1 The text below describes the locking rules for VFS-related methods.
2 It is (believed to be) up-to-date. *Please*, if you change anything in
3 prototypes or locking protocols - update this file. And update the relevant
4 instances in the tree, don't leave that to maintainers of filesystems/devices/
5 etc. At the very least, put the list of dubious cases in the end of this file.
6 Don't turn it into log - maintainers of out-of-the-tree code are supposed to
7 be able to use diff(1).
8 Thing currently missing here: socket operations. Alexey?
10 --------------------------- dentry_operations --------------------------
12 int (*d_revalidate)(struct dentry *, int);
13 int (*d_hash) (struct dentry *, struct qstr *);
14 int (*d_compare) (struct dentry *, struct qstr *, struct qstr *);
15 int (*d_delete)(struct dentry *);
16 void (*d_release)(struct dentry *);
17 void (*d_iput)(struct dentry *, struct inode *);
21 dcache_lock rename_lock ->d_lock may block
22 d_revalidate: no no no yes
24 d_compare: no yes no no
25 d_delete: yes no yes no
26 d_release: no no no yes
29 --------------------------- inode_operations ---------------------------
31 int (*create) (struct inode *,struct dentry *,int, struct nameidata *);
32 struct dentry * (*lookup) (struct inode *,struct dentry *, struct nameid
34 int (*link) (struct dentry *,struct inode *,struct dentry *);
35 int (*unlink) (struct inode *,struct dentry *);
36 int (*symlink) (struct inode *,struct dentry *,const char *);
37 int (*mkdir) (struct inode *,struct dentry *,int);
38 int (*rmdir) (struct inode *,struct dentry *);
39 int (*mknod) (struct inode *,struct dentry *,int,dev_t);
40 int (*rename) (struct inode *, struct dentry *,
41 struct inode *, struct dentry *);
42 int (*readlink) (struct dentry *, char __user *,int);
43 int (*follow_link) (struct dentry *, struct nameidata *);
44 void (*truncate) (struct inode *);
45 int (*permission) (struct inode *, int, struct nameidata *);
46 int (*setattr) (struct dentry *, struct iattr *);
47 int (*getattr) (struct vfsmount *, struct dentry *, struct kstat *);
48 int (*setxattr) (struct dentry *, const char *,const void *,size_t,int);
49 ssize_t (*getxattr) (struct dentry *, const char *, void *, size_t);
50 ssize_t (*listxattr) (struct dentry *, char *, size_t);
51 int (*removexattr) (struct dentry *, const char *);
54 all may block, none have BKL
63 rmdir: yes (both) (see below)
64 rename: yes (all) (see below)
67 truncate: yes (see below)
75 Additionally, ->rmdir(), ->unlink() and ->rename() have ->i_sem on
77 cross-directory ->rename() has (per-superblock) ->s_vfs_rename_sem.
78 ->truncate() is never called directly - it's a callback, not a
79 method. It's called by vmtruncate() - library function normally used by
80 ->setattr(). Locking information above applies to that call (i.e. is
81 inherited from ->setattr() - vmtruncate() is used when ATTR_SIZE had been
84 See Documentation/filesystems/directory-locking for more detailed discussion
85 of the locking scheme for directory operations.
87 --------------------------- super_operations ---------------------------
89 struct inode *(*alloc_inode)(struct super_block *sb);
90 void (*destroy_inode)(struct inode *);
91 void (*read_inode) (struct inode *);
92 void (*dirty_inode) (struct inode *);
93 int (*write_inode) (struct inode *, int);
94 void (*put_inode) (struct inode *);
95 void (*drop_inode) (struct inode *);
96 void (*delete_inode) (struct inode *);
97 void (*put_super) (struct super_block *);
98 void (*write_super) (struct super_block *);
99 int (*sync_fs)(struct super_block *sb, int wait);
100 void (*write_super_lockfs) (struct super_block *);
101 void (*unlockfs) (struct super_block *);
102 int (*statfs) (struct super_block *, struct kstatfs *);
103 int (*remount_fs) (struct super_block *, int *, char *);
104 void (*clear_inode) (struct inode *);
105 void (*umount_begin) (struct super_block *);
106 int (*show_options)(struct seq_file *, struct vfsmount *);
107 ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
108 ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
113 alloc_inode: no no no
115 read_inode: no (see below)
116 dirty_inode: no (must not sleep)
119 drop_inode: no !!!inode_lock!!!
121 put_super: yes yes no
122 write_super: no yes read
124 write_super_lockfs: ?
127 remount_fs: no yes maybe (see below)
129 umount_begin: yes no no
130 show_options: no (vfsmount->sem)
131 quota_read: no no no (see below)
132 quota_write: no no no (see below)
134 ->read_inode() is not a method - it's a callback used in iget().
