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 *, unsigned int);
13 int (*d_weak_revalidate)(struct dentry *, unsigned int);
14 int (*d_hash)(const struct dentry *, struct qstr *);
15 int (*d_compare)(const struct dentry *, const struct dentry *,
16 unsigned int, const char *, const struct qstr *);
17 int (*d_delete)(struct dentry *);
18 void (*d_release)(struct dentry *);
19 void (*d_iput)(struct dentry *, struct inode *);
20 char *(*d_dname)((struct dentry *dentry, char *buffer, int buflen);
21 struct vfsmount *(*d_automount)(struct path *path);
22 int (*d_manage)(struct dentry *, bool);
25 rename_lock ->d_lock may block rcu-walk
26 d_revalidate: no no yes (ref-walk) maybe
27 d_weak_revalidate:no no yes no
29 d_compare: yes no no maybe
30 d_delete: no yes no no
31 d_release: no no yes no
35 d_automount: no no yes no
36 d_manage: no no yes (ref-walk) maybe
38 --------------------------- inode_operations ---------------------------
40 int (*create) (struct inode *,struct dentry *,umode_t, bool);
41 struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
42 int (*link) (struct dentry *,struct inode *,struct dentry *);
43 int (*unlink) (struct inode *,struct dentry *);
44 int (*symlink) (struct inode *,struct dentry *,const char *);
45 int (*mkdir) (struct inode *,struct dentry *,umode_t);
46 int (*rmdir) (struct inode *,struct dentry *);
47 int (*mknod) (struct inode *,struct dentry *,umode_t,dev_t);
48 int (*rename) (struct inode *, struct dentry *,
49 struct inode *, struct dentry *);
50 int (*rename2) (struct inode *, struct dentry *,
51 struct inode *, struct dentry *, unsigned int);
52 int (*readlink) (struct dentry *, char __user *,int);
53 const char *(*get_link) (struct dentry *, struct inode *, void **);
54 void (*truncate) (struct inode *);
55 int (*permission) (struct inode *, int, unsigned int);
56 int (*get_acl)(struct inode *, int);
57 int (*setattr) (struct dentry *, struct iattr *);
58 int (*getattr) (struct vfsmount *, struct dentry *, struct kstat *);
59 int (*setxattr) (struct dentry *, const char *,const void *,size_t,int);
60 ssize_t (*getxattr) (struct dentry *, const char *, void *, size_t);
61 ssize_t (*listxattr) (struct dentry *, char *, size_t);
62 int (*removexattr) (struct dentry *, const char *);
63 int (*fiemap)(struct inode *, struct fiemap_extent_info *, u64 start, u64 len);
64 void (*update_time)(struct inode *, struct timespec *, int);
65 int (*atomic_open)(struct inode *, struct dentry *,
66 struct file *, unsigned open_flag,
67 umode_t create_mode, int *opened);
68 int (*tmpfile) (struct inode *, struct dentry *, umode_t);
69 int (*dentry_open)(struct dentry *, struct file *, const struct cred *);
81 rmdir: yes (both) (see below)
82 rename: yes (all) (see below)
83 rename2: yes (all) (see below)
87 permission: no (may not block if called in rcu-walk mode)
100 Additionally, ->rmdir(), ->unlink() and ->rename() have ->i_mutex on
102 cross-directory ->rename() and rename2() has (per-superblock)
105 See Documentation/filesystems/directory-locking for more detailed discussion
106 of the locking scheme for directory operations.
108 --------------------------- super_operations ---------------------------
110 struct inode *(*alloc_inode)(struct super_block *sb);
111 void (*destroy_inode)(struct inode *);
112 void (*dirty_inode) (struct inode *, int flags);
113 int (*write_inode) (struct inode *, struct writeback_control *wbc);
114 int (*drop_inode) (struct inode *);
115 void (*evict_inode) (struct inode *);
116 void (*put_super) (struct super_block *);
117 int (*sync_fs)(struct super_block *sb, int wait);
118 int (*freeze_fs) (struct super_block *);
119 int (*unfreeze_fs) (struct super_block *);
120 int (*statfs) (struct dentry *, struct kstatfs *);
121 int (*remount_fs) (struct super_block *, int *, char *);
122 void (*umount_begin) (struct super_block *);
123 int (*show_options)(struct seq_file *, struct dentry *);
124 ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
125 ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
126 int (*bdev_try_to_free_page)(struct super_block*, struct page*, gfp_t);
129 All may block [not true, see below]
135 drop_inode: !!!inode->i_lock!!!
