, made sense to be put on the same
+hierarchy.  This often meant that userland ended up managing multiple
+similar hierarchies repeating the same steps on each hierarchy
+whenever a hierarchy management operation was necessary.
+
+Furthermore, support for multiple hierarchies came at a steep cost.
+It greatly complicated cgroup core implementation but more importantly
+the support for multiple hierarchies restricted how cgroup could be
+used in general and what controllers was able to do.
+
+There was no limit on how many hierarchies there might be, which meant
+that a thread's cgroup membership couldn't be described in finite
+length.  The key might contain any number of entries and was unlimited
+in length, which made it highly awkward to manipulate and led to
+addition of controllers which existed only to identify membership,
+which in turn exacerbated the original problem of proliferating number
+of hierarchies.
+
+Also, as a controller couldn't have any expectation regarding the
+topologies of hierarchies other controllers might be on, each
+controller had to assume that all other controllers were attached to
+completely orthogonal hierarchies.  This made it impossible, or at
+least very cumbersome, for controllers to cooperate with each other.
+
+In most use cases, putting controllers on hierarchies which are
+completely orthogonal to each other isn't necessary.  What usually is
+called for is the ability to have differing levels of granularity
+depending on the specific controller.  In other words, hierarchy may
+be collapsed from leaf towards root when viewed from specific
+controllers.  For example, a given configuration might not care about
+how memory is distributed beyond a certain level while still wanting
+to control how CPU cycles are distributed.
+
+
+R-2. Thread Granularity
+
+cgroup v1 allowed threads of a process to belong to different cgroups.
+This didn't make sense for some controllers and those controllers
+ended up implementing different ways to ignore such situations but
+much more importantly it blurred the line between API exposed to
+individual applications and system management interface.
+
+Generally, in-process knowledge is available only to the process
+itself; thus, unlike service-level organization of processes,
+categorizing threads of a process requires active participation from
+the application which owns the target process.
+
+cgroup v1 had an ambiguously defined delegation model which got abused
+in combination with thread granularity.  cgroups were delegated to
+individual applications so that they can create and manage their own
+sub-hierarchies and control resource distributions along them.  This
+effectively raised cgroup to the status of a syscall-like API exposed
+to lay programs.
+
+First of all, cgroup has a fundamentally inadequate interface to be
+exposed this way.  For a process to access its own knobs, it has to
+extract the path on the target hierarchy from /proc/self/cgroup,
+construct the path by appending the name of the knob to the path, open
+and then read and/or write to it.  This is not only extremely clunky
+and unusual but also inherently racy.  There is no conventional way to
+define transaction across the required steps and nothing can guarantee
+that the process would actually be operating on its own sub-hierarchy.
+
+cgroup controllers implemented a number of knobs which would never be
+accepted as public APIs because they were just adding control knobs to
+system-management pseudo filesystem.  cgroup ended up with interface
+knobs which were not properly abstracted or refined and directly
+revealed kernel internal details.  These knobs got exposed to
+individual applications through the ill-defined delegation mechanism
+effectively abusing cgroup as a shortcut to implementing public APIs
+without going through the required scrutiny.
+
+This was painful for both userland and kernel.  Userland ended up with
+misbehaving and poorly abstracted interfaces and kernel exposing and
+locked into constructs inadvertently.
+
+
+R-3. Competition Between Inner Nodes and Threads
+
+cgroup v1 allowed threads to be in any cgroups which created an
+interesting problem where threads belonging to a parent cgroup and its
+children cgroups competed for resources.  This was nasty as two
+different types of entities competed and there was no obvious way to
+settle it.  Different controllers did different things.
+
+The cpu controller considered threads and cgroups as equivalents and
+mapped nice levels to cgroup weights.  This worked for some cases but
+fell flat when children wanted to be allocated specific ratios of CPU
+cycles and the number of internal threads fluctuated - the ratios
+constantly changed as the number of competing entities fluctuated.
+There also were other issues.  The mapping from nice level to weight
+wasn't obvious or universal, and there were various other knobs which
+simply weren't available for threads.
+
+The io controller implicitly created a hidden leaf node for each
+cgroup to host the threads.  The hidden leaf had its own copies of all
+the knobs with "leaf_" prefixed.  While this allowed equivalent
+control over internal threads, it was with serious drawbacks.  It
+always added an extra layer of nesting which wouldn't be necessary
+otherwise, made the interface messy and significantly complicated the
+implementation.
