Documentation/filesystems/Locking

	The text below describes the locking rules for VFS-related methods.
It is (believed to be) up-to-date. *Please*, if you change anything in
prototypes or locking protocols - update this file. And update the relevant
instances in the tree, don't leave that to maintainers of filesystems/devices/
etc. At the very least, put the list of dubious cases in the end of this file.
Don't turn it into log - maintainers of out-of-the-tree code are supposed to
be able to use diff(1).
	Thing currently missing here: socket operations. Alexey?

--------------------------- dentry_operations --------------------------
prototypes:
	int (*d_revalidate)(struct dentry *, int);
	int (*d_hash) (struct dentry *, struct qstr *);
	int (*d_compare) (struct dentry *, struct qstr *, struct qstr *);
	int (*d_delete)(struct dentry *);
	void (*d_release)(struct dentry *);
	void (*d_iput)(struct dentry *, struct inode *);

locking rules:
	none have BKL
		dcache_lock	may block
d_revalidate:	no		yes
d_hash		no		yes
d_compare:	no		no 
d_delete:	yes		no
d_release:	no		yes
d_iput:		no		yes

--------------------------- inode_operations --------------------------- 
prototypes:
	int (*create) (struct inode *,struct dentry *,int);
	struct dentry * (*lookup) (struct inode *,struct dentry *);
	int (*link) (struct dentry *,struct inode *,struct dentry *);
	int (*unlink) (struct inode *,struct dentry *);
	int (*symlink) (struct inode *,struct dentry *,const char *);
	int (*mkdir) (struct inode *,struct dentry *,int);
	int (*rmdir) (struct inode *,struct dentry *);
	int (*mknod) (struct inode *,struct dentry *,int,dev_t);
	int (*rename) (struct inode *, struct dentry *,
			struct inode *, struct dentry *);
	int (*readlink) (struct dentry *, char *,int);
	int (*follow_link) (struct dentry *, struct nameidata *);
	void (*truncate) (struct inode *);
	int (*permission) (struct inode *, int);
	int (*setattr) (struct dentry *, struct iattr *);
	int (*getattr) (struct vfsmount *, struct dentry *, struct kstat *);
	int (*setxattr) (struct dentry *, const char *, void *, size_t, int);
	ssize_t (*getxattr) (struct dentry *, const char *, void *, size_t);
	ssize_t (*listxattr) (struct dentry *, char *, size_t);
	int (*removexattr) (struct dentry *, const char *);

locking rules:
	all may block, none have BKL
		i_sem(inode)
lookup:		yes
create:		yes
link:		yes (both)
mknod:		yes
symlink:	yes
mkdir:		yes
unlink:		yes (both)
rmdir:		yes (both)	(see below)
rename:		yes (all)	(see below)
readlink:	no
follow_link:	no
truncate:	yes		(see below)
setattr:	yes
permission:	no
getattr:	no
setxattr:	yes
getxattr:	yes
listxattr:	yes
removexattr:	yes
	Additionally, ->rmdir(), ->unlink() and ->rename() have ->i_sem on
victim.
	cross-directory ->rename() has (per-superblock) ->s_vfs_rename_sem.
	->truncate() is never called directly - it's a callback, not a
method. It's called by vmtruncate() - library function normally used by
->setattr(). Locking information above applies to that call (i.e. is
inherited from ->setattr() - vmtruncate() is used when ATTR_SIZE had been
passed).

See Documentation/filesystems/directory-locking for more detailed discussion
of the locking scheme for directory operations.