141 statfs: maybe(read) (see below)
144 show_options: no (namespace_sem)
145 quota_read: no (see below)
146 quota_write: no (see below)
147 bdev_try_to_free_page: no (see below)
149 ->statfs() has s_umount (shared) when called by ustat(2) (native or
150 compat), but that's an accident of bad API; s_umount is used to pin
151 the superblock down when we only have dev_t given us by userland to
152 identify the superblock. Everything else (statfs(), fstatfs(), etc.)
153 doesn't hold it when calling ->statfs() - superblock is pinned down
154 by resolving the pathname passed to syscall.
155 ->quota_read() and ->quota_write() functions are both guaranteed to
156 be the only ones operating on the quota file by the quota code (via
157 dqio_sem) (unless an admin really wants to screw up something and
158 writes to quota files with quotas on). For other details about locking
159 see also dquot_operations section.
160 ->bdev_try_to_free_page is called from the ->releasepage handler of
161 the block device inode. See there for more details.
163 --------------------------- file_system_type ---------------------------
165 struct dentry *(*mount) (struct file_system_type *, int,
166 const char *, void *);
167 void (*kill_sb) (struct super_block *);
173 ->mount() returns ERR_PTR or the root dentry; its superblock should be locked
175 ->kill_sb() takes a write-locked superblock, does all shutdown work on it,
176 unlocks and drops the reference.
178 --------------------------- address_space_operations --------------------------
180 int (*writepage)(struct page *page, struct writeback_control *wbc);
181 int (*readpage)(struct file *, struct page *);
182 int (*sync_page)(struct page *);
183 int (*writepages)(struct address_space *, struct writeback_control *);
184 int (*set_page_dirty)(struct page *page);
185 int (*readpages)(struct file *filp, struct address_space *mapping,
186 struct list_head *pages, unsigned nr_pages);
187 int (*write_begin)(struct file *, struct address_space *mapping,
188 loff_t pos, unsigned len, unsigned flags,
189 struct page **pagep, void **fsdata);
190 int (*write_end)(struct file *, struct address_space *mapping,
191 loff_t pos, unsigned len, unsigned copied,
192 struct page *page, void *fsdata);
193 sector_t (*bmap)(struct address_space *, sector_t);
194 void (*invalidatepage) (struct page *, unsigned int, unsigned int);
195 int (*releasepage) (struct page *, int);
196 void (*freepage)(struct page *);
197 int (*direct_IO)(struct kiocb *, struct iov_iter *iter);
198 bool (*isolate_page) (struct page *, isolate_mode_t);
199 int (*migratepage)(struct address_space *, struct page *, struct page *);
200 void (*putback_page) (struct page *);
201 int (*launder_page)(struct page *);
202 int (*is_partially_uptodate)(struct page *, unsigned long, unsigned long);
203 int (*error_remove_page)(struct address_space *, struct page *);
204 int (*swap_activate)(struct file *);
205 int (*swap_deactivate)(struct file *);
208 All except set_page_dirty and freepage may block
210 PageLocked(page) i_mutex
211 writepage: yes, unlocks (see below)
212 readpage: yes, unlocks
217 write_begin: locks the page yes
218 write_end: yes, unlocks yes
225 migratepage: yes (both)
228 is_partially_uptodate: yes
229 error_remove_page: yes
233 ->write_begin(), ->write_end(), ->sync_page() and ->readpage()
234 may be called from the request handler (/dev/loop).
236 ->readpage() unlocks the page, either synchronously or via I/O
239 ->readpages() populates the pagecache with the passed pages and starts
240 I/O against them. They come unlocked upon I/O completion.
242 ->writepage() is used for two purposes: for "memory cleansing" and for
243 "sync". These are quite different operations and the behaviour may differ
244 depending upon the mode.
246 If writepage is called for sync (wbc->sync_mode != WBC_SYNC_NONE) then
247 it *must* start I/O against the page, even if that would involve
248 blocking on in-progress I/O.
250 If writepage is called for memory cleansing (sync_mode ==
251 WBC_SYNC_NONE) then its role is to get as much writeout underway as
252 possible. So writepage should try to avoid blocking against
253 currently-in-progress I/O.
