chrdev: allocate dynamic chardevs in all unused holes
This is a duct-tape-and-chewing-gum solution to the problem
with the major numbers running out when allocating major
numbers dynamically.
To avoid collisions in the major space, we supply a bitmap with
"holes" that exist in the lower range of major numbers [0-254]
and pick numbers from there, beginning with the unused char
device 8 and moving up through 26, 40, 60-63, 93-94, 102,
120-127, 159, 213-215, 222-223 and 234-254.
The algorithm will behave like the old dynamic assignment
to begin with: dynamic majors will be assigned starting with
254, 253, ... but when it reaches 234 it will make a jump
and assign 223 and so on.
It will also FAIL if we actually fill up all free major
numbers. This seems to me like the reasonable thing to do
since the other numbers are, after all, reserved.
This also deletes the comment /* temporary */ which must be
one of the biggest lies ever.
This also updates the Documentation/devices.txt document to
reflect that all these numbers are used for dynamic assignment.
Reported-by: Ying Huang <ying.huang@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Alan Cox <alan@linux.intel.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
---
ChangeLog v4->v5:
- Create a new macro, BITS32() in <linux/bitops.> that allow
us to make an intuitive bitmask for the existing major
number. This reuses GENMASK(), clamps the arguments and
switch them around: to me it is atleast most intuitive to
have the lower bit before the higher one.
- Use BITS32() and BIT_MASK() to define the bitlist for
available devices.
- Assign dynamic majors from the top down (254, 253...) like
the old code was doing. I tried assigning bottom up
(8, 20,...) but that gave rise to "interesting" phenomena
on Intels test servers running random QEMU: character
devices in the lower range would stop probing properly,
even if they have a major number not colliding with the
dynamic assignment. At one time IDE tape (major 37) and
one time ISDN (major 45). No clue as to why.
ChangeLog v3->v4:
- Create the BITS() macro in a separate patch. This was
more tangled up than I thought, as the nice build servers
quickly told me.
ChangeLog v2->v3:
- Of course I had a dangling hunk for pr_dbg()->pr_debug()
in my working tree. Mea culpa.
- Fix the redefinition of BITS() in select.c to use the
more precise name FDS_BITS() over the generic plural.
ChangeLog v1->v2:
- Follow-up on the previous RFC patch, this uses Torvald's
suggested bitmap approach to allocate devices instead of
a list of free numbers.
- As a result of using find_first_zero_bit(), the major
numbers are assigned from low to high instead from high
to low. It's a bit scarier but I guess drivers using
dynamic numbers should be all right with it, I'm more
worried about userspaces expecting dynamic majors to
be in the [234,254] range. Input welcome, maybe I'm
just chicken.
- This still needs to be applied on top of the previous
fix to start warning about going below major 234. If
you prefer to just get this patch and get rid of the
problem then tell me.
diff --git a/Documentation/devices.txt b/Documentation/devices.txt
index 4035eca..2a4242b 100644
--- a/Documentation/devices.txt
+++ b/Documentation/devices.txt
@@ -248,6 +248,8 @@
associated with block devices. The binding to the
loop devices is handled by mount(8) or losetup(8).
+ 8 char RESERVED FOR DYNAMIC ALLOCATION OF MAJOR NUMBERS
+
8 block SCSI disk devices (0-15)
0 = /dev/sda First SCSI disk whole disk
16 = /dev/sdb Second SCSI disk whole disk
@@ -620,7 +622,7 @@
2 = /dev/sbpcd2 Panasonic CD-ROM controller 0 unit 2
3 = /dev/sbpcd3 Panasonic CD-ROM controller 0 unit 3
- 26 char
+ 26 char RESERVED FOR DYNAMIC ALLOCATION OF MAJOR NUMBERS
26 block Second Matsushita (Panasonic/SoundBlaster) CD-ROM
0 = /dev/sbpcd4 Panasonic CD-ROM controller 1 unit 0
@@ -863,7 +865,7 @@
...
39 block
- 40 char
+ 40 char RESERVED FOR DYNAMIC ALLOCATION OF MAJOR NUMBERS
40 block
@@ -1127,7 +1129,7 @@
NAMING CONFLICT -- PROPOSED REVISED NAME /dev/rpda0 etc
- 60-63 char LOCAL/EXPERIMENTAL USE
+ 60-63 char RESERVED FOR DYNAMIC ALLOCATION OF MAJOR NUMBERS
60-63 block LOCAL/EXPERIMENTAL USE
Allocated for local/experimental use. For devices not
@@ -1641,7 +1643,7 @@
disks (see major number 3) except that the limit on
partitions is 15.
- 93 char
+ 93 char RESERVED FOR DYNAMIC ALLOCATION OF MAJOR NUMBERS
93 block NAND Flash Translation Layer filesystem
0 = /dev/nftla First NFTL layer
@@ -1649,7 +1651,7 @@
...
240 = /dev/nftlp 16th NTFL layer
- 94 char
+ 94 char RESERVED FOR DYNAMIC ALLOCATION OF MAJOR NUMBERS
94 block IBM S/390 DASD block storage
0 = /dev/dasda First DASD device, major
@@ -1742,7 +1744,7 @@
...
15 = /dev/amiraid/ar?p15 15th partition
-102 char
+102 char RESERVED FOR DYNAMIC ALLOCATION OF MAJOR NUMBERS
102 block Compressed block device
0 = /dev/cbd/a First compressed block device, whole device
@@ -2027,7 +2029,7 @@
1 = /dev/vnet1 2nd virtual network
...