+
+The memory controller didn't have a way to control what happened
+between internal tasks and child cgroups and the behavior was not
+clearly defined.  There were attempts to add ad-hoc behaviors and
+knobs to tailor the behavior to specific workloads which would have
+led to problems extremely difficult to resolve in the long term.
+
+Multiple controllers struggled with internal tasks and came up with
+different ways to deal with it; unfortunately, all the approaches were
+severely flawed and, furthermore, the widely different behaviors
+made cgroup as a whole highly inconsistent.
+
+This clearly is a problem which needs to be addressed from cgroup core
+in a uniform way.
+
+
+R-4. Other Interface Issues
+
+cgroup v1 grew without oversight and developed a large number of
+idiosyncrasies and inconsistencies.  One issue on the cgroup core side
+was how an empty cgroup was notified - a userland helper binary was
+forked and executed for each event.  The event delivery wasn't
+recursive or delegatable.  The limitations of the mechanism also led
+to in-kernel event delivery filtering mechanism further complicating
+the interface.
+
+Controller interfaces were problematic too.  An extreme example is
+controllers completely ignoring hierarchical organization and treating
+all cgroups as if they were all located directly under the root
+cgroup.  Some controllers exposed a large amount of inconsistent
+implementation details to userland.
+
+There also was no consistency across controllers.  When a new cgroup
+was created, some controllers defaulted to not imposing extra
+restrictions while others disallowed any resource usage until
+explicitly configured.  Configuration knobs for the same type of
+control used widely differing naming schemes and formats.  Statistics
+and information knobs were named arbitrarily and used different
+formats and units even in the same controller.
+
+cgroup v2 establishes common conventions where appropriate and updates
+controllers so that they expose minimal and consistent interfaces.
+
+
+R-5. Controller Issues and Remedies
+
+R-5-1. Memory
+
+The original lower boundary, the soft limit, is defined as a limit
+that is per default unset.  As a result, the set of cgroups that
+global reclaim prefers is opt-in, rather than opt-out.  The costs for
+optimizing these mostly negative lookups are so high that the
+implementation, despite its enormous size, does not even provide the
+basic desirable behavior.  First off, the soft limit has no
+hierarchical meaning.  All configured groups are organized in a global
+rbtree and treated like equal peers, regardless where they are located
+in the hierarchy.  This makes subtree delegation impossible.  Second,
+the soft limit reclaim pass is so aggressive that it not just
+introduces high allocation latencies into the system, but also impacts
+system performance due to overreclaim, to the point where the feature
+becomes self-defeating.
+
+The memory.low boundary on the other hand is a top-down allocated
+reserve.  A cgroup enjoys reclaim protection when it and all its
+ancestors are below their low boundaries, which makes delegation of
+subtrees possible.  Secondly, new cgroups have no reserve per default
+and in the common case most cgroups are eligible for the preferred
+reclaim pass.  This allows the new low boundary to be efficiently
+implemented with just a minor addition to the generic reclaim code,
+without the need for out-of-band data structures and reclaim passes.
+Because the generic reclaim code considers all cgroups except for the
+ones running low in the preferred first reclaim pass, overreclaim of
+individual groups is eliminated as well, resulting in much better
+overall workload performance.
+
+The original high boundary, the hard limit, is defined as a strict
+limit that can not budge, even if the OOM killer has to be called.
+But this generally goes against the goal of making the most out of the
+available memory.  The memory consumption of workloads varies during
+runtime, and that requires users to overcommit.  But doing that with a
+strict upper limit requires either a fairly accurate prediction of the
+working set size or adding slack to the limit.  Since working set size
+estimation is hard and error prone, and getting it wrong results in
+OOM kills, most users tend to err on the side of a looser limit and
+end up wasting precious resources.
+
+The memory.high boundary on the other hand can be set much more
+conservatively.  When hit, it throttles allocations by forcing them
+into direct reclaim to work off the excess, but it never invokes the
+OOM killer.  As a result, a high boundary that is chosen too
+aggressively will not terminate the processes, but instead it will
+lead to gradual performance degradation.  The user can monitor this
+and make corrections until the minimal memory footprint that still
+gives acceptable performance is found.
+
+In extreme cases, with many concurrent allocations and a complete
+breakdown of reclaim progress within the group, the high boundary can
+be exceeded.  But even then it's mostly better to satisfy the
+allocation from the slack available in other groups or the rest of the
+system than killing the group.  Otherwise, memory.max is there to
+limit this type of spillover and ultimately contain buggy or even
+malicious applications.
