| blob | eea71025621845e9ebbff82eac37aa49ffb10908 |
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 --------------------------
11 prototypes:
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 *);
18 char *(*d_dname)((struct dentry *dentry, char *buffer, int buflen);
20 locking rules:
21 none have BKL
22 dcache_lock rename_lock ->d_lock may block
23 d_revalidate: no no no yes
24 d_hash no no no yes
25 d_compare: no yes no no
26 d_delete: yes no yes no
27 d_release: no no no yes
28 d_iput: no no no yes
29 d_dname: no no no no
31 --------------------------- inode_operations ---------------------------
32 prototypes:
33 int (*create) (struct inode *,struct dentry *,int, struct nameidata *);
34 struct dentry * (*lookup) (struct inode *,struct dentry *, struct nameid
35 ata *);
36 int (*link) (struct dentry *,struct inode *,struct dentry *);
37 int (*unlink) (struct inode *,struct dentry *);
38 int (*symlink) (struct inode *,struct dentry *,const char *);
39 int (*mkdir) (struct inode *,struct dentry *,int);
40 int (*rmdir) (struct inode *,struct dentry *);
41 int (*mknod) (struct inode *,struct dentry *,int,dev_t);
42 int (*rename) (struct inode *, struct dentry *,
43 struct inode *, struct dentry *);
44 int (*readlink) (struct dentry *, char __user *,int);
45 int (*follow_link) (struct dentry *, struct nameidata *);
46 void (*truncate) (struct inode *);
47 int (*permission) (struct inode *, int, struct nameidata *);
48 int (*setattr) (struct dentry *, struct iattr *);
49 int (*getattr) (struct vfsmount *, struct dentry *, struct kstat *);
50 int (*setxattr) (struct dentry *, const char *,const void *,size_t,int);
51 ssize_t (*getxattr) (struct dentry *, const char *, void *, size_t);
52 ssize_t (*listxattr) (struct dentry *, char *, size_t);
53 int (*removexattr) (struct dentry *, const char *);
55 locking rules:
56 all may block, none have BKL
57 i_mutex(inode)
58 lookup: yes
59 create: yes
60 link: yes (both)
61 mknod: yes
62 symlink: yes
63 mkdir: yes
64 unlink: yes (both)
65 rmdir: yes (both) (see below)
66 rename: yes (all) (see below)
67 readlink: no
68 follow_link: no
69 truncate: yes (see below)
70 setattr: yes
71 permission: no
72 getattr: no
73 setxattr: yes
74 getxattr: no
75 listxattr: no
76 removexattr: yes
77 Additionally, ->rmdir(), ->unlink() and ->rename() have ->i_mutex on
78 victim.
79 cross-directory ->rename() has (per-superblock) ->s_vfs_rename_sem.
80 ->truncate() is never called directly - it's a callback, not a
81 method. It's called by vmtruncate() - library function normally used by
82 ->setattr(). Locking information above applies to that call (i.e. is
83 inherited from ->setattr() - vmtruncate() is used when ATTR_SIZE had been
84 passed).
86 See Documentation/filesystems/directory-locking for more detailed discussion
87 of the locking scheme for directory operations.
89 --------------------------- super_operations ---------------------------
90 prototypes:
91 struct inode *(*alloc_inode)(struct super_block *sb);
92 void (*destroy_inode)(struct inode *);
93 void (*dirty_inode) (struct inode *);
94 int (*write_inode) (struct inode *, int);
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 int (*freeze_fs) (struct super_block *);
101 int (*unfreeze_fs) (struct super_block *);
102 int (*statfs) (struct dentry *, 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);
110 locking rules:
111 All may block.
112 BKL s_lock s_umount
113 alloc_inode: no no no
114 destroy_inode: no
115 dirty_inode: no (must not sleep)
116 write_inode: no
117 drop_inode: no !!!inode_lock!!!
118 delete_inode: no
119 put_super: yes yes no
120 write_super: no yes read
121 sync_fs: no no read
122 freeze_fs: ?
123 unfreeze_fs: ?
124 statfs: no no no
125 remount_fs: yes yes maybe (see below)
126 clear_inode: no
127 umount_begin: yes no no
128 show_options: no (vfsmount->sem)
129 quota_read: no no no (see below)
130 quota_write: no no no (see below)
132 ->remount_fs() will have the s_umount lock if it's already mounted.