--------------------------- super_operations ---------------------------
prototypes:
	void (*read_inode) (struct inode *);
	void (*write_inode) (struct inode *, int);
	void (*put_inode) (struct inode *);
	void (*drop_inode) (struct inode *);
	void (*delete_inode) (struct inode *);
	void (*put_super) (struct super_block *);
	void (*write_super) (struct super_block *);
	int (*sync_fs) (struct super_block *sb, int wait);
	int (*statfs) (struct super_block *, struct statfs *);
	int (*remount_fs) (struct super_block *, int *, char *);
	void (*clear_inode) (struct inode *);
	void (*umount_begin) (struct super_block *);

locking rules:
	All may block.
		BKL	s_lock	mount_sem
read_inode:	yes				(see below)
write_inode:	no	
put_inode:	no	
drop_inode:	no				!!!inode_lock!!!
delete_inode:	no	
clear_inode:	no	
put_super:	yes	yes	maybe		(see below)
write_super:	no	yes	maybe		(see below)
sync_fs:	no	no	maybe		(see below)
statfs:		no	no	no
remount_fs:	yes	yes	maybe		(see below)
umount_begin:	yes	no	maybe		(see below)

->read_inode() is not a method - it's a callback used in iget().
rules for mount_sem are not too nice - it is going to die and be replaced
by better scheme anyway.

--------------------------- file_system_type ---------------------------
prototypes:
	struct super_block *(*get_sb) (struct file_system_type *, int, char *, void *);
	void (*kill_sb) (struct super_block *);
locking rules:
		may block	BKL
get_sb		yes		yes
kill_sb		yes		yes

->get_sb() returns error or a locked superblock (exclusive on ->s_umount).
->kill_sb() takes a locked superblock, does all shutdown work on it,
unlocks and drops the reference.

--------------------------- address_space_operations --------------------------
prototypes:
	int (*writepage)(struct page *page, struct writeback_control *wbc);
	int (*readpage)(struct file *, struct page *);
	int (*sync_page)(struct page *);
	int (*writepages)(struct address_space *, struct writeback_control *);
	int (*set_page_dirty)(struct page *page);
	int (*prepare_write)(struct file *, struct page *, unsigned, unsigned);
	int (*commit_write)(struct file *, struct page *, unsigned, unsigned);
	int (*bmap)(struct address_space *, long);
	int (*invalidatepage) (struct page *, unsigned long);
	int (*releasepage) (struct page *, int);
	int (*direct_IO)(int, struct inode *, struct kiobuf *, unsigned long, int);

locking rules:
	All except set_page_dirty may block

			BKL	PageLocked(page)
writepage:		no	yes, unlocks (see below)
readpage:		no	yes, unlocks
readpages:		no
sync_page:		no	maybe
writepages:		no
set_page_dirty		no	no
prepare_write:		no	yes
commit_write:		no	yes
bmap:			yes
invalidatepage:		no	yes
releasepage:		no	yes

	->prepare_write(), ->commit_write(), ->sync_page() and ->readpage()
may be called from the request handler (/dev/loop).

	->readpage() unlocks the page, either synchronously or via I/O
completion.

	->readpages() populates the pagecache with the passed pages and starts
I/O against them.  They come unlocked upon I/O completion.

	->writepage() is used for two purposes: for "memory cleansing" and for
"sync".  These are quite different operations and the behaviour may differ
depending upon the mode.

If writepage is called for sync (wbc->sync_mode != WBC_SYNC_NONE) then
it *must* start I/O against the page, even if that would involve
blocking on in-progress I/O.

If writepage is called for memory cleansing (sync_mode ==
WBC_SYNC_NONE) then its role is to get as much writeout underway as
possible.  So writepage should try to avoid blocking against
currently-in-progress I/O.

If the filesystem is not called for "sync" and it determines that it
would need to block against in-progress I/O to be able to start new I/O
against the page the filesystem shoud redirty the page (usually with
__set_page_dirty_nobuffers()), then unlock the page and return zero. 
This may also be done to avoid internal deadlocks, but rarely.

If the filesytem is called for sync then it must wait on any
in-progress I/O and then start new I/O.

The filesystem should unlock the page synchronously, before returning
to the caller.  If the page has write I/O underway against it,
writepage() should run SetPageWriteback() against the page prior to
unlocking it.  The write I/O completion handler should run
end_page_writeback() against the page.