+
+The combined memory+swap accounting and limiting is replaced by real
+control over swap space.
+
+The main argument for a combined memory+swap facility in the original
+cgroup design was that global or parental pressure would always be
+able to swap all anonymous memory of a child group, regardless of the
+child's own (possibly untrusted) configuration.  However, untrusted
+groups can sabotage swapping by other means - such as referencing its
+anonymous memory in a tight loop - and an admin can not assume full
+swappability when overcommitting untrusted jobs.
+
+For trusted jobs, on the other hand, a combined counter is not an
+intuitive userspace interface, and it flies in the face of the idea
+that cgroup controllers should account and limit specific physical
+resources.  Swap space is a resource like all others in the system,
+and that's why unified hierarchy allows distributing it separately.
diff --git a/Documentation/cgroups/00-INDEX b/Documentation/cgroups/00-INDEX
deleted file mode 100644
index 3f5a40f..0000000
--- a/Documentation/cgroups/00-INDEX
+++ /dev/null
@@ -1,30 +0,0 @@
-00-INDEX
-	- this file
-blkio-controller.txt
-	- Description for Block IO Controller, implementation and usage details.
-cgroups.txt
-	- Control Groups definition, implementation details, examples and API.
-cpuacct.txt
-	- CPU Accounting Controller; account CPU usage for groups of tasks.
-cpusets.txt
-	- documents the cpusets feature; assign CPUs and Mem to a set of tasks.
-devices.txt
-	- Device Whitelist Controller; description, interface and security.
-freezer-subsystem.txt
-	- checkpointing; rationale to not use signals, interface.
-hugetlb.txt
-	- HugeTLB Controller implementation and usage details.
-memcg_test.txt
-	- Memory Resource Controller; implementation details.
-memory.txt
-	- Memory Resource Controller; design, accounting, interface, testing.
-net_cls.txt
-	- Network classifier cgroups details and usages.
-net_prio.txt
-	- Network priority cgroups details and usages.
-pids.txt
-	- Process number cgroups details and usages.
-resource_counter.txt
-	- Resource Counter API.
-unified-hierarchy.txt
-	- Description the new/next cgroup interface.
diff --git a/Documentation/cgroups/blkio-controller.txt b/Documentation/cgroups/blkio-controller.txt
deleted file mode 100644
index 52fa9f3..0000000
--- a/Documentation/cgroups/blkio-controller.txt
+++ /dev/null
@@ -1,455 +0,0 @@
-				Block IO Controller
-				===================
-Overview
-========
-cgroup subsys "blkio" implements the block io controller. There seems to be
-a need of various kinds of IO control policies (like proportional BW, max BW)
-both at leaf nodes as well as at intermediate nodes in a storage hierarchy.
-Plan is to use the same cgroup based management interface for blkio controller
-and based on user options switch IO policies in the background.
-
-Currently two IO control policies are implemented. First one is proportional
-weight time based division of disk policy. It is implemented in CFQ. Hence
-this policy takes effect only on leaf nodes when CFQ is being used. The second
-one is throttling policy which can be used to specify upper IO rate limits
-on devices. This policy is implemented in generic block layer and can be
-used on leaf nodes as well as higher level logical devices like device mapper.
-
-HOWTO
-=====
-Proportional Weight division of bandwidth
------------------------------------------
-You can do a very simple testing of running two dd threads in two different
-cgroups. Here is what you can do.
-
-- Enable Block IO controller
-	CONFIG_BLK_CGROUP=y
-
-- Enable group scheduling in CFQ
-	CONFIG_CFQ_GROUP_IOSCHED=y
-
-- Compile and boot into kernel and mount IO controller (blkio); see
-  cgroups.txt, Why are cgroups needed?.
-
-	mount -t tmpfs cgroup_root /sys/fs/cgroup
-	mkdir /sys/fs/cgroup/blkio
-	mount -t cgroup -o blkio none /sys/fs/cgroup/blkio
-
-- Create two cgroups
-	mkdir -p /sys/fs/cgroup/blkio/test1/ /sys/fs/cgroup/blkio/test2
-
-- Set weights of group test1 and test2
-	echo 1000 > /sys/fs/cgroup/blkio/test1/blkio.weight
-	echo 500 > /sys/fs/cgroup/blkio/test2/blkio.weight
-
-- Create two same size files (say 512MB each) on same disk (file1, file2) and
-  launch two dd threads in different cgroup to read those files.