255 If the filesystem is not called for "sync" and it determines that it
256 would need to block against in-progress I/O to be able to start new I/O
257 against the page the filesystem should redirty the page with
258 redirty_page_for_writepage(), then unlock the page and return zero.
259 This may also be done to avoid internal deadlocks, but rarely.
261 If the filesystem is called for sync then it must wait on any
262 in-progress I/O and then start new I/O.
264 The filesystem should unlock the page synchronously, before returning to the
265 caller, unless ->writepage() returns special WRITEPAGE_ACTIVATE
266 value. WRITEPAGE_ACTIVATE means that page cannot really be written out
267 currently, and VM should stop calling ->writepage() on this page for some
268 time. VM does this by moving page to the head of the active list, hence the
271 Unless the filesystem is going to redirty_page_for_writepage(), unlock the page
272 and return zero, writepage *must* run set_page_writeback() against the page,
273 followed by unlocking it. Once set_page_writeback() has been run against the
274 page, write I/O can be submitted and the write I/O completion handler must run
275 end_page_writeback() once the I/O is complete. If no I/O is submitted, the
276 filesystem must run end_page_writeback() against the page before returning from
279 That is: after 2.5.12, pages which are under writeout are *not* locked. Note,
280 if the filesystem needs the page to be locked during writeout, that is ok, too,
281 the page is allowed to be unlocked at any point in time between the calls to
282 set_page_writeback() and end_page_writeback().
284 Note, failure to run either redirty_page_for_writepage() or the combination of
285 set_page_writeback()/end_page_writeback() on a page submitted to writepage
286 will leave the page itself marked clean but it will be tagged as dirty in the
287 radix tree. This incoherency can lead to all sorts of hard-to-debug problems
288 in the filesystem like having dirty inodes at umount and losing written data.
290 ->sync_page() locking rules are not well-defined - usually it is called
291 with lock on page, but that is not guaranteed. Considering the currently
292 existing instances of this method ->sync_page() itself doesn't look
295 ->writepages() is used for periodic writeback and for syscall-initiated
296 sync operations. The address_space should start I/O against at least
297 *nr_to_write pages. *nr_to_write must be decremented for each page which is
298 written. The address_space implementation may write more (or less) pages
299 than *nr_to_write asks for, but it should try to be reasonably close. If
300 nr_to_write is NULL, all dirty pages must be written.
302 writepages should _only_ write pages which are present on
305 ->set_page_dirty() is called from various places in the kernel
306 when the target page is marked as needing writeback. It may be called
307 under spinlock (it cannot block) and is sometimes called with the page
310 ->bmap() is currently used by legacy ioctl() (FIBMAP) provided by some
311 filesystems and by the swapper. The latter will eventually go away. Please,
312 keep it that way and don't breed new callers.
314 ->invalidatepage() is called when the filesystem must attempt to drop
315 some or all of the buffers from the page when it is being truncated. It
316 returns zero on success. If ->invalidatepage is zero, the kernel uses
317 block_invalidatepage() instead.
319 ->releasepage() is called when the kernel is about to try to drop the
320 buffers from the page in preparation for freeing it. It returns zero to
321 indicate that the buffers are (or may be) freeable. If ->releasepage is zero,
322 the kernel assumes that the fs has no private interest in the buffers.
324 ->freepage() is called when the kernel is done dropping the page
327 ->launder_page() may be called prior to releasing a page if
328 it is still found to be dirty. It returns zero if the page was successfully
329 cleaned, or an error value if not. Note that in order to prevent the page
330 getting mapped back in and redirtied, it needs to be kept locked
331 across the entire operation.
333 ->swap_activate will be called with a non-zero argument on
334 files backing (non block device backed) swapfiles. A return value
335 of zero indicates success, in which case this file can be used for
336 backing swapspace. The swapspace operations will be proxied to the
337 address space operations.
339 ->swap_deactivate() will be called in the sys_swapoff()
340 path after ->swap_activate() returned success.
342 ----------------------- file_lock_operations ------------------------------
344 void (*fl_copy_lock)(struct file_lock *, struct file_lock *);
345 void (*fl_release_private)(struct file_lock *);
349 inode->i_lock may block
351 fl_release_private: maybe maybe[1]
353 [1]: ->fl_release_private for flock or POSIX locks is currently allowed
354 to block. Leases however can still be freed while the i_lock is held and
355 so fl_release_private called on a lease should not block.