That is: after 2.5.12, pages which are under writeout are *not* locked.

	->sync_page() locking rules are not well-defined - usually it is called
with lock on page, but that is not guaranteed. Considering the currently
existing instances of this method ->sync_page() itself doesn't look
well-defined...

	->writepages() is used for periodic writeback and for syscall-initiated
sync operations.  The address_space should start I/O against at least
*nr_to_write pages.  *nr_to_write must be decremented for each page which is
written.  The address_space implementation may write more (or less) pages
than *nr_to_write asks for, but it should try to be reasonably close.  If
nr_to_write is NULL, all dirty pages must be written.

writepages should _only_ write pages which are present on
mapping->io_pages.

	->set_page_dirty() is called from various places in the kernel
when the target page is marked as needing writeback.  It may be called
under spinlock (it cannot block) and is sometimes called with the page
not locked.

	->bmap() is currently used by legacy ioctl() (FIBMAP) provided by some
filesystems and by the swapper. The latter will eventually go away. All
instances do not actually need the BKL. Please, keep it that way and don't
breed new callers.

	->invalidatepage() is called when the filesystem must attempt to drop
some or all of the buffers from the page when it is being truncated.  It
returns zero on success.  If ->invalidatepage is zero, the kernel uses
block_invalidatepage() instead.

	->releasepage() is called when the kernel is about to try to drop the
buffers from the page in preparation for freeing it.  It returns zero to
indicate that the buffers are (or may be) freeable.  If ->releasepage is zero,
the kernel assumes that the fs has no private interest in the buffers.

	Note: currently almost all instances of address_space methods are
using BKL for internal serialization and that's one of the worst sources
of contention. Normally they are calling library functions (in fs/buffer.c)
and pass foo_get_block() as a callback (on local block-based filesystems,
indeed). BKL is not needed for library stuff and is usually taken by
foo_get_block(). It's an overkill, since block bitmaps can be protected by
internal fs locking and real critical areas are much smaller than the areas
filesystems protect now.

--------------------------- file_lock ------------------------------------
prototypes:
	void (*fl_notify)(struct file_lock *);	/* unblock callback */
	void (*fl_insert)(struct file_lock *);	/* lock insertion callback */
	void (*fl_remove)(struct file_lock *);	/* lock removal callback */

locking rules:
		BKL	may block
fl_notify:	yes	no
fl_insert:	yes	maybe
fl_remove:	yes	maybe
	Currently only NLM provides instances of this class. None of the
them block. If you have out-of-tree instances - please, show up. Locking
in that area will change.

--------------------------- buffer_head -----------------------------------
prototypes:
	void (*b_end_io)(struct buffer_head *bh, int uptodate);

locking rules:
	called from interrupts. In other words, extreme care is needed here.
bh is locked, but that's all warranties we have here. Currently only RAID1,
highmem and fs/buffer.c are providing these. Block devices call this method
upon the IO completion.

--------------------------- block_device_operations -----------------------
prototypes:
	int (*open) (struct inode *, struct file *);
	int (*release) (struct inode *, struct file *);
	int (*ioctl) (struct inode *, struct file *, unsigned, unsigned long);
	int (*check_media_change) (kdev_t);
	int (*revalidate) (kdev_t);
locking rules:
			BKL	bd_sem
open:			yes	yes
release:		yes	yes
ioctl:			yes	no
check_media_change:	yes	no
revalidate:		yes	no

The last two are called only from check_disk_change(). Prototypes are very
bad - as soon as we'll get disk_struct they will change (and methods will
become per-disk instead of per-partition).

