5 The text below describes the locking rules for VFS-related methods.
6 It is (believed to be) up-to-date. *Please*, if you change anything in
7 prototypes or locking protocols - update this file. And update the relevant
8 instances in the tree, don't leave that to maintainers of filesystems/devices/
9 etc. At the very least, put the list of dubious cases in the end of this file.
10 Don't turn it into log - maintainers of out-of-the-tree code are supposed to
11 be able to use diff(1).
13 Thing currently missing here: socket operations. Alexey?
20 int (*d_revalidate)(struct dentry *, unsigned int);
21 int (*d_weak_revalidate)(struct dentry *, unsigned int);
22 int (*d_hash)(const struct dentry *, struct qstr *);
23 int (*d_compare)(const struct dentry *,
24 unsigned int, const char *, const struct qstr *);
25 int (*d_delete)(struct dentry *);
26 int (*d_init)(struct dentry *);
27 void (*d_release)(struct dentry *);
28 void (*d_iput)(struct dentry *, struct inode *);
29 char *(*d_dname)((struct dentry *dentry, char *buffer, int buflen);
30 struct vfsmount *(*d_automount)(struct path *path);
31 int (*d_manage)(const struct path *, bool);
32 struct dentry *(*d_real)(struct dentry *, const struct inode *);
36 ================== =========== ======== ============== ========
37 ops rename_lock ->d_lock may block rcu-walk
38 ================== =========== ======== ============== ========
39 d_revalidate: no no yes (ref-walk) maybe
40 d_weak_revalidate: no no yes no
42 d_compare: yes no no maybe
43 d_delete: no yes no no
45 d_release: no no yes no
49 d_automount: no no yes no
50 d_manage: no no yes (ref-walk) maybe
52 ================== =========== ======== ============== ========
59 int (*create) (struct inode *,struct dentry *,umode_t, bool);
60 struct dentry * (*lookup) (struct inode *,struct dentry *, unsigned int);
61 int (*link) (struct dentry *,struct inode *,struct dentry *);
62 int (*unlink) (struct inode *,struct dentry *);
63 int (*symlink) (struct inode *,struct dentry *,const char *);
64 int (*mkdir) (struct inode *,struct dentry *,umode_t);
65 int (*rmdir) (struct inode *,struct dentry *);
66 int (*mknod) (struct inode *,struct dentry *,umode_t,dev_t);
67 int (*rename) (struct inode *, struct dentry *,
68 struct inode *, struct dentry *, unsigned int);
69 int (*readlink) (struct dentry *, char __user *,int);
70 const char *(*get_link) (struct dentry *, struct inode *, struct delayed_call *);
71 void (*truncate) (struct inode *);
72 int (*permission) (struct inode *, int, unsigned int);
73 int (*get_acl)(struct inode *, int);
74 int (*setattr) (struct dentry *, struct iattr *);
75 int (*getattr) (const struct path *, struct kstat *, u32, unsigned int);
76 ssize_t (*listxattr) (struct dentry *, char *, size_t);
77 int (*fiemap)(struct inode *, struct fiemap_extent_info *, u64 start, u64 len);
78 void (*update_time)(struct inode *, struct timespec *, int);
79 int (*atomic_open)(struct inode *, struct dentry *,
80 struct file *, unsigned open_flag,
82 int (*tmpfile) (struct inode *, struct dentry *, umode_t);
87 ============ =============================================
89 ============ =============================================
92 link: exclusive (both)
96 unlink: exclusive (both)
97 rmdir: exclusive (both)(see below)
98 rename: exclusive (both parents, some children) (see below)
102 permission: no (may not block if called in rcu-walk mode)
108 atomic_open: shared (exclusive if O_CREAT is set in open flags)
110 ============ =============================================
113 Additionally, ->rmdir(), ->unlink() and ->rename() have ->i_rwsem
115 cross-directory ->rename() has (per-superblock) ->s_vfs_rename_sem.
116 ->unlink() and ->rename() have ->i_rwsem exclusive on all non-directories
118 ->rename() has ->i_rwsem exclusive on any subdirectory that changes parent.
120 See Documentation/filesystems/directory-locking.rst for more detailed discussion
121 of the locking scheme for directory operations.