357 ----------------------- lock_manager_operations ---------------------------
359 int (*lm_compare_owner)(struct file_lock *, struct file_lock *);
360 unsigned long (*lm_owner_key)(struct file_lock *);
361 void (*lm_notify)(struct file_lock *); /* unblock callback */
362 int (*lm_grant)(struct file_lock *, struct file_lock *, int);
363 void (*lm_break)(struct file_lock *); /* break_lease callback */
364 int (*lm_change)(struct file_lock **, int);
368 inode->i_lock blocked_lock_lock may block
369 lm_compare_owner: yes[1] maybe no
370 lm_owner_key yes[1] yes no
371 lm_notify: yes yes no
376 [1]: ->lm_compare_owner and ->lm_owner_key are generally called with
377 *an* inode->i_lock held. It may not be the i_lock of the inode
378 associated with either file_lock argument! This is the case with deadlock
379 detection, since the code has to chase down the owners of locks that may
380 be entirely unrelated to the one on which the lock is being acquired.
381 For deadlock detection however, the blocked_lock_lock is also held. The
382 fact that these locks are held ensures that the file_locks do not
383 disappear out from under you while doing the comparison or generating an
386 --------------------------- buffer_head -----------------------------------
388 void (*b_end_io)(struct buffer_head *bh, int uptodate);
391 called from interrupts. In other words, extreme care is needed here.
392 bh is locked, but that's all warranties we have here. Currently only RAID1,
393 highmem, fs/buffer.c, and fs/ntfs/aops.c are providing these. Block devices
394 call this method upon the IO completion.
396 --------------------------- block_device_operations -----------------------
398 int (*open) (struct block_device *, fmode_t);
399 int (*release) (struct gendisk *, fmode_t);
400 int (*ioctl) (struct block_device *, fmode_t, unsigned, unsigned long);
401 int (*compat_ioctl) (struct block_device *, fmode_t, unsigned, unsigned long);
402 int (*direct_access) (struct block_device *, sector_t, void __pmem **,
404 int (*media_changed) (struct gendisk *);
405 void (*unlock_native_capacity) (struct gendisk *);
406 int (*revalidate_disk) (struct gendisk *);
407 int (*getgeo)(struct block_device *, struct hd_geometry *);
408 void (*swap_slot_free_notify) (struct block_device *, unsigned long);
418 unlock_native_capacity: no
421 swap_slot_free_notify: no (see below)
423 media_changed, unlock_native_capacity and revalidate_disk are called only from
426 swap_slot_free_notify is called with swap_lock and sometimes the page lock
430 --------------------------- file_operations -------------------------------
432 loff_t (*llseek) (struct file *, loff_t, int);
433 ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
434 ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
435 ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
436 ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
437 int (*iterate) (struct file *, struct dir_context *);
438 unsigned int (*poll) (struct file *, struct poll_table_struct *);
439 long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
440 long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
441 int (*mmap) (struct file *, struct vm_area_struct *);
442 int (*open) (struct inode *, struct file *);
443 int (*flush) (struct file *);
444 int (*release) (struct inode *, struct file *);
445 int (*fsync) (struct file *, loff_t start, loff_t end, int datasync);
446 int (*aio_fsync) (struct kiocb *, int datasync);
447 int (*fasync) (int, struct file *, int);
448 int (*lock) (struct file *, int, struct file_lock *);
449 ssize_t (*readv) (struct file *, const struct iovec *, unsigned long,
451 ssize_t (*writev) (struct file *, const struct iovec *, unsigned long,
453 ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t,
455 ssize_t (*sendpage) (struct file *, struct page *, int, size_t,
457 unsigned long (*get_unmapped_area)(struct file *, unsigned long,
458 unsigned long, unsigned long, unsigned long);
459 int (*check_flags)(int);
460 int (*flock) (struct file *, int, struct file_lock *);
461 ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *,
462 size_t, unsigned int);
463 ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *,
464 size_t, unsigned int);
465 int (*setlease)(struct file *, long, struct file_lock **, void **);
466 long (*fallocate)(struct file *, int, loff_t, loff_t);
472 ->llseek() locking has moved from llseek to the individual llseek
473 implementations. If your fs is not using generic_file_llseek, you
474 need to acquire and release the appropriate locks in your ->llseek().
475 For many filesystems, it is probably safe to acquire the inode
476 mutex or just to use i_size_read() instead.
477 Note: this does not protect the file->f_pos against concurrent modifications
478 since this is something the userspace has to take care about.