-
-	sync
-	echo 3 > /proc/sys/vm/drop_caches
-
-	dd if=/mnt/sdb/zerofile1 of=/dev/null &
-	echo $! > /sys/fs/cgroup/blkio/test1/tasks
-	cat /sys/fs/cgroup/blkio/test1/tasks
-
-	dd if=/mnt/sdb/zerofile2 of=/dev/null &
-	echo $! > /sys/fs/cgroup/blkio/test2/tasks
-	cat /sys/fs/cgroup/blkio/test2/tasks
-
-- At macro level, first dd should finish first. To get more precise data, keep
-  on looking at (with the help of script), at blkio.disk_time and
-  blkio.disk_sectors files of both test1 and test2 groups. This will tell how
-  much disk time (in milliseconds), each group got and how many sectors each
-  group dispatched to the disk. We provide fairness in terms of disk time, so
-  ideally io.disk_time of cgroups should be in proportion to the weight.
-
-Throttling/Upper Limit policy
------------------------------
-- Enable Block IO controller
-	CONFIG_BLK_CGROUP=y
-
-- Enable throttling in block layer
-	CONFIG_BLK_DEV_THROTTLING=y
-
-- Mount blkio controller (see cgroups.txt, Why are cgroups needed?)
-        mount -t cgroup -o blkio none /sys/fs/cgroup/blkio
-
-- Specify a bandwidth rate on particular device for root group. The format
-  for policy is "<major>:<minor>  <bytes_per_second>".
-
-        echo "8:16  1048576" > /sys/fs/cgroup/blkio/blkio.throttle.read_bps_device
-
-  Above will put a limit of 1MB/second on reads happening for root group
-  on device having major/minor number 8:16.
-
-- Run dd to read a file and see if rate is throttled to 1MB/s or not.
-
-		# dd if=/mnt/common/zerofile of=/dev/null bs=4K count=1024
-		# iflag=direct
-        1024+0 records in
-        1024+0 records out
-        4194304 bytes (4.2 MB) copied, 4.0001 s, 1.0 MB/s
-
- Limits for writes can be put using blkio.throttle.write_bps_device file.
-
-Hierarchical Cgroups
-====================
-
-Both CFQ and throttling implement hierarchy support; however,
-throttling's hierarchy support is enabled iff "sane_behavior" is
-enabled from cgroup side, which currently is a development option and
-not publicly available.
-
-If somebody created a hierarchy like as follows.
-
-			root
-			/  \
-		     test1 test2
-			|
-		     test3
-
-CFQ by default and throttling with "sane_behavior" will handle the
-hierarchy correctly.  For details on CFQ hierarchy support, refer to
-Documentation/block/cfq-iosched.txt.  For throttling, all limits apply
-to the whole subtree while all statistics are local to the IOs
-directly generated by tasks in that cgroup.
-
-Throttling without "sane_behavior" enabled from cgroup side will
-practically treat all groups at same level as if it looks like the
-following.
-
-				pivot
-			     /  /   \  \
-			root  test1 test2  test3
-
-Various user visible config options
-===================================
-CONFIG_BLK_CGROUP
-	- Block IO controller.
-
-CONFIG_DEBUG_BLK_CGROUP
-	- Debug help. Right now some additional stats file show up in cgroup
-	  if this option is enabled.
-
-CONFIG_CFQ_GROUP_IOSCHED
-	- Enables group scheduling in CFQ. Currently only 1 level of group
-	  creation is allowed.
-
-CONFIG_BLK_DEV_THROTTLING
-	- Enable block device throttling support in block layer.
-
-Details of cgroup files
-=======================
-Proportional weight policy files
---------------------------------
-- blkio.weight
-	- Specifies per cgroup weight. This is default weight of the group
-	  on all the devices until and unless overridden by per device rule.
-	  (See blkio.weight_device).
-	  Currently allowed range of weights is from 10 to 1000.
-
-- blkio.weight_device
-	- One can specify per cgroup per device rules using this interface.
-	  These rules override the default value of group weight as specified
-	  by blkio.weight.
-
-	  Following is the format.
