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 void (*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 *);
111 alloc_inode: no no no
113 read_inode: no (see below)
114 dirty_inode: no (must not sleep)
117 drop_inode: no !!!inode_lock!!!
119 put_super: yes yes no
120 write_super: no yes read
122 write_super_lockfs: ?
125 remount_fs: no yes maybe (see below)
127 umount_begin: yes no no
128 show_options: no (vfsmount->sem)
130 ->read_inode() is not a method - it's a callback used in iget().
131 ->remount_fs() will have the s_umount lock if it's already mounted.
132 When called from get_sb_single, it does NOT have the s_umount lock.
134 --------------------------- file_system_type ---------------------------
136 struct super_block *(*get_sb) (struct file_system_type *, int,
137 const char *, void *);
138 void (*kill_sb) (struct super_block *);
144 ->get_sb() returns error or a locked superblock (exclusive on ->s_umount).
145 ->kill_sb() takes a write-locked superblock, does all shutdown work on it,
146 unlocks and drops the reference.
148 --------------------------- address_space_operations --------------------------
150 int (*writepage)(struct page *page, struct writeback_control *wbc);
151 int (*readpage)(struct file *, struct page *);
152 int (*sync_page)(struct page *);
153 int (*writepages)(struct address_space *, struct writeback_control *);
154 int (*set_page_dirty)(struct page *page);
155 int (*readpages)(struct file *filp, struct address_space *mapping,
156 struct list_head *pages, unsigned nr_pages);
157 int (*prepare_write)(struct file *, struct page *, unsigned, unsigned);
158 int (*commit_write)(struct file *, struct page *, unsigned, unsigned);
159 sector_t (*bmap)(struct address_space *, sector_t);
160 int (*invalidatepage) (struct page *, unsigned long);
161 int (*releasepage) (struct page *, int);
162 int (*direct_IO)(int, struct kiocb *, const struct iovec *iov,
163 loff_t offset, unsigned long nr_segs);
166 All except set_page_dirty may block
169 writepage: no yes, unlocks (see below)
170 readpage: no yes, unlocks
175 prepare_write: no yes
178 invalidatepage: no yes
182 ->prepare_write(), ->commit_write(), ->sync_page() and ->readpage()
183 may be called from the request handler (/dev/loop).
185 ->readpage() unlocks the page, either synchronously or via I/O
188 ->readpages() populates the pagecache with the passed pages and starts
189 I/O against them. They come unlocked upon I/O completion.
191 ->writepage() is used for two purposes: for "memory cleansing" and for
192 "sync". These are quite different operations and the behaviour may differ
193 depending upon the mode.
195 If writepage is called for sync (wbc->sync_mode != WBC_SYNC_NONE) then
196 it *must* start I/O against the page, even if that would involve
197 blocking on in-progress I/O.
199 If writepage is called for memory cleansing (sync_mode ==
200 WBC_SYNC_NONE) then its role is to get as much writeout underway as
201 possible. So writepage should try to avoid blocking against
202 currently-in-progress I/O.
204 If the filesystem is not called for "sync" and it determines that it
205 would need to block against in-progress I/O to be able to start new I/O
206 against the page the filesystem shoud redirty the page (usually with
207 __set_page_dirty_nobuffers()), then unlock the page and return zero.
208 This may also be done to avoid internal deadlocks, but rarely.
210 If the filesytem is called for sync then it must wait on any
211 in-progress I/O and then start new I/O.
213 The filesystem should unlock the page synchronously, before returning
214 to the caller. If the page has write I/O underway against it,
215 writepage() should run SetPageWriteback() against the page prior to
216 unlocking it. The write I/O completion handler should run
217 end_page_writeback() against the page.
219 That is: after 2.5.12, pages which are under writeout are *not* locked.
221 ->sync_page() locking rules are not well-defined - usually it is called
222 with lock on page, but that is not guaranteed. Considering the currently
223 existing instances of this method ->sync_page() itself doesn't look
226 ->writepages() is used for periodic writeback and for syscall-initiated
227 sync operations. The address_space should start I/O against at least
228 *nr_to_write pages. *nr_to_write must be decremented for each page which is
229 written. The address_space implementation may write more (or less) pages
230 than *nr_to_write asks for, but it should try to be reasonably close. If
231 nr_to_write is NULL, all dirty pages must be written.