480 ->fasync() is responsible for maintaining the FASYNC bit in filp->f_flags.
481 Most instances call fasync_helper(), which does that maintenance, so it's
482 not normally something one needs to worry about. Return values > 0 will be
483 mapped to zero in the VFS layer.
485 ->readdir() and ->ioctl() on directories must be changed. Ideally we would
486 move ->readdir() to inode_operations and use a separate method for directory
487 ->ioctl() or kill the latter completely. One of the problems is that for
488 anything that resembles union-mount we won't have a struct file for all
489 components. And there are other reasons why the current interface is a mess...
491 ->read on directories probably must go away - we should just enforce -EISDIR
492 in sys_read() and friends.
494 ->setlease operations should call generic_setlease() before or after setting
495 the lease within the individual filesystem to record the result of the
498 --------------------------- dquot_operations -------------------------------
500 int (*write_dquot) (struct dquot *);
501 int (*acquire_dquot) (struct dquot *);
502 int (*release_dquot) (struct dquot *);
503 int (*mark_dirty) (struct dquot *);
504 int (*write_info) (struct super_block *, int);
506 These operations are intended to be more or less wrapping functions that ensure
507 a proper locking wrt the filesystem and call the generic quota operations.
509 What filesystem should expect from the generic quota functions:
511 FS recursion Held locks when called
512 write_dquot: yes dqonoff_sem or dqptr_sem
513 acquire_dquot: yes dqonoff_sem or dqptr_sem
514 release_dquot: yes dqonoff_sem or dqptr_sem
516 write_info: yes dqonoff_sem
518 FS recursion means calling ->quota_read() and ->quota_write() from superblock
521 More details about quota locking can be found in fs/dquot.c.
523 --------------------------- vm_operations_struct -----------------------------
525 void (*open)(struct vm_area_struct*);
526 void (*close)(struct vm_area_struct*);
527 int (*fault)(struct vm_area_struct*, struct vm_fault *);
528 int (*page_mkwrite)(struct vm_area_struct *, struct vm_fault *);
529 int (*pfn_mkwrite)(struct vm_area_struct *, struct vm_fault *);
530 int (*access)(struct vm_area_struct *, unsigned long, void*, int, int);
533 mmap_sem PageLocked(page)
536 fault: yes can return with page locked
538 page_mkwrite: yes can return with page locked
542 ->fault() is called when a previously not present pte is about
543 to be faulted in. The filesystem must find and return the page associated
544 with the passed in "pgoff" in the vm_fault structure. If it is possible that
545 the page may be truncated and/or invalidated, then the filesystem must lock
546 the page, then ensure it is not already truncated (the page lock will block
547 subsequent truncate), and then return with VM_FAULT_LOCKED, and the page
548 locked. The VM will unlock the page.
550 ->map_pages() is called when VM asks to map easy accessible pages.
551 Filesystem should find and map pages associated with offsets from "pgoff"
552 till "max_pgoff". ->map_pages() is called with page table locked and must
553 not block. If it's not possible to reach a page without blocking,
554 filesystem should skip it. Filesystem should use do_set_pte() to setup
555 page table entry. Pointer to entry associated with offset "pgoff" is
556 passed in "pte" field in vm_fault structure. Pointers to entries for other
557 offsets should be calculated relative to "pte".
559 ->page_mkwrite() is called when a previously read-only pte is
560 about to become writeable. The filesystem again must ensure that there are
561 no truncate/invalidate races, and then return with the page locked. If
562 the page has been truncated, the filesystem should not look up a new page
563 like the ->fault() handler, but simply return with VM_FAULT_NOPAGE, which
564 will cause the VM to retry the fault.
566 ->pfn_mkwrite() is the same as page_mkwrite but when the pte is
567 VM_PFNMAP or VM_MIXEDMAP with a page-less entry. Expected return is
568 VM_FAULT_NOPAGE. Or one of the VM_FAULT_ERROR types. The default behavior
569 after this call is to make the pte read-write, unless pfn_mkwrite returns
572 ->access() is called when get_user_pages() fails in
573 access_process_vm(), typically used to debug a process through
574 /proc/pid/mem or ptrace. This function is needed only for
575 VM_IO | VM_PFNMAP VMAs.
577 ================================================================================
580 (if you break something or notice that it is broken and do not fix it yourself
581 - at least put it here)