133 When called from get_sb_single, it does NOT have the s_umount lock.
134 ->quota_read() and ->quota_write() functions are both guaranteed to
135 be the only ones operating on the quota file by the quota code (via
136 dqio_sem) (unless an admin really wants to screw up something and
137 writes to quota files with quotas on). For other details about locking
138 see also dquot_operations section.
140 --------------------------- file_system_type ---------------------------
141 prototypes:
142 int (*get_sb) (struct file_system_type *, int,
143 const char *, void *, struct vfsmount *);
144 void (*kill_sb) (struct super_block *);
145 locking rules:
146 may block BKL
147 get_sb yes no
148 kill_sb yes no
150 ->get_sb() returns error or 0 with locked superblock attached to the vfsmount
151 (exclusive on ->s_umount).
152 ->kill_sb() takes a write-locked superblock, does all shutdown work on it,
153 unlocks and drops the reference.
155 --------------------------- address_space_operations --------------------------
156 prototypes:
157 int (*writepage)(struct page *page, struct writeback_control *wbc);
158 int (*readpage)(struct file *, struct page *);
159 int (*sync_page)(struct page *);
160 int (*writepages)(struct address_space *, struct writeback_control *);
161 int (*set_page_dirty)(struct page *page);
162 int (*readpages)(struct file *filp, struct address_space *mapping,
163 struct list_head *pages, unsigned nr_pages);
164 int (*write_begin)(struct file *, struct address_space *mapping,
165 loff_t pos, unsigned len, unsigned flags,
166 struct page **pagep, void **fsdata);
167 int (*write_end)(struct file *, struct address_space *mapping,
168 loff_t pos, unsigned len, unsigned copied,
169 struct page *page, void *fsdata);
170 sector_t (*bmap)(struct address_space *, sector_t);
171 int (*invalidatepage) (struct page *, unsigned long);
172 int (*releasepage) (struct page *, int);
173 int (*direct_IO)(int, struct kiocb *, const struct iovec *iov,
174 loff_t offset, unsigned long nr_segs);
175 int (*launder_page) (struct page *);
177 locking rules:
178 All except set_page_dirty may block
180 BKL PageLocked(page) i_sem
181 writepage: no yes, unlocks (see below)
182 readpage: no yes, unlocks
183 sync_page: no maybe
184 writepages: no
185 set_page_dirty no no
186 readpages: no
187 write_begin: no locks the page yes
188 write_end: no yes, unlocks yes
189 perform_write: no n/a yes
190 bmap: yes
191 invalidatepage: no yes
192 releasepage: no yes
193 direct_IO: no
194 launder_page: no yes
196 ->write_begin(), ->write_end(), ->sync_page() and ->readpage()
197 may be called from the request handler (/dev/loop).
199 ->readpage() unlocks the page, either synchronously or via I/O
200 completion.
202 ->readpages() populates the pagecache with the passed pages and starts
203 I/O against them. They come unlocked upon I/O completion.
205 ->writepage() is used for two purposes: for "memory cleansing" and for
206 "sync". These are quite different operations and the behaviour may differ
207 depending upon the mode.
209 If writepage is called for sync (wbc->sync_mode != WBC_SYNC_NONE) then
210 it *must* start I/O against the page, even if that would involve
211 blocking on in-progress I/O.
213 If writepage is called for memory cleansing (sync_mode ==
214 WBC_SYNC_NONE) then its role is to get as much writeout underway as
215 possible. So writepage should try to avoid blocking against
216 currently-in-progress I/O.
218 If the filesystem is not called for "sync" and it determines that it
219 would need to block against in-progress I/O to be able to start new I/O
220 against the page the filesystem should redirty the page with
221 redirty_page_for_writepage(), then unlock the page and return zero.
222 This may also be done to avoid internal deadlocks, but rarely.
224 If the filesystem is called for sync then it must wait on any
225 in-progress I/O and then start new I/O.
227 The filesystem should unlock the page synchronously, before returning to the
228 caller, unless ->writepage() returns special WRITEPAGE_ACTIVATE
229 value. WRITEPAGE_ACTIVATE means that page cannot really be written out
230 currently, and VM should stop calling ->writepage() on this page for some
231 time. VM does this by moving page to the head of the active list, hence the
232 name.