123 xattr_handler operations
124 ========================
128 bool (*list)(struct dentry *dentry);
129 int (*get)(const struct xattr_handler *handler, struct dentry *dentry,
130 struct inode *inode, const char *name, void *buffer,
132 int (*set)(const struct xattr_handler *handler, struct dentry *dentry,
133 struct inode *inode, const char *name, const void *buffer,
134 size_t size, int flags);
152 struct inode *(*alloc_inode)(struct super_block *sb);
153 void (*free_inode)(struct inode *);
154 void (*destroy_inode)(struct inode *);
155 void (*dirty_inode) (struct inode *, int flags);
156 int (*write_inode) (struct inode *, struct writeback_control *wbc);
157 int (*drop_inode) (struct inode *);
158 void (*evict_inode) (struct inode *);
159 void (*put_super) (struct super_block *);
160 int (*sync_fs)(struct super_block *sb, int wait);
161 int (*freeze_fs) (struct super_block *);
162 int (*unfreeze_fs) (struct super_block *);
163 int (*statfs) (struct dentry *, struct kstatfs *);
164 int (*remount_fs) (struct super_block *, int *, char *);
165 void (*umount_begin) (struct super_block *);
166 int (*show_options)(struct seq_file *, struct dentry *);
167 ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
168 ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
169 int (*bdev_try_to_free_page)(struct super_block*, struct page*, gfp_t);
172 All may block [not true, see below]
174 ====================== ============ ========================
176 ====================== ============ ========================
178 free_inode: called from RCU callback
182 drop_inode: !!!inode->i_lock!!!
188 statfs: maybe(read) (see below)
191 show_options: no (namespace_sem)
192 quota_read: no (see below)
193 quota_write: no (see below)
194 bdev_try_to_free_page: no (see below)
195 ====================== ============ ========================
197 ->statfs() has s_umount (shared) when called by ustat(2) (native or
198 compat), but that's an accident of bad API; s_umount is used to pin
199 the superblock down when we only have dev_t given us by userland to
200 identify the superblock. Everything else (statfs(), fstatfs(), etc.)
201 doesn't hold it when calling ->statfs() - superblock is pinned down
202 by resolving the pathname passed to syscall.
204 ->quota_read() and ->quota_write() functions are both guaranteed to
205 be the only ones operating on the quota file by the quota code (via
206 dqio_sem) (unless an admin really wants to screw up something and
207 writes to quota files with quotas on). For other details about locking
208 see also dquot_operations section.
210 ->bdev_try_to_free_page is called from the ->releasepage handler of
211 the block device inode. See there for more details.
218 struct dentry *(*mount) (struct file_system_type *, int,
219 const char *, void *);
220 void (*kill_sb) (struct super_block *);
231 ->mount() returns ERR_PTR or the root dentry; its superblock should be locked
234 ->kill_sb() takes a write-locked superblock, does all shutdown work on it,
235 unlocks and drops the reference.
237 address_space_operations
238 ========================
241 int (*writepage)(struct page *page, struct writeback_control *wbc);
242 int (*readpage)(struct file *, struct page *);
243 int (*writepages)(struct address_space *, struct writeback_control *);
244 int (*set_page_dirty)(struct page *page);
245 void (*readahead)(struct readahead_control *);
246 int (*readpages)(struct file *filp, struct address_space *mapping,
247 struct list_head *pages, unsigned nr_pages);
248 int (*write_begin)(struct file *, struct address_space *mapping,
249 loff_t pos, unsigned len, unsigned flags,
250 struct page **pagep, void **fsdata);
251 int (*write_end)(struct file *, struct address_space *mapping,
252 loff_t pos, unsigned len, unsigned copied,
253 struct page *page, void *fsdata);
254 sector_t (*bmap)(struct address_space *, sector_t);
255 void (*invalidatepage) (struct page *, unsigned int, unsigned int);
256 int (*releasepage) (struct page *, int);
257 void (*freepage)(struct page *);
258 int (*direct_IO)(struct kiocb *, struct iov_iter *iter);
259 bool (*isolate_page) (struct page *, isolate_mode_t);
260 int (*migratepage)(struct address_space *, struct page *, struct page *);
261 void (*putback_page) (struct page *);
262 int (*launder_page)(struct page *);
263 int (*is_partially_uptodate)(struct page *, unsigned long, unsigned long);
264 int (*error_remove_page)(struct address_space *, struct page *);
265 int (*swap_activate)(struct file *);
266 int (*swap_deactivate)(struct file *);
269 All except set_page_dirty and freepage may block
271 ====================== ======================== =========
272 ops PageLocked(page) i_rwsem
273 ====================== ======================== =========
274 writepage: yes, unlocks (see below)
275 readpage: yes, unlocks
278 readahead: yes, unlocks
280 write_begin: locks the page exclusive
281 write_end: yes, unlocks exclusive
288 migratepage: yes (both)
291 is_partially_uptodate: yes
292 error_remove_page: yes
295 ====================== ======================== =========
297 ->write_begin(), ->write_end() and ->readpage() may be called from
298 the request handler (/dev/loop).