-
-	  # echo dev_maj:dev_minor weight > blkio.weight_device
-	  Configure weight=300 on /dev/sdb (8:16) in this cgroup
-	  # echo 8:16 300 > blkio.weight_device
-	  # cat blkio.weight_device
-	  dev     weight
-	  8:16    300
-
-	  Configure weight=500 on /dev/sda (8:0) in this cgroup
-	  # echo 8:0 500 > blkio.weight_device
-	  # cat blkio.weight_device
-	  dev     weight
-	  8:0     500
-	  8:16    300
-
-	  Remove specific weight for /dev/sda in this cgroup
-	  # echo 8:0 0 > blkio.weight_device
-	  # cat blkio.weight_device
-	  dev     weight
-	  8:16    300
-
-- blkio.leaf_weight[_device]
-	- Equivalents of blkio.weight[_device] for the purpose of
-          deciding how much weight tasks in the given cgroup has while
-          competing with the cgroup's child cgroups. For details,
-          please refer to Documentation/block/cfq-iosched.txt.
-
-- blkio.time
-	- disk time allocated to cgroup per device in milliseconds. First
-	  two fields specify the major and minor number of the device and
-	  third field specifies the disk time allocated to group in
-	  milliseconds.
-
-- blkio.sectors
-	- number of sectors transferred to/from disk by the group. First
-	  two fields specify the major and minor number of the device and
-	  third field specifies the number of sectors transferred by the
-	  group to/from the device.
-
-- blkio.io_service_bytes
-	- Number of bytes transferred to/from the disk by the group. These
-	  are further divided by the type of operation - read or write, sync
-	  or async. First two fields specify the major and minor number of the
-	  device, third field specifies the operation type and the fourth field
-	  specifies the number of bytes.
-
-- blkio.io_serviced
-	- Number of IOs (bio) issued to the disk by the group. These
-	  are further divided by the type of operation - read or write, sync
-	  or async. First two fields specify the major and minor number of the
-	  device, third field specifies the operation type and the fourth field
-	  specifies the number of IOs.
-
-- blkio.io_service_time
-	- Total amount of time between request dispatch and request completion
-	  for the IOs done by this cgroup. This is in nanoseconds to make it
-	  meaningful for flash devices too. For devices with queue depth of 1,
-	  this time represents the actual service time. When queue_depth > 1,
-	  that is no longer true as requests may be served out of order. This
-	  may cause the service time for a given IO to include the service time
-	  of multiple IOs when served out of order which may result in total
-	  io_service_time > actual time elapsed. This time is further divided by
-	  the type of operation - read or write, sync or async. First two fields
-	  specify the major and minor number of the device, third field
-	  specifies the operation type and the fourth field specifies the
-	  io_service_time in ns.
-
-- blkio.io_wait_time
-	- Total amount of time the IOs for this cgroup spent waiting in the
-	  scheduler queues for service. This can be greater than the total time
-	  elapsed since it is cumulative io_wait_time for all IOs. It is not a
-	  measure of total time the cgroup spent waiting but rather a measure of
-	  the wait_time for its individual IOs. For devices with queue_depth > 1
-	  this metric does not include the time spent waiting for service once
-	  the IO is dispatched to the device but till it actually gets serviced
-	  (there might be a time lag here due to re-ordering of requests by the
-	  device). This is in nanoseconds to make it meaningful for flash
-	  devices too. This time is further divided by the type of operation -
-	  read or write, sync or async. First two fields specify the major and
-	  minor number of the device, third field specifies the operation type
-	  and the fourth field specifies the io_wait_time in ns.
-
-- blkio.io_merged
-	- Total number of bios/requests merged into requests belonging to this
-	  cgroup. This is further divided by the type of operation - read or
-	  write, sync or async.
-
-- blkio.io_queued
-	- Total number of requests queued up at any given instant for this
-	  cgroup. This is further divided by the type of operation - read or
-	  write, sync or async.
-
-- blkio.avg_queue_size
-	- Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y.
-	  The average queue size for this cgroup over the entire time of this
-	  cgroup's existence. Queue size samples are taken each time one of the
-	  queues of this cgroup gets a timeslice.
-
-- blkio.group_wait_time
-	- Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y.
-	  This is the amount of time the cgroup had to wait since it became busy
-	  (i.e., went from 0 to 1 request queued) to get a timeslice for one of
-	  its queues. This is different from the io_wait_time which is the
-	  cumulative total of the amount of time spent by each IO in that cgroup
-	  waiting in the scheduler queue. This is in nanoseconds. If this is
-	  read when the cgroup is in a waiting (for timeslice) state, the stat
-	  will only report the group_wait_time accumulated till the last time it
-	  got a timeslice and will not include the current delta.
-
-- blkio.empty_time
-	- Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y.