135 ->remount_fs() will have the s_umount lock if it's already mounted.
136 When called from get_sb_single, it does NOT have the s_umount lock.
137 ->quota_read() and ->quota_write() functions are both guaranteed to
138 be the only ones operating on the quota file by the quota code (via
139 dqio_sem) (unless an admin really wants to screw up something and
140 writes to quota files with quotas on). For other details about locking
141 see also dquot_operations section.
143 --------------------------- file_system_type ---------------------------
145 struct super_block *(*get_sb) (struct file_system_type *, int,
146 const char *, void *);
147 void (*kill_sb) (struct super_block *);
153 ->get_sb() returns error or a locked superblock (exclusive on ->s_umount).
154 ->kill_sb() takes a write-locked superblock, does all shutdown work on it,
155 unlocks and drops the reference.
157 --------------------------- address_space_operations --------------------------
159 int (*writepage)(struct page *page, struct writeback_control *wbc);
160 int (*readpage)(struct file *, struct page *);
161 int (*sync_page)(struct page *);
162 int (*writepages)(struct address_space *, struct writeback_control *);
163 int (*set_page_dirty)(struct page *page);
164 int (*readpages)(struct file *filp, struct address_space *mapping,
165 struct list_head *pages, unsigned nr_pages);
166 int (*prepare_write)(struct file *, struct page *, unsigned, unsigned);
167 int (*commit_write)(struct file *, struct page *, unsigned, unsigned);
168 sector_t (*bmap)(struct address_space *, sector_t);
169 int (*invalidatepage) (struct page *, unsigned long);
170 int (*releasepage) (struct page *, int);
171 int (*direct_IO)(int, struct kiocb *, const struct iovec *iov,
172 loff_t offset, unsigned long nr_segs);
175 All except set_page_dirty may block
178 writepage: no yes, unlocks (see below)
179 readpage: no yes, unlocks
184 prepare_write: no yes
187 invalidatepage: no yes
191 ->prepare_write(), ->commit_write(), ->sync_page() and ->readpage()
192 may be called from the request handler (/dev/loop).
194 ->readpage() unlocks the page, either synchronously or via I/O
197 ->readpages() populates the pagecache with the passed pages and starts
198 I/O against them. They come unlocked upon I/O completion.
200 ->writepage() is used for two purposes: for "memory cleansing" and for
201 "sync". These are quite different operations and the behaviour may differ
202 depending upon the mode.
204 If writepage is called for sync (wbc->sync_mode != WBC_SYNC_NONE) then
205 it *must* start I/O against the page, even if that would involve
206 blocking on in-progress I/O.
208 If writepage is called for memory cleansing (sync_mode ==
209 WBC_SYNC_NONE) then its role is to get as much writeout underway as
210 possible. So writepage should try to avoid blocking against
211 currently-in-progress I/O.
213 If the filesystem is not called for "sync" and it determines that it
214 would need to block against in-progress I/O to be able to start new I/O
215 against the page the filesystem should redirty the page with
216 redirty_page_for_writepage(), then unlock the page and return zero.
217 This may also be done to avoid internal deadlocks, but rarely.
219 If the filesytem is called for sync then it must wait on any
220 in-progress I/O and then start new I/O.
222 The filesystem should unlock the page synchronously, before returning
225 Unless the filesystem is going to redirty_page_for_writepage(), unlock the page
226 and return zero, writepage *must* run set_page_writeback() against the page,
227 followed by unlocking it. Once set_page_writeback() has been run against the
228 page, write I/O can be submitted and the write I/O completion handler must run
229 end_page_writeback() once the I/O is complete. If no I/O is submitted, the
230 filesystem must run end_page_writeback() against the page before returning from
233 That is: after 2.5.12, pages which are under writeout are *not* locked. Note,
234 if the filesystem needs the page to be locked during writeout, that is ok, too,
235 the page is allowed to be unlocked at any point in time between the calls to
236 set_page_writeback() and end_page_writeback().
238 Note, failure to run either redirty_page_for_writepage() or the combination of
239 set_page_writeback()/end_page_writeback() on a page submitted to writepage
240 will leave the page itself marked clean but it will be tagged as dirty in the
241 radix tree. This incoherency can lead to all sorts of hard-to-debug problems
242 in the filesystem like having dirty inodes at umount and losing written data.