-120-127 char LOCAL/EXPERIMENTAL USE
+120-127 char RESERVED FOR DYNAMIC ALLOCATION OF MAJOR NUMBERS
120-127 block LOCAL/EXPERIMENTAL USE
Allocated for local/experimental use. For devices not
@@ -2341,7 +2343,7 @@
1 = /dev/gfax1 GammaLink channel 1
...
-159 char RESERVED
+159 char RESERVED FOR DYNAMIC ALLOCATION OF MAJOR NUMBERS
159 block RESERVED
@@ -2929,6 +2931,8 @@
...
196 = /dev/dvb/adapter3/video0 first video decoder of fourth card
+213-215 char RESERVED FOR DYNAMIC ALLOCATION OF MAJOR NUMBERS
+
216 char Bluetooth RFCOMM TTY devices
0 = /dev/rfcomm0 First Bluetooth RFCOMM TTY device
1 = /dev/rfcomm1 Second Bluetooth RFCOMM TTY device
@@ -2971,6 +2975,8 @@
same interface. For interface documentation see
http://www.vmelinux.org/.
+222-223 char RESERVED FOR DYNAMIC ALLOCATION OF MAJOR NUMBERS
+
224 char A2232 serial card
0 = /dev/ttyY0 First A2232 port
1 = /dev/ttyY1 Second A2232 port
diff --git a/fs/char_dev.c b/fs/char_dev.c
index 687471d..b647cc8 100644
--- a/fs/char_dev.c
+++ b/fs/char_dev.c
@@ -21,6 +21,8 @@
#include <linux/mutex.h>
#include <linux/backing-dev.h>
#include <linux/tty.h>
+#include <linux/bitmap.h>
+#include <linux/bitops.h>
#include "internal.h"
@@ -37,6 +39,31 @@
struct cdev *cdev; /* will die */
} *chrdevs[CHRDEV_MAJOR_HASH_SIZE];
+/*
+ * A bitmap for the 255 lower device numbers. A "1" means the
+ * major number is taken, a "0" means it is available. We first
+ * need to define all the assigned devices as taken so that
+ * dynamic device allocation will not go in and steal them.
+ */
+static unsigned long majors_map[] = {
+ /* 8 and 26 are free */
+ BITS32(0, 7) | BITS32(9, 25) | BITS32(27, 31),
+ /* 40 and 60-63 are free */
+ BITS32(32, 39) | BITS32(41, 59),
+ /* 93 and 94 are free */
+ BITS32(64, 92) | BIT_MASK(95),
+ /* 102 and 120-127 are free */
+ BITS32(96, 101) | BITS32(103, 119),
+ /* 159 is free */
+ BITS32(128, 158),
+ /* No free numbers */
+ ~0x0U,
+ /* 213-215 and 222-223 are free */
+ BITS32(192, 212) | BITS32(216, 221),
+ /* 234-254 are free */
+ BITS32(224, 233) | BIT_MASK(255)
+};
+
/* index in the above */
static inline int major_to_index(unsigned major)
{
@@ -84,22 +111,25 @@
mutex_lock(&chrdevs_lock);
- /* temporary */
if (major == 0) {
- for (i = ARRAY_SIZE(chrdevs)-1; i > 0; i--) {
- if (chrdevs[i] == NULL)
- break;
- }
-
- if (i < CHRDEV_MAJOR_DYN_END)
- pr_warn("CHRDEV \"%s\" major number %d goes below the dynamic allocation range",
- name, i);
-
- if (i == 0) {
+ i = find_first_zero_bit(majors_map, CHRDEV_MAJOR_HASH_SIZE);
+ if (i == CHRDEV_MAJOR_HASH_SIZE) {
+ pr_warn("CHRDEV: \"%s\" out of major numbers",
+ name);
ret = -EBUSY;
goto out;
}
- major = i;
+ /* Allocate from the top (254 and down) */
+ while (i != CHRDEV_MAJOR_HASH_SIZE) {
+ major = i;
+ i++;
+ i = find_next_zero_bit(majors_map,
+ CHRDEV_MAJOR_HASH_SIZE,
+ i);
+ }
+ set_bit(major, majors_map);
+ pr_debug("CHRDEV: \"%s\" using major %d\n",
+ name, major);
}
cd->major = major;
diff --git a/include/linux/bitops.h b/include/linux/bitops.h
index 299e76b..eebd4b1 100644
--- a/include/linux/bitops.h
+++ b/include/linux/bitops.h
@@ -24,6 +24,12 @@
#define GENMASK_ULL(h, l) \
(((~0ULL) << (l)) & (~0ULL >> (BITS_PER_LONG_LONG - 1 - (h))))
+/*
+ * BITS32 creates a 32-bit mask starting at bit @s and ending at bit
+ * @e, clamped to the 32-bit bitspace if exceeding 31.
+ */
+#define BITS32(s, e) GENMASK((e % BITS_PER_LONG), (s % BITS_PER_LONG))
+
extern unsigned int __sw_hweight8(unsigned int w);
extern unsigned int __sw_hweight16(unsigned int w);
extern unsigned int __sw_hweight32(unsigned int w);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index b8b3c72..10d3d8f 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2427,8 +2427,6 @@
/* fs/char_dev.c */
#define CHRDEV_MAJOR_HASH_SIZE 255
-/* Marks the bottom of the first segment of free char majors */
-#define CHRDEV_MAJOR_DYN_END 234
extern int alloc_chrdev_region(dev_t *, unsigned, unsigned, const char *);
extern int register_chrdev_region(dev_t, unsigned, const char *);
extern int __register_chrdev(unsigned int major, unsigned int baseminor,