234 Unless the filesystem is going to redirty_page_for_writepage(), unlock the page
235 and return zero, writepage *must* run set_page_writeback() against the page,
236 followed by unlocking it. Once set_page_writeback() has been run against the
237 page, write I/O can be submitted and the write I/O completion handler must run
238 end_page_writeback() once the I/O is complete. If no I/O is submitted, the
239 filesystem must run end_page_writeback() against the page before returning from
240 writepage.
242 That is: after 2.5.12, pages which are under writeout are *not* locked. Note,
243 if the filesystem needs the page to be locked during writeout, that is ok, too,
244 the page is allowed to be unlocked at any point in time between the calls to
245 set_page_writeback() and end_page_writeback().
247 Note, failure to run either redirty_page_for_writepage() or the combination of
248 set_page_writeback()/end_page_writeback() on a page submitted to writepage
249 will leave the page itself marked clean but it will be tagged as dirty in the
250 radix tree. This incoherency can lead to all sorts of hard-to-debug problems
251 in the filesystem like having dirty inodes at umount and losing written data.
253 ->sync_page() locking rules are not well-defined - usually it is called
254 with lock on page, but that is not guaranteed. Considering the currently
255 existing instances of this method ->sync_page() itself doesn't look
256 well-defined...
258 ->writepages() is used for periodic writeback and for syscall-initiated
259 sync operations. The address_space should start I/O against at least
260 *nr_to_write pages. *nr_to_write must be decremented for each page which is
261 written. The address_space implementation may write more (or less) pages
262 than *nr_to_write asks for, but it should try to be reasonably close. If
263 nr_to_write is NULL, all dirty pages must be written.
265 writepages should _only_ write pages which are present on
266 mapping->io_pages.
268 ->set_page_dirty() is called from various places in the kernel
269 when the target page is marked as needing writeback. It may be called
270 under spinlock (it cannot block) and is sometimes called with the page
271 not locked.
273 ->bmap() is currently used by legacy ioctl() (FIBMAP) provided by some
274 filesystems and by the swapper. The latter will eventually go away. All
275 instances do not actually need the BKL. Please, keep it that way and don't
276 breed new callers.
278 ->invalidatepage() is called when the filesystem must attempt to drop
279 some or all of the buffers from the page when it is being truncated. It
280 returns zero on success. If ->invalidatepage is zero, the kernel uses
281 block_invalidatepage() instead.
283 ->releasepage() is called when the kernel is about to try to drop the
284 buffers from the page in preparation for freeing it. It returns zero to
285 indicate that the buffers are (or may be) freeable. If ->releasepage is zero,
286 the kernel assumes that the fs has no private interest in the buffers.
288 ->launder_page() may be called prior to releasing a page if
289 it is still found to be dirty. It returns zero if the page was successfully
290 cleaned, or an error value if not. Note that in order to prevent the page
291 getting mapped back in and redirtied, it needs to be kept locked
292 across the entire operation.
294 Note: currently almost all instances of address_space methods are
295 using BKL for internal serialization and that's one of the worst sources
296 of contention. Normally they are calling library functions (in fs/buffer.c)
297 and pass foo_get_block() as a callback (on local block-based filesystems,
298 indeed). BKL is not needed for library stuff and is usually taken by
299 foo_get_block(). It's an overkill, since block bitmaps can be protected by
300 internal fs locking and real critical areas are much smaller than the areas
301 filesystems protect now.
303 ----------------------- file_lock_operations ------------------------------
304 prototypes:
305 void (*fl_insert)(struct file_lock *); /* lock insertion callback */
306 void (*fl_remove)(struct file_lock *); /* lock removal callback */
307 void (*fl_copy_lock)(struct file_lock *, struct file_lock *);
308 void (*fl_release_private)(struct file_lock *);
311 locking rules:
312 BKL may block
313 fl_insert: yes no
314 fl_remove: yes no
315 fl_copy_lock: yes no
316 fl_release_private: yes yes
318 ----------------------- lock_manager_operations ---------------------------
319 prototypes:
320 int (*fl_compare_owner)(struct file_lock *, struct file_lock *);
321 void (*fl_notify)(struct file_lock *); /* unblock callback */
322 void (*fl_copy_lock)(struct file_lock *, struct file_lock *);
323 void (*fl_release_private)(struct file_lock *);
324 void (*fl_break)(struct file_lock *); /* break_lease callback */
326 locking rules:
327 BKL may block
328 fl_compare_owner: yes no
329 fl_notify: yes no
330 fl_copy_lock: yes no
331 fl_release_private: yes yes
332 fl_break: yes no
334 Currently only NFSD and NLM provide instances of this class. None of the
335 them block. If you have out-of-tree instances - please, show up. Locking
336 in that area will change.