300 ->readpage() unlocks the page, either synchronously or via I/O
303 ->readahead() unlocks the pages that I/O is attempted on like ->readpage().
305 ->readpages() populates the pagecache with the passed pages and starts
306 I/O against them. They come unlocked upon I/O completion.
308 ->writepage() is used for two purposes: for "memory cleansing" and for
309 "sync". These are quite different operations and the behaviour may differ
310 depending upon the mode.
312 If writepage is called for sync (wbc->sync_mode != WBC_SYNC_NONE) then
313 it *must* start I/O against the page, even if that would involve
314 blocking on in-progress I/O.
316 If writepage is called for memory cleansing (sync_mode ==
317 WBC_SYNC_NONE) then its role is to get as much writeout underway as
318 possible. So writepage should try to avoid blocking against
319 currently-in-progress I/O.
321 If the filesystem is not called for "sync" and it determines that it
322 would need to block against in-progress I/O to be able to start new I/O
323 against the page the filesystem should redirty the page with
324 redirty_page_for_writepage(), then unlock the page and return zero.
325 This may also be done to avoid internal deadlocks, but rarely.
327 If the filesystem is called for sync then it must wait on any
328 in-progress I/O and then start new I/O.
330 The filesystem should unlock the page synchronously, before returning to the
331 caller, unless ->writepage() returns special WRITEPAGE_ACTIVATE
332 value. WRITEPAGE_ACTIVATE means that page cannot really be written out
333 currently, and VM should stop calling ->writepage() on this page for some
334 time. VM does this by moving page to the head of the active list, hence the
337 Unless the filesystem is going to redirty_page_for_writepage(), unlock the page
338 and return zero, writepage *must* run set_page_writeback() against the page,
339 followed by unlocking it. Once set_page_writeback() has been run against the
340 page, write I/O can be submitted and the write I/O completion handler must run
341 end_page_writeback() once the I/O is complete. If no I/O is submitted, the
342 filesystem must run end_page_writeback() against the page before returning from
345 That is: after 2.5.12, pages which are under writeout are *not* locked. Note,
346 if the filesystem needs the page to be locked during writeout, that is ok, too,
347 the page is allowed to be unlocked at any point in time between the calls to
348 set_page_writeback() and end_page_writeback().
350 Note, failure to run either redirty_page_for_writepage() or the combination of
351 set_page_writeback()/end_page_writeback() on a page submitted to writepage
352 will leave the page itself marked clean but it will be tagged as dirty in the
353 radix tree. This incoherency can lead to all sorts of hard-to-debug problems
354 in the filesystem like having dirty inodes at umount and losing written data.
356 ->writepages() is used for periodic writeback and for syscall-initiated
357 sync operations. The address_space should start I/O against at least
358 ``*nr_to_write`` pages. ``*nr_to_write`` must be decremented for each page
359 which is written. The address_space implementation may write more (or less)
360 pages than ``*nr_to_write`` asks for, but it should try to be reasonably close.
361 If nr_to_write is NULL, all dirty pages must be written.
363 writepages should _only_ write pages which are present on
366 ->set_page_dirty() is called from various places in the kernel
367 when the target page is marked as needing writeback. It may be called
368 under spinlock (it cannot block) and is sometimes called with the page
371 ->bmap() is currently used by legacy ioctl() (FIBMAP) provided by some
372 filesystems and by the swapper. The latter will eventually go away. Please,
373 keep it that way and don't breed new callers.
375 ->invalidatepage() is called when the filesystem must attempt to drop
376 some or all of the buffers from the page when it is being truncated. It
377 returns zero on success. If ->invalidatepage is zero, the kernel uses
378 block_invalidatepage() instead.
380 ->releasepage() is called when the kernel is about to try to drop the
381 buffers from the page in preparation for freeing it. It returns zero to
382 indicate that the buffers are (or may be) freeable. If ->releasepage is zero,
383 the kernel assumes that the fs has no private interest in the buffers.
385 ->freepage() is called when the kernel is done dropping the page
388 ->launder_page() may be called prior to releasing a page if
389 it is still found to be dirty. It returns zero if the page was successfully
390 cleaned, or an error value if not. Note that in order to prevent the page
391 getting mapped back in and redirtied, it needs to be kept locked
392 across the entire operation.
394 ->swap_activate will be called with a non-zero argument on
395 files backing (non block device backed) swapfiles. A return value
396 of zero indicates success, in which case this file can be used for
397 backing swapspace. The swapspace operations will be proxied to the
398 address space operations.