233 writepages should _only_ write pages which are present on
236 ->set_page_dirty() is called from various places in the kernel
237 when the target page is marked as needing writeback. It may be called
238 under spinlock (it cannot block) and is sometimes called with the page
241 ->bmap() is currently used by legacy ioctl() (FIBMAP) provided by some
242 filesystems and by the swapper. The latter will eventually go away. All
243 instances do not actually need the BKL. Please, keep it that way and don't
246 ->invalidatepage() is called when the filesystem must attempt to drop
247 some or all of the buffers from the page when it is being truncated. It
248 returns zero on success. If ->invalidatepage is zero, the kernel uses
249 block_invalidatepage() instead.
251 ->releasepage() is called when the kernel is about to try to drop the
252 buffers from the page in preparation for freeing it. It returns zero to
253 indicate that the buffers are (or may be) freeable. If ->releasepage is zero,
254 the kernel assumes that the fs has no private interest in the buffers.
256 Note: currently almost all instances of address_space methods are
257 using BKL for internal serialization and that's one of the worst sources
258 of contention. Normally they are calling library functions (in fs/buffer.c)
259 and pass foo_get_block() as a callback (on local block-based filesystems,
260 indeed). BKL is not needed for library stuff and is usually taken by
261 foo_get_block(). It's an overkill, since block bitmaps can be protected by
262 internal fs locking and real critical areas are much smaller than the areas
263 filesystems protect now.
265 --------------------------- file_lock ------------------------------------
267 void (*fl_notify)(struct file_lock *); /* unblock callback */
268 void (*fl_insert)(struct file_lock *); /* lock insertion callback */
269 void (*fl_remove)(struct file_lock *); /* lock removal callback */
276 Currently only NLM provides instances of this class. None of the
277 them block. If you have out-of-tree instances - please, show up. Locking
278 in that area will change.
280 --------------------------- buffer_head -----------------------------------
282 void (*b_end_io)(struct buffer_head *bh, int uptodate);
285 called from interrupts. In other words, extreme care is needed here.
286 bh is locked, but that's all warranties we have here. Currently only RAID1,
287 highmem and fs/buffer.c are providing these. Block devices call this method
288 upon the IO completion.
290 --------------------------- block_device_operations -----------------------
292 int (*open) (struct inode *, struct file *);
293 int (*release) (struct inode *, struct file *);
294 int (*ioctl) (struct inode *, struct file *, unsigned, unsigned long);
295 int (*media_changed) (struct gendisk *);
296 int (*revalidate_disk) (struct gendisk *);
304 revalidate_disk: no no
306 The last two are called only from check_disk_change().
308 --------------------------- file_operations -------------------------------
310 loff_t (*llseek) (struct file *, loff_t, int);
311 ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
312 ssize_t (*aio_read) (struct kiocb *, char __user *, size_t, loff_t);
313 ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
314 ssize_t (*aio_write) (struct kiocb *, const char __user *, size_t,
316 int (*readdir) (struct file *, void *, filldir_t);
317 unsigned int (*poll) (struct file *, struct poll_table_struct *);
318 int (*ioctl) (struct inode *, struct file *, unsigned int,
320 int (*mmap) (struct file *, struct vm_area_struct *);
321 int (*open) (struct inode *, struct file *);
322 int (*flush) (struct file *);
323 int (*release) (struct inode *, struct file *);
324 int (*fsync) (struct file *, struct dentry *, int datasync);
325 int (*aio_fsync) (struct kiocb *, int datasync);
326 int (*fasync) (int, struct file *, int);
327 int (*lock) (struct file *, int, struct file_lock *);
328 ssize_t (*readv) (struct file *, const struct iovec *, unsigned long,
330 ssize_t (*writev) (struct file *, const struct iovec *, unsigned long,
332 ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t,
334 ssize_t (*sendpage) (struct file *, struct page *, int, size_t,
336 unsigned long (*get_unmapped_area)(struct file *, unsigned long,
337 unsigned long, unsigned long, unsigned long);
341 All except ->poll() may block.
343 llseek: no (see below)
350 ioctl: yes (see below)
352 open: maybe (see below)
355 fsync: no (see below)
357 fasync: yes (see below)
363 get_unmapped_area: no
365 ->llseek() locking has moved from llseek to the individual llseek
366 implementations. If your fs is not using generic_file_llseek, you
367 need to acquire and release the appropriate locks in your ->llseek().