244 ->sync_page() locking rules are not well-defined - usually it is called
245 with lock on page, but that is not guaranteed. Considering the currently
246 existing instances of this method ->sync_page() itself doesn't look
249 ->writepages() is used for periodic writeback and for syscall-initiated
250 sync operations. The address_space should start I/O against at least
251 *nr_to_write pages. *nr_to_write must be decremented for each page which is
252 written. The address_space implementation may write more (or less) pages
253 than *nr_to_write asks for, but it should try to be reasonably close. If
254 nr_to_write is NULL, all dirty pages must be written.
256 writepages should _only_ write pages which are present on
259 ->set_page_dirty() is called from various places in the kernel
260 when the target page is marked as needing writeback. It may be called
261 under spinlock (it cannot block) and is sometimes called with the page
264 ->bmap() is currently used by legacy ioctl() (FIBMAP) provided by some
265 filesystems and by the swapper. The latter will eventually go away. All
266 instances do not actually need the BKL. Please, keep it that way and don't
269 ->invalidatepage() is called when the filesystem must attempt to drop
270 some or all of the buffers from the page when it is being truncated. It
271 returns zero on success. If ->invalidatepage is zero, the kernel uses
272 block_invalidatepage() instead.
274 ->releasepage() is called when the kernel is about to try to drop the
275 buffers from the page in preparation for freeing it. It returns zero to
276 indicate that the buffers are (or may be) freeable. If ->releasepage is zero,
277 the kernel assumes that the fs has no private interest in the buffers.
279 Note: currently almost all instances of address_space methods are
280 using BKL for internal serialization and that's one of the worst sources
281 of contention. Normally they are calling library functions (in fs/buffer.c)
282 and pass foo_get_block() as a callback (on local block-based filesystems,
283 indeed). BKL is not needed for library stuff and is usually taken by
284 foo_get_block(). It's an overkill, since block bitmaps can be protected by
285 internal fs locking and real critical areas are much smaller than the areas
286 filesystems protect now.
288 ----------------------- file_lock_operations ------------------------------
290 void (*fl_insert)(struct file_lock *); /* lock insertion callback */
291 void (*fl_remove)(struct file_lock *); /* lock removal callback */
292 void (*fl_copy_lock)(struct file_lock *, struct file_lock *);
293 void (*fl_release_private)(struct file_lock *);
301 fl_release_private: yes yes
303 ----------------------- lock_manager_operations ---------------------------
305 int (*fl_compare_owner)(struct file_lock *, struct file_lock *);
306 void (*fl_notify)(struct file_lock *); /* unblock callback */
307 void (*fl_copy_lock)(struct file_lock *, struct file_lock *);
308 void (*fl_release_private)(struct file_lock *);
309 void (*fl_break)(struct file_lock *); /* break_lease callback */
313 fl_compare_owner: yes no
316 fl_release_private: yes yes
319 Currently only NFSD and NLM provide instances of this class. None of the
320 them block. If you have out-of-tree instances - please, show up. Locking
321 in that area will change.
322 --------------------------- buffer_head -----------------------------------
324 void (*b_end_io)(struct buffer_head *bh, int uptodate);
327 called from interrupts. In other words, extreme care is needed here.
328 bh is locked, but that's all warranties we have here. Currently only RAID1,
329 highmem, fs/buffer.c, and fs/ntfs/aops.c are providing these. Block devices
330 call this method upon the IO completion.
332 --------------------------- block_device_operations -----------------------
334 int (*open) (struct inode *, struct file *);
335 int (*release) (struct inode *, struct file *);
336 int (*ioctl) (struct inode *, struct file *, unsigned, unsigned long);
337 int (*media_changed) (struct gendisk *);
338 int (*revalidate_disk) (struct gendisk *);
346 revalidate_disk: no no
348 The last two are called only from check_disk_change().
350 --------------------------- file_operations -------------------------------
352 loff_t (*llseek) (struct file *, loff_t, int);
353 ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
354 ssize_t (*aio_read) (struct kiocb *, char __user *, size_t, loff_t);
355 ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
356 ssize_t (*aio_write) (struct kiocb *, const char __user *, size_t,
358 int (*readdir) (struct file *, void *, filldir_t);
359 unsigned int (*poll) (struct file *, struct poll_table_struct *);
360 int (*ioctl) (struct inode *, struct file *, unsigned int,
362 long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
363 long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
364 int (*mmap) (struct file *, struct vm_area_struct *);
365 int (*open) (struct inode *, struct file *);
366 int (*flush) (struct file *);
367 int (*release) (struct inode *, struct file *);
368 int (*fsync) (struct file *, struct dentry *, int datasync);
369 int (*aio_fsync) (struct kiocb *, int datasync);
370 int (*fasync) (int, struct file *, int);
371 int (*lock) (struct file *, int, struct file_lock *);
372 ssize_t (*readv) (struct file *, const struct iovec *, unsigned long,
374 ssize_t (*writev) (struct file *, const struct iovec *, unsigned long,
376 ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t,
378 ssize_t (*sendpage) (struct file *, struct page *, int, size_t,
380 unsigned long (*get_unmapped_area)(struct file *, unsigned long,
381 unsigned long, unsigned long, unsigned long);
382 int (*check_flags)(int);
383 int (*dir_notify)(struct file *, unsigned long);