337 --------------------------- buffer_head -----------------------------------
338 prototypes:
339 void (*b_end_io)(struct buffer_head *bh, int uptodate);
341 locking rules:
342 called from interrupts. In other words, extreme care is needed here.
343 bh is locked, but that's all warranties we have here. Currently only RAID1,
344 highmem, fs/buffer.c, and fs/ntfs/aops.c are providing these. Block devices
345 call this method upon the IO completion.
347 --------------------------- block_device_operations -----------------------
348 prototypes:
349 int (*open) (struct inode *, struct file *);
350 int (*release) (struct inode *, struct file *);
351 int (*ioctl) (struct inode *, struct file *, unsigned, unsigned long);
352 int (*media_changed) (struct gendisk *);
353 int (*revalidate_disk) (struct gendisk *);
355 locking rules:
356 BKL bd_sem
357 open: yes yes
358 release: yes yes
359 ioctl: yes no
360 media_changed: no no
361 revalidate_disk: no no
363 The last two are called only from check_disk_change().
365 --------------------------- file_operations -------------------------------
366 prototypes:
367 loff_t (*llseek) (struct file *, loff_t, int);
368 ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
369 ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
370 ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
371 ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
372 int (*readdir) (struct file *, void *, filldir_t);
373 unsigned int (*poll) (struct file *, struct poll_table_struct *);
374 int (*ioctl) (struct inode *, struct file *, unsigned int,
375 unsigned long);
376 long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
377 long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
378 int (*mmap) (struct file *, struct vm_area_struct *);
379 int (*open) (struct inode *, struct file *);
380 int (*flush) (struct file *);
381 int (*release) (struct inode *, struct file *);
382 int (*fsync) (struct file *, struct dentry *, int datasync);
383 int (*aio_fsync) (struct kiocb *, int datasync);
384 int (*fasync) (int, struct file *, int);
385 int (*lock) (struct file *, int, struct file_lock *);
386 ssize_t (*readv) (struct file *, const struct iovec *, unsigned long,
387 loff_t *);
388 ssize_t (*writev) (struct file *, const struct iovec *, unsigned long,
389 loff_t *);
390 ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t,
391 void __user *);
392 ssize_t (*sendpage) (struct file *, struct page *, int, size_t,
393 loff_t *, int);
394 unsigned long (*get_unmapped_area)(struct file *, unsigned long,
395 unsigned long, unsigned long, unsigned long);
396 int (*check_flags)(int);
397 };
399 locking rules:
400 All may block.
401 BKL
402 llseek: no (see below)
403 read: no
404 aio_read: no
405 write: no
406 aio_write: no
407 readdir: no
408 poll: no
409 ioctl: yes (see below)
410 unlocked_ioctl: no (see below)
411 compat_ioctl: no
412 mmap: no
413 open: no
414 flush: no
415 release: no
416 fsync: no (see below)
417 aio_fsync: no
418 fasync: no
419 lock: yes
420 readv: no
421 writev: no
422 sendfile: no
423 sendpage: no
424 get_unmapped_area: no
425 check_flags: no
427 ->llseek() locking has moved from llseek to the individual llseek
428 implementations. If your fs is not using generic_file_llseek, you
429 need to acquire and release the appropriate locks in your ->llseek().
430 For many filesystems, it is probably safe to acquire the inode
431 semaphore. Note some filesystems (i.e. remote ones) provide no
432 protection for i_size so you will need to use the BKL.
434 Note: ext2_release() was *the* source of contention on fs-intensive
435 loads and dropping BKL on ->release() helps to get rid of that (we still
436 grab BKL for cases when we close a file that had been opened r/w, but that
437 can and should be done using the internal locking with smaller critical areas).
438 Current worst offender is ext2_get_block()...
440 ->fasync() is a mess. This area needs a big cleanup and that will probably
441 affect locking.