-	  This is the amount of time a cgroup spends without any pending
-	  requests when not being served, i.e., it does not include any time
-	  spent idling for one of the queues of the cgroup. This is in
-	  nanoseconds. If this is read when the cgroup is in an empty state,
-	  the stat will only report the empty_time accumulated till the last
-	  time it had a pending request and will not include the current delta.
-
-- blkio.idle_time
-	- Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y.
-	  This is the amount of time spent by the IO scheduler idling for a
-	  given cgroup in anticipation of a better request than the existing ones
-	  from other queues/cgroups. This is in nanoseconds. If this is read
-	  when the cgroup is in an idling state, the stat will only report the
-	  idle_time accumulated till the last idle period and will not include
-	  the current delta.
-
-- blkio.dequeue
-	- Debugging aid only enabled if CONFIG_DEBUG_BLK_CGROUP=y. This
-	  gives the statistics about how many a times a group was dequeued
-	  from service tree of the device. First two fields specify the major
-	  and minor number of the device and third field specifies the number
-	  of times a group was dequeued from a particular device.
-
-- blkio.*_recursive
-	- Recursive version of various stats. These files show the
-          same information as their non-recursive counterparts but
-          include stats from all the descendant cgroups.
-
-Throttling/Upper limit policy files
------------------------------------
-- blkio.throttle.read_bps_device
-	- Specifies upper limit on READ rate from the device. IO rate is
-	  specified in bytes per second. Rules are per device. Following is
-	  the format.
-
-  echo "<major>:<minor>  <rate_bytes_per_second>" > /cgrp/blkio.throttle.read_bps_device
-
-- blkio.throttle.write_bps_device
-	- Specifies upper limit on WRITE rate to the device. IO rate is
-	  specified in bytes per second. Rules are per device. Following is
-	  the format.
-
-  echo "<major>:<minor>  <rate_bytes_per_second>" > /cgrp/blkio.throttle.write_bps_device
-
-- blkio.throttle.read_iops_device
-	- Specifies upper limit on READ rate from the device. IO rate is
-	  specified in IO per second. Rules are per device. Following is
-	  the format.
-
-  echo "<major>:<minor>  <rate_io_per_second>" > /cgrp/blkio.throttle.read_iops_device
-
-- blkio.throttle.write_iops_device
-	- Specifies upper limit on WRITE rate to the device. IO rate is
-	  specified in io per second. Rules are per device. Following is
-	  the format.
-
-  echo "<major>:<minor>  <rate_io_per_second>" > /cgrp/blkio.throttle.write_iops_device
-
-Note: If both BW and IOPS rules are specified for a device, then IO is
-      subjected to both the constraints.
-
-- blkio.throttle.io_serviced
-	- Number of IOs (bio) issued to the disk by the group. These
-	  are further divided by the type of operation - read or write, sync
-	  or async. First two fields specify the major and minor number of the
-	  device, third field specifies the operation type and the fourth field
-	  specifies the number of IOs.
-
-- blkio.throttle.io_service_bytes
-	- Number of bytes transferred to/from the disk by the group. These
-	  are further divided by the type of operation - read or write, sync
-	  or async. First two fields specify the major and minor number of the
-	  device, third field specifies the operation type and the fourth field
-	  specifies the number of bytes.
-
-Common files among various policies
------------------------------------
-- blkio.reset_stats
-	- Writing an int to this file will result in resetting all the stats
-	  for that cgroup.
-
-CFQ sysfs tunable
-=================
-/sys/block/<disk>/queue/iosched/slice_idle
-------------------------------------------
-On a faster hardware CFQ can be slow, especially with sequential workload.
-This happens because CFQ idles on a single queue and single queue might not
-drive deeper request queue depths to keep the storage busy. In such scenarios
-one can try setting slice_idle=0 and that would switch CFQ to IOPS
-(IO operations per second) mode on NCQ supporting hardware.
-
-That means CFQ will not idle between cfq queues of a cfq group and hence be
-able to driver higher queue depth and achieve better throughput. That also
-means that cfq provides fairness among groups in terms of IOPS and not in
-terms of disk time.
-
-/sys/block/<disk>/queue/iosched/group_idle
-------------------------------------------
-If one disables idling on individual cfq queues and cfq service trees by
-setting slice_idle=0, group_idle kicks in. That means CFQ will still idle
-on the group in an attempt to provide fairness among groups.
-
-By default group_idle is same as slice_idle and does not do anything if
-slice_idle is enabled.
-
-One can experience an overall throughput drop if you have created multiple
-groups and put applications in that group which are not driving enough
-IO to keep disk busy. In that case set group_idle=0, and CFQ will not idle
-on individual groups and throughput should improve.