400 ->swap_deactivate() will be called in the sys_swapoff()
401 path after ->swap_activate() returned success.
408 void (*fl_copy_lock)(struct file_lock *, struct file_lock *);
409 void (*fl_release_private)(struct file_lock *);
414 =================== ============= =========
415 ops inode->i_lock may block
416 =================== ============= =========
418 fl_release_private: maybe maybe[1]_
419 =================== ============= =========
422 ->fl_release_private for flock or POSIX locks is currently allowed
423 to block. Leases however can still be freed while the i_lock is held and
424 so fl_release_private called on a lease should not block.
426 lock_manager_operations
427 =======================
431 void (*lm_notify)(struct file_lock *); /* unblock callback */
432 int (*lm_grant)(struct file_lock *, struct file_lock *, int);
433 void (*lm_break)(struct file_lock *); /* break_lease callback */
434 int (*lm_change)(struct file_lock **, int);
435 bool (*lm_breaker_owns_lease)(struct file_lock *);
439 ====================== ============= ================= =========
440 ops inode->i_lock blocked_lock_lock may block
441 ====================== ============= ================= =========
442 lm_notify: yes yes no
446 lm_breaker_owns_lease: no no no
447 ====================== ============= ================= =========
454 void (*b_end_io)(struct buffer_head *bh, int uptodate);
458 called from interrupts. In other words, extreme care is needed here.
459 bh is locked, but that's all warranties we have here. Currently only RAID1,
460 highmem, fs/buffer.c, and fs/ntfs/aops.c are providing these. Block devices
461 call this method upon the IO completion.
463 block_device_operations
464 =======================
467 int (*open) (struct block_device *, fmode_t);
468 int (*release) (struct gendisk *, fmode_t);
469 int (*ioctl) (struct block_device *, fmode_t, unsigned, unsigned long);
470 int (*compat_ioctl) (struct block_device *, fmode_t, unsigned, unsigned long);
471 int (*direct_access) (struct block_device *, sector_t, void **,
473 void (*unlock_native_capacity) (struct gendisk *);
474 int (*revalidate_disk) (struct gendisk *);
475 int (*getgeo)(struct block_device *, struct hd_geometry *);
476 void (*swap_slot_free_notify) (struct block_device *, unsigned long);
480 ======================= ===================
482 ======================= ===================
488 unlock_native_capacity: no
491 swap_slot_free_notify: no (see below)
492 ======================= ===================
494 swap_slot_free_notify is called with swap_lock and sometimes the page lock
503 loff_t (*llseek) (struct file *, loff_t, int);
504 ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
505 ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
506 ssize_t (*read_iter) (struct kiocb *, struct iov_iter *);
507 ssize_t (*write_iter) (struct kiocb *, struct iov_iter *);
508 int (*iterate) (struct file *, struct dir_context *);
509 int (*iterate_shared) (struct file *, struct dir_context *);
510 __poll_t (*poll) (struct file *, struct poll_table_struct *);
511 long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
512 long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
513 int (*mmap) (struct file *, struct vm_area_struct *);
514 int (*open) (struct inode *, struct file *);
515 int (*flush) (struct file *);
516 int (*release) (struct inode *, struct file *);
517 int (*fsync) (struct file *, loff_t start, loff_t end, int datasync);
518 int (*fasync) (int, struct file *, int);
519 int (*lock) (struct file *, int, struct file_lock *);
520 ssize_t (*readv) (struct file *, const struct iovec *, unsigned long,
522 ssize_t (*writev) (struct file *, const struct iovec *, unsigned long,
524 ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t,
526 ssize_t (*sendpage) (struct file *, struct page *, int, size_t,
528 unsigned long (*get_unmapped_area)(struct file *, unsigned long,
529 unsigned long, unsigned long, unsigned long);
530 int (*check_flags)(int);
531 int (*flock) (struct file *, int, struct file_lock *);
532 ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *,
533 size_t, unsigned int);
534 ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *,
535 size_t, unsigned int);
536 int (*setlease)(struct file *, long, struct file_lock **, void **);
537 long (*fallocate)(struct file *, int, loff_t, loff_t);
542 ->llseek() locking has moved from llseek to the individual llseek
543 implementations. If your fs is not using generic_file_llseek, you
544 need to acquire and release the appropriate locks in your ->llseek().
545 For many filesystems, it is probably safe to acquire the inode
546 mutex or just to use i_size_read() instead.
547 Note: this does not protect the file->f_pos against concurrent modifications
548 since this is something the userspace has to take care about.