443 ->readdir() and ->ioctl() on directories must be changed. Ideally we would
444 move ->readdir() to inode_operations and use a separate method for directory
445 ->ioctl() or kill the latter completely. One of the problems is that for
446 anything that resembles union-mount we won't have a struct file for all
447 components. And there are other reasons why the current interface is a mess...
449 ->ioctl() on regular files is superceded by the ->unlocked_ioctl() that
450 doesn't take the BKL.
452 ->read on directories probably must go away - we should just enforce -EISDIR
453 in sys_read() and friends.
455 ->fsync() has i_mutex on inode.
457 --------------------------- dquot_operations -------------------------------
458 prototypes:
459 int (*initialize) (struct inode *, int);
460 int (*drop) (struct inode *);
461 int (*alloc_space) (struct inode *, qsize_t, int);
462 int (*alloc_inode) (const struct inode *, unsigned long);
463 int (*free_space) (struct inode *, qsize_t);
464 int (*free_inode) (const struct inode *, unsigned long);
465 int (*transfer) (struct inode *, struct iattr *);
466 int (*write_dquot) (struct dquot *);
467 int (*acquire_dquot) (struct dquot *);
468 int (*release_dquot) (struct dquot *);
469 int (*mark_dirty) (struct dquot *);
470 int (*write_info) (struct super_block *, int);
472 These operations are intended to be more or less wrapping functions that ensure
473 a proper locking wrt the filesystem and call the generic quota operations.
475 What filesystem should expect from the generic quota functions:
477 FS recursion Held locks when called
478 initialize: yes maybe dqonoff_sem
479 drop: yes -
480 alloc_space: ->mark_dirty() -
481 alloc_inode: ->mark_dirty() -
482 free_space: ->mark_dirty() -
483 free_inode: ->mark_dirty() -
484 transfer: yes -
485 write_dquot: yes dqonoff_sem or dqptr_sem
486 acquire_dquot: yes dqonoff_sem or dqptr_sem
487 release_dquot: yes dqonoff_sem or dqptr_sem
488 mark_dirty: no -
489 write_info: yes dqonoff_sem
491 FS recursion means calling ->quota_read() and ->quota_write() from superblock
492 operations.
494 ->alloc_space(), ->alloc_inode(), ->free_space(), ->free_inode() are called
495 only directly by the filesystem and do not call any fs functions only
496 the ->mark_dirty() operation.
498 More details about quota locking can be found in fs/dquot.c.
500 --------------------------- vm_operations_struct -----------------------------
501 prototypes:
502 void (*open)(struct vm_area_struct*);
503 void (*close)(struct vm_area_struct*);
504 int (*fault)(struct vm_area_struct*, struct vm_fault *);
505 int (*page_mkwrite)(struct vm_area_struct *, struct vm_fault *);
506 int (*access)(struct vm_area_struct *, unsigned long, void*, int, int);
508 locking rules:
509 BKL mmap_sem PageLocked(page)
510 open: no yes
511 close: no yes
512 fault: no yes can return with page locked
513 page_mkwrite: no yes can return with page locked
514 access: no yes
516 ->fault() is called when a previously not present pte is about
517 to be faulted in. The filesystem must find and return the page associated
518 with the passed in "pgoff" in the vm_fault structure. If it is possible that
519 the page may be truncated and/or invalidated, then the filesystem must lock
520 the page, then ensure it is not already truncated (the page lock will block
521 subsequent truncate), and then return with VM_FAULT_LOCKED, and the page
522 locked. The VM will unlock the page.
524 ->page_mkwrite() is called when a previously read-only pte is
525 about to become writeable. The filesystem again must ensure that there are
526 no truncate/invalidate races, and then return with the page locked. If
527 the page has been truncated, the filesystem should not look up a new page
528 like the ->fault() handler, but simply return with VM_FAULT_NOPAGE, which
529 will cause the VM to retry the fault.
531 ->access() is called when get_user_pages() fails in
532 acces_process_vm(), typically used to debug a process through
533 /proc/pid/mem or ptrace. This function is needed only for
534 VM_IO | VM_PFNMAP VMAs.
536 ================================================================================
537 Dubious stuff
539 (if you break something or notice that it is broken and do not fix it yourself
540 - at least put it here)
542 ipc/shm.c::shm_delete() - may need BKL.
543 ->read() and ->write() in many drivers are (probably) missing BKL.