387 All except ->poll() may block.
389 llseek: no (see below)
396 ioctl: yes (see below)
397 unlocked_ioctl: no (see below)
400 open: maybe (see below)
403 fsync: no (see below)
405 fasync: yes (see below)
411 get_unmapped_area: no
415 ->llseek() locking has moved from llseek to the individual llseek
416 implementations. If your fs is not using generic_file_llseek, you
417 need to acquire and release the appropriate locks in your ->llseek().
418 For many filesystems, it is probably safe to acquire the inode
419 semaphore. Note some filesystems (i.e. remote ones) provide no
420 protection for i_size so you will need to use the BKL.
422 ->open() locking is in-transit: big lock partially moved into the methods.
423 The only exception is ->open() in the instances of file_operations that never
424 end up in ->i_fop/->proc_fops, i.e. ones that belong to character devices
425 (chrdev_open() takes lock before replacing ->f_op and calling the secondary
426 method. As soon as we fix the handling of module reference counters all
427 instances of ->open() will be called without the BKL.
429 Note: ext2_release() was *the* source of contention on fs-intensive
430 loads and dropping BKL on ->release() helps to get rid of that (we still
431 grab BKL for cases when we close a file that had been opened r/w, but that
432 can and should be done using the internal locking with smaller critical areas).
433 Current worst offender is ext2_get_block()...
435 ->fasync() is a mess. This area needs a big cleanup and that will probably
438 ->readdir() and ->ioctl() on directories must be changed. Ideally we would
439 move ->readdir() to inode_operations and use a separate method for directory
440 ->ioctl() or kill the latter completely. One of the problems is that for
441 anything that resembles union-mount we won't have a struct file for all
442 components. And there are other reasons why the current interface is a mess...
444 ->ioctl() on regular files is superceded by the ->unlocked_ioctl() that
445 doesn't take the BKL.
447 ->read on directories probably must go away - we should just enforce -EISDIR
448 in sys_read() and friends.
450 ->fsync() has i_sem on inode.
452 --------------------------- dquot_operations -------------------------------
454 int (*initialize) (struct inode *, int);
455 int (*drop) (struct inode *);
456 int (*alloc_space) (struct inode *, qsize_t, int);
457 int (*alloc_inode) (const struct inode *, unsigned long);
458 int (*free_space) (struct inode *, qsize_t);
459 int (*free_inode) (const struct inode *, unsigned long);
460 int (*transfer) (struct inode *, struct iattr *);
461 int (*write_dquot) (struct dquot *);
462 int (*acquire_dquot) (struct dquot *);
463 int (*release_dquot) (struct dquot *);
464 int (*mark_dirty) (struct dquot *);
465 int (*write_info) (struct super_block *, int);
467 These operations are intended to be more or less wrapping functions that ensure
468 a proper locking wrt the filesystem and call the generic quota operations.
470 What filesystem should expect from the generic quota functions:
472 FS recursion Held locks when called
473 initialize: yes maybe dqonoff_sem
475 alloc_space: ->mark_dirty() -
476 alloc_inode: ->mark_dirty() -
477 free_space: ->mark_dirty() -
478 free_inode: ->mark_dirty() -
480 write_dquot: yes dqonoff_sem or dqptr_sem
481 acquire_dquot: yes dqonoff_sem or dqptr_sem
482 release_dquot: yes dqonoff_sem or dqptr_sem
484 write_info: yes dqonoff_sem
486 FS recursion means calling ->quota_read() and ->quota_write() from superblock
489 ->alloc_space(), ->alloc_inode(), ->free_space(), ->free_inode() are called
490 only directly by the filesystem and do not call any fs functions only
491 the ->mark_dirty() operation.
493 More details about quota locking can be found in fs/dquot.c.
495 --------------------------- vm_operations_struct -----------------------------
497 void (*open)(struct vm_area_struct*);
498 void (*close)(struct vm_area_struct*);
499 struct page *(*nopage)(struct vm_area_struct*, unsigned long, int *);
507 ================================================================================
510 (if you break something or notice that it is broken and do not fix it yourself
511 - at least put it here)
513 ipc/shm.c::shm_delete() - may need BKL.
514 ->read() and ->write() in many drivers are (probably) missing BKL.
515 drivers/sgi/char/graphics.c::sgi_graphics_nopage() - may need BKL.