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:	yes		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,int);
	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 (*revalidate) (struct dentry *);
	int (*setattr) (struct dentry *, struct iattr *);
	int (*getattr) (struct dentry *, struct iattr *);

locking rules:
	all may block
		BKL	i_sem(inode)	i_zombie(inode)
lookup:		yes	yes		no
create:		yes	yes		yes
link:		yes	yes		yes
mknod:		yes	yes		yes
mkdir:		yes	yes		yes
unlink:		yes	yes		yes
rmdir:		yes	yes		yes		(see below)
rename:		yes	yes (both)	yes (both)	(see below)
readlink:	no	no		no
follow_link:	no	no		no
truncate:	yes	yes		no		(see below)
setattr:	yes	if ATTR_SIZE	no
permssion:	yes	no		no
getattr:						(see below)
revalidate:	no					(see below)
	Additionally, ->rmdir() has i_zombie on victim and so does ->rename()
in case when target exists and is a directory.
	->rename() on directories has (per-superblock) ->s_vfs_rename_sem.
	->revalidate(), it may be called both with and without the i_sem
on dentry->d_inode. VFS never calls it with i_zombie on dentry->d_inode,
but watch for other methods directly calling this one...
	->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).
	->getattr() is currently unused.

--------------------------- super_operations ---------------------------
prototypes:
	void (*read_inode) (struct inode *);
	void (*write_inode) (struct inode *, int);
	void (*put_inode) (struct inode *);
	void (*delete_inode) (struct inode *);
	void (*put_super) (struct super_block *);
	void (*write_super) (struct super_block *);
	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	
delete_inode:	no	
clear_inode:	no	
put_super:	yes	yes	maybe		(see below)
write_super:	yes	yes	maybe		(see below)
statfs:		yes	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()/iget4().
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 *(*read_super) (struct super_block *, void *, int);
locking rules:
may block	BKL	->s_lock	mount_sem
yes		yes	yes		maybe

--------------------------- address_space_operations --------------------------
prototypes:
	int (*writepage)(struct page *);
	int (*readpage)(struct file *, struct page *);
	int (*sync_page)(struct 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);
locking rules:
	All may block
		BKL	PageLocked(page)
writepage:	no	yes, unlocks
readpage:	no	yes, unlocks
sync_page:	no	maybe
prepare_write:	no	yes
commit_write:	no	yes
bmap:		yes

	->prepare_write(), ->commit_write(), ->sync_page() and ->readpage()
may be called from the request handler (/dev/loop).
	->readpage() and ->writepage() unlock the page.
	->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...
	->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.
	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:	yes	(see below)
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.