locking

linux 内核源代码

	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 *);
	char *(*d_dname)((struct dentry *dentry, char *buffer, int buflen);

locking rules:
	none have BKL
		dcache_lock	rename_lock	->d_lock	may block
d_revalidate:	no		no		no		yes
d_hash		no		no		no		yes
d_compare:	no		yes		no		no 
d_delete:	yes		no		yes		no
d_release:	no		no		no		yes
d_iput:		no		no		no		yes
d_dname:	no		no		no		no

--------------------------- inode_operations --------------------------- 
prototypes:
	int (*create) (struct inode *,struct dentry *,int, struct nameidata *);
	struct dentry * (*lookup) (struct inode *,struct dentry *, struct nameid
ata *);
	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 __user *,int);
	int (*follow_link) (struct dentry *, struct nameidata *);
	void (*truncate) (struct inode *);
	int (*permission) (struct inode *, int, struct nameidata *);
	int (*setattr) (struct dentry *, struct iattr *);
	int (*getattr) (struct vfsmount *, struct dentry *, struct kstat *);
	int (*setxattr) (struct dentry *, const char *,const 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_mutex(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:	no
listxattr:	no
removexattr:	yes
	Additionally, ->rmdir(), ->unlink() and ->rename() have ->i_mutex 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:
	struct inode *(*alloc_inode)(struct super_block *sb);
	void (*destroy_inode)(struct inode *);
	void (*read_inode) (struct inode *);
	void (*dirty_inode) (struct inode *);
	int (*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);
	void (*write_super_lockfs) (struct super_block *);
	void (*unlockfs) (struct super_block *);
	int (*statfs) (struct dentry *, struct kstatfs *);
	int (*remount_fs) (struct super_block *, int *, char *);
	void (*clear_inode) (struct inode *);
	void (*umount_begin) (struct super_block *);
	int (*show_options)(struct seq_file *, struct vfsmount *);
	ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
	ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);

locking rules:
	All may block.
			BKL	s_lock	s_umount
alloc_inode:		no	no	no
destroy_inode:		no
read_inode:		no				(see below)
dirty_inode:		no				(must not sleep)
write_inode:		no
put_inode:		no
drop_inode:		no				!!!inode_lock!!!
delete_inode:		no
put_super:		yes	yes	no
write_super:		no	yes	read
sync_fs:		no	no	read
write_super_lockfs:	?
unlockfs:		?
statfs:			no	no	no
remount_fs:		yes	yes	maybe		(see below)
clear_inode:		no
umount_begin:		yes	no	no
show_options:		no				(vfsmount->sem)
quota_read:		no	no	no		(see below)
quota_write:		no	no	no		(see below)

->read_inode() is not a method - it's a callback used in iget().
->remount_fs() will have the s_umount lock if it's already mounted.
When called from get_sb_single, it does NOT have the s_umount lock.
->quota_read() and ->quota_write() functions are both guaranteed to
be the only ones operating on the quota file by the quota code (via
dqio_sem) (unless an admin really wants to screw up something and
writes to quota files with quotas on). For other details about locking
see also dquot_operations section.

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

->get_sb() returns error or 0 with locked superblock attached to the vfsmount
(exclusive on ->s_umount).
->kill_sb() takes a write-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 (*readpages)(struct file *filp, struct address_space *mapping,
			struct list_head *pages, unsigned nr_pages);
	int (*prepare_write)(struct file *, struct page *, unsigned, unsigned);
	int (*commit_write)(struct file *, struct page *, unsigned, unsigned);
	sector_t (*bmap)(struct address_space *, sector_t);
	int (*invalidatepage) (struct page *, unsigned long);
	int (*releasepage) (struct page *, int);
	int (*direct_IO)(int, struct kiocb *, const struct iovec *iov,
			loff_t offset, unsigned long nr_segs);
	int (*launder_page) (struct page *);

locking rules:
	All except set_page_dirty may block

			BKL	PageLocked(page)	i_sem
writepage:		no	yes, unlocks (see below)
readpage:		no	yes, unlocks
sync_page:		no	maybe
writepages:		no
set_page_dirty		no	no
readpages:		no
prepare_write:		no	yes			yes
commit_write:		no	yes			yes
write_begin:		no	locks the page		yes
write_end:		no	yes, unlocks		yes
perform_write:		no	n/a			yes
bmap:			yes
invalidatepage:		no	yes
releasepage:		no	yes
direct_IO:		no
launder_page:		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 should redirty the page with
redirty_page_for_writepage(), then unlock the page and return zero.
This may also be done to avoid internal deadlocks, but rarely.

If the filesystem 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, unless ->writepage() returns special WRITEPAGE_ACTIVATE
value. WRITEPAGE_ACTIVATE means that page cannot really be written out
currently, and VM should stop calling ->writepage() on this page for some
time. VM does this by moving page to the head of the active list, hence the
name.

Unless the filesystem is going to redirty_page_for_writepage(), unlock the page
and return zero, writepage *must* run set_page_writeback() against the page,
followed by unlocking it.  Once set_page_writeback() has been run against the
page, write I/O can be submitted and the write I/O completion handler must run
end_page_writeback() once the I/O is complete.  If no I/O is submitted, the
filesystem must run end_page_writeback() against the page before returning from
writepage.

That is: after 2.5.12, pages which are under writeout are *not* locked.  Note,
if the filesystem needs the page to be locked during writeout, that is ok, too,
the page is allowed to be unlocked at any point in time between the calls to
set_page_writeback() and end_page_writeback().

Note, failure to run either redirty_page_for_writepage() or the combination of
set_page_writeback()/end_page_writeback() on a page submitted to writepage
will leave the page itself marked clean but it will be tagged as dirty in the
radix tree.  This incoherency can lead to all sorts of hard-to-debug problems
in the filesystem like having dirty inodes at umount and losing written data.

	->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