-
-Writeback
-=========
-
-Page cache is dirtied through buffered writes and shared mmaps and
-written asynchronously to the backing filesystem by the writeback
-mechanism.  Writeback sits between the memory and IO domains and
-regulates the proportion of dirty memory by balancing dirtying and
-write IOs.
-
-On traditional cgroup hierarchies, relationships between different
-controllers cannot be established making it impossible for writeback
-to operate accounting for cgroup resource restrictions and all
-writeback IOs are attributed to the root cgroup.
-
-If both the blkio and memory controllers are used on the v2 hierarchy
-and the filesystem supports cgroup writeback, writeback operations
-correctly follow the resource restrictions imposed by both memory and
-blkio controllers.
-
-Writeback examines both system-wide and per-cgroup dirty memory status
-and enforces the more restrictive of the two.  Also, writeback control
-parameters which are absolute values - vm.dirty_bytes and
-vm.dirty_background_bytes - are distributed across cgroups according
-to their current writeback bandwidth.
-
-There's a peculiarity stemming from the discrepancy in ownership
-granularity between memory controller and writeback.  While memory
-controller tracks ownership per page, writeback operates on inode
-basis.  cgroup writeback bridges the gap by tracking ownership by
-inode but migrating ownership if too many foreign pages, pages which
-don't match the current inode ownership, have been encountered while
-writing back the inode.
-
-This is a conscious design choice as writeback operations are
-inherently tied to inodes making strictly following page ownership
-complicated and inefficient.  The only use case which suffers from
-this compromise is multiple cgroups concurrently dirtying disjoint
-regions of the same inode, which is an unlikely use case and decided
-to be unsupported.  Note that as memory controller assigns page
-ownership on the first use and doesn't update it until the page is
-released, even if cgroup writeback strictly follows page ownership,
-multiple cgroups dirtying overlapping areas wouldn't work as expected.
-In general, write-sharing an inode across multiple cgroups is not well
-supported.
-
-Filesystem support for cgroup writeback
----------------------------------------
-
-A filesystem can make writeback IOs cgroup-aware by updating
-address_space_operations->writepage[s]() to annotate bio's using the
-following two functions.
-
-* wbc_init_bio(@wbc, @bio)
-
-  Should be called for each bio carrying writeback data and associates
-  the bio with the inode's owner cgroup.  Can be called anytime
-  between bio allocation and submission.
-
-* wbc_account_io(@wbc, @page, @bytes)
-
-  Should be called for each data segment being written out.  While
-  this function doesn't care exactly when it's called during the
-  writeback session, it's the easiest and most natural to call it as
-  data segments are added to a bio.
-
-With writeback bio's annotated, cgroup support can be enabled per
-super_block by setting MS_CGROUPWB in ->s_flags.  This allows for
-selective disabling of cgroup writeback support which is helpful when
-certain filesystem features, e.g. journaled data mode, are
-incompatible.
-
-wbc_init_bio() binds the specified bio to its cgroup.  Depending on
-the configuration, the bio may be executed at a lower priority and if
-the writeback session is holding shared resources, e.g. a journal
-entry, may lead to priority inversion.  There is no one easy solution
-for the problem.  Filesystems can try to work around specific problem
-cases by skipping wbc_init_bio() or using bio_associate_blkcg()
-directly.
diff --git a/Documentation/cgroups/unified-hierarchy.txt b/Documentation/cgroups/unified-hierarchy.txt
deleted file mode 100644
index 781b1d4..0000000
--- a/Documentation/cgroups/unified-hierarchy.txt
+++ /dev/null
@@ -1,647 +0,0 @@
-
-Cgroup unified hierarchy
-
-April, 2014		Tejun Heo <tj@kernel.org>
-
-This document describes the changes made by unified hierarchy and
-their rationales.  It will eventually be merged into the main cgroup
-documentation.
-
-CONTENTS
-
-1. Background
-2. Basic Operation
-  2-1. Mounting
-  2-2. cgroup.subtree_control
-  2-3. cgroup.controllers
-3. Structural Constraints
-  3-1. Top-down
-  3-2. No internal tasks
-4. Delegation
-  4-1. Model of delegation
-  4-2. Common ancestor rule
-5. Other Changes
-  5-1. [Un]populated Notification
-  5-2. Other Core Changes
-  5-3. Controller File Conventions
-    5-3-1. Format
-    5-3-2. Control Knobs
-  5-4. Per-Controller Changes
-    5-4-1. io
-    5-4-2. cpuset
-    5-4-3. memory
-6. Planned Changes
-  6-1. CAP for resource control
-
-
-1. Background
-
-cgroup allows an arbitrary number of hierarchies and each hierarchy
-can host any number of controllers.  While this seems to provide a
-high level of flexibility, it isn't quite useful in practice.