550 ->iterate() is called with i_rwsem exclusive.
552 ->iterate_shared() is called with i_rwsem at least shared.
554 ->fasync() is responsible for maintaining the FASYNC bit in filp->f_flags.
555 Most instances call fasync_helper(), which does that maintenance, so it's
556 not normally something one needs to worry about. Return values > 0 will be
557 mapped to zero in the VFS layer.
559 ->readdir() and ->ioctl() on directories must be changed. Ideally we would
560 move ->readdir() to inode_operations and use a separate method for directory
561 ->ioctl() or kill the latter completely. One of the problems is that for
562 anything that resembles union-mount we won't have a struct file for all
563 components. And there are other reasons why the current interface is a mess...
565 ->read on directories probably must go away - we should just enforce -EISDIR
566 in sys_read() and friends.
568 ->setlease operations should call generic_setlease() before or after setting
569 the lease within the individual filesystem to record the result of the
577 int (*write_dquot) (struct dquot *);
578 int (*acquire_dquot) (struct dquot *);
579 int (*release_dquot) (struct dquot *);
580 int (*mark_dirty) (struct dquot *);
581 int (*write_info) (struct super_block *, int);
583 These operations are intended to be more or less wrapping functions that ensure
584 a proper locking wrt the filesystem and call the generic quota operations.
586 What filesystem should expect from the generic quota functions:
588 ============== ============ =========================
589 ops FS recursion Held locks when called
590 ============== ============ =========================
591 write_dquot: yes dqonoff_sem or dqptr_sem
592 acquire_dquot: yes dqonoff_sem or dqptr_sem
593 release_dquot: yes dqonoff_sem or dqptr_sem
595 write_info: yes dqonoff_sem
596 ============== ============ =========================
598 FS recursion means calling ->quota_read() and ->quota_write() from superblock
601 More details about quota locking can be found in fs/dquot.c.
608 void (*open)(struct vm_area_struct*);
609 void (*close)(struct vm_area_struct*);
610 vm_fault_t (*fault)(struct vm_area_struct*, struct vm_fault *);
611 vm_fault_t (*page_mkwrite)(struct vm_area_struct *, struct vm_fault *);
612 vm_fault_t (*pfn_mkwrite)(struct vm_area_struct *, struct vm_fault *);
613 int (*access)(struct vm_area_struct *, unsigned long, void*, int, int);
617 ============= ========= ===========================
618 ops mmap_lock PageLocked(page)
619 ============= ========= ===========================
622 fault: yes can return with page locked
624 page_mkwrite: yes can return with page locked
627 ============= ========= ===========================
629 ->fault() is called when a previously not present pte is about
630 to be faulted in. The filesystem must find and return the page associated
631 with the passed in "pgoff" in the vm_fault structure. If it is possible that
632 the page may be truncated and/or invalidated, then the filesystem must lock
633 the page, then ensure it is not already truncated (the page lock will block
634 subsequent truncate), and then return with VM_FAULT_LOCKED, and the page
635 locked. The VM will unlock the page.
637 ->map_pages() is called when VM asks to map easy accessible pages.
638 Filesystem should find and map pages associated with offsets from "start_pgoff"
639 till "end_pgoff". ->map_pages() is called with page table locked and must
640 not block. If it's not possible to reach a page without blocking,
641 filesystem should skip it. Filesystem should use do_set_pte() to setup
642 page table entry. Pointer to entry associated with the page is passed in
643 "pte" field in vm_fault structure. Pointers to entries for other offsets
644 should be calculated relative to "pte".
646 ->page_mkwrite() is called when a previously read-only pte is
647 about to become writeable. The filesystem again must ensure that there are
648 no truncate/invalidate races, and then return with the page locked. If
649 the page has been truncated, the filesystem should not look up a new page
650 like the ->fault() handler, but simply return with VM_FAULT_NOPAGE, which
651 will cause the VM to retry the fault.
653 ->pfn_mkwrite() is the same as page_mkwrite but when the pte is
654 VM_PFNMAP or VM_MIXEDMAP with a page-less entry. Expected return is
655 VM_FAULT_NOPAGE. Or one of the VM_FAULT_ERROR types. The default behavior
656 after this call is to make the pte read-write, unless pfn_mkwrite returns
659 ->access() is called when get_user_pages() fails in
660 access_process_vm(), typically used to debug a process through
661 /proc/pid/mem or ptrace. This function is needed only for
662 VM_IO | VM_PFNMAP VMAs.
664 --------------------------------------------------------------------------------
668 (if you break something or notice that it is broken and do not fix it yourself
669 - at least put it here)