--------------------------- file_operations -------------------------------
prototypes:
	loff_t (*llseek) (struct file *, loff_t, int);
	ssize_t (*read) (struct file *, char *, size_t, loff_t *);
	ssize_t (*write) (struct file *, const char *, size_t, loff_t *);
	int (*readdir) (struct file *, void *, filldir_t);
	unsigned int (*poll) (struct file *, struct poll_table_struct *);
	int (*ioctl) (struct inode *, struct file *, unsigned int, unsigned long);
	int (*mmap) (struct file *, struct vm_area_struct *);
	int (*open) (struct inode *, struct file *);
	int (*flush) (struct file *);
	int (*release) (struct inode *, struct file *);
	int (*fsync) (struct file *, struct dentry *, int datasync);
	int (*fasync) (int, struct file *, int);
	int (*lock) (struct file *, int, struct file_lock *);
	ssize_t (*readv) (struct file *, const struct iovec *, unsigned long, loff_t *);
	ssize_t (*writev) (struct file *, const struct iovec *, unsigned long, loff_t *);
};

locking rules:
	All except ->poll() may block.
		BKL
llseek:		yes	(see below)
read:		no
write:		no
readdir: 	no	
poll:		no
ioctl:		yes	(see below)
mmap:		no
open:		maybe	(see below)
flush:		yes
release:	no
fsync:		yes	(see below)
fasync:		yes	(see below)
lock:		yes
readv:		no
writev:		no

->llseek() locking has moved from llseek to the individual llseek
implementations.  If your fs is not using generic_file_llseek, you
need to acquire and release the appropriate locks in your ->llseek().
For many filesystems, it is probably safe to acquire the inode
semaphore.  Note some filesystems (i.e. remote ones) provide no
protection for i_size so you will need to use the BKL.

->open() locking is in-transit: big lock partially moved into the methods.
The only exception is ->open() in the instances of file_operations that never
end up in ->i_fop/->proc_fops, i.e. ones that belong to character devices
(chrdev_open() takes lock before replacing ->f_op and calling the secondary
method. As soon as we fix the handling of module reference counters all
instances of ->open() will be called without the BKL.

Note: ext2_release() was *the* source of contention on fs-intensive
loads and dropping BKL on ->release() helps to get rid of that (we still
grab BKL for cases when we close a file that had been opened r/w, but that
can and should be done using the internal locking with smaller critical areas).
Current worst offender is ext2_get_block()...

->fasync() is a mess. This area needs a big cleanup and that will probably
affect locking.

->readdir() and ->ioctl() on directories must be changed. Ideally we would
move ->readdir() to inode_operations and use a separate method for directory
->ioctl() or kill the latter completely. One of the problems is that for
anything that resembles union-mount we won't have a struct file for all
components. And there are other reasons why the current interface is a mess...

->read on directories probably must go away - we should just enforce -EISDIR
in sys_read() and friends.

->fsync() has i_sem on inode.

--------------------------- dquot_operations -------------------------------
prototypes:
	void (*initialize) (struct inode *, short);
	void (*drop) (struct inode *);
	int (*alloc_block) (const struct inode *, unsigned long, char);
	int (*alloc_inode) (const struct inode *, unsigned long);
	void (*free_block) (const struct inode *, unsigned long);
	void (*free_inode) (const struct inode *, unsigned long);
	int (*transfer) (struct dentry *, struct iattr *);

locking rules:
		BKL
initialize:	no
drop:		no
alloc_block:	yes
alloc_inode:	yes
free_block:	yes
free_inode:	yes
transfer:	no

--------------------------- vm_operations_struct -----------------------------
prototypes:
	void (*open)(struct vm_area_struct*);
	void (*close)(struct vm_area_struct*);
	struct page *(*nopage)(struct vm_area_struct*, unsigned long, int);

locking rules:
		BKL	mmap_sem
open:		no	yes
close:		no	yes
nopage:		no	yes

================================================================================
			Dubious stuff

(if you break something or notice that it is broken and do not fix it yourself
- at least put it here)

ipc/shm.c::shm_delete() - may need BKL.
->read() and ->write() in many drivers are (probably) missing BKL.
drivers/sgi/char/graphics.c::sgi_graphics_nopage() - may need BKL.