-
-For example, as there is only one instance of each controller, utility
-type controllers such as freezer which can be useful in all
-hierarchies can only be used in one.  The issue is exacerbated by the
-fact that controllers can't be moved around once hierarchies are
-populated.  Another issue is that all controllers bound to a hierarchy
-are forced to have exactly the same view of the hierarchy.  It isn't
-possible to vary the granularity depending on the specific controller.
-
-In practice, these issues heavily limit which controllers can be put
-on the same hierarchy and most configurations resort to putting each
-controller on its own hierarchy.  Only closely related ones, such as
-the cpu and cpuacct controllers, make sense to put on the same
-hierarchy.  This often means that userland ends up managing multiple
-similar hierarchies repeating the same steps on each hierarchy
-whenever a hierarchy management operation is necessary.
-
-Unfortunately, support for multiple hierarchies comes at a steep cost.
-Internal implementation in cgroup core proper is dazzlingly
-complicated but more importantly the support for multiple hierarchies
-restricts how cgroup is used in general and what controllers can do.
-
-There's no limit on how many hierarchies there may be, which means
-that a task's cgroup membership can't be described in finite length.
-The key may contain any varying number of entries and is unlimited in
-length, which makes it highly awkward to handle and leads to addition
-of controllers which exist only to identify membership, which in turn
-exacerbates the original problem.
-
-Also, as a controller can't have any expectation regarding what shape
-of hierarchies other controllers would be on, each controller has to
-assume that all other controllers are operating on completely
-orthogonal hierarchies.  This makes it impossible, or at least very
-cumbersome, for controllers to cooperate with each other.
-
-In most use cases, putting controllers on hierarchies which are
-completely orthogonal to each other isn't necessary.  What usually is
-called for is the ability to have differing levels of granularity
-depending on the specific controller.  In other words, hierarchy may
-be collapsed from leaf towards root when viewed from specific
-controllers.  For example, a given configuration might not care about
-how memory is distributed beyond a certain level while still wanting
-to control how CPU cycles are distributed.
-
-Unified hierarchy is the next version of cgroup interface.  It aims to
-address the aforementioned issues by having more structure while
-retaining enough flexibility for most use cases.  Various other
-general and controller-specific interface issues are also addressed in
-the process.
-
-
-2. Basic Operation
-
-2-1. Mounting
-
-Currently, unified hierarchy can be mounted with the following mount
-command.  Note that this is still under development and scheduled to
-change soon.
-
- mount -t cgroup -o __DEVEL__sane_behavior cgroup $MOUNT_POINT
-
-All controllers which support the unified hierarchy and are not bound
-to other hierarchies are automatically bound to unified hierarchy and
-show up at the root of it.  Controllers which are enabled only in the
-root of unified hierarchy can be bound to other hierarchies.  This
-allows mixing unified hierarchy with the traditional multiple
-hierarchies in a fully backward compatible way.
-
-A controller can be moved across hierarchies only after the controller
-is no longer referenced in its current hierarchy.  Because per-cgroup
-controller states are destroyed asynchronously and controllers may
-have lingering references, a controller may not show up immediately on
-the unified hierarchy after the final umount of the previous
-hierarchy.  Similarly, a controller should be fully disabled to be
-moved out of the unified hierarchy and it may take some time for the
-disabled controller to become available for other hierarchies;
-furthermore, due to dependencies among controllers, other controllers
-may need to be disabled too.
-
-While useful for development and manual configurations, dynamically
-moving controllers between the unified and other hierarchies is
-strongly discouraged for production use.  It is recommended to decide
-the hierarchies and controller associations before starting using the
-controllers.
-
-
-2-2. cgroup.subtree_control
-
-All cgroups on unified hierarchy have a "cgroup.subtree_control" file
-which governs which controllers are enabled on the children of the
-cgroup.  Let's assume a hierarchy like the following.
-
-  root - A - B - C
-               \ D
-
-root's "cgroup.subtree_control" file determines which controllers are
-enabled on A.  A's on B.  B's on C and D.  This coincides with the
-fact that controllers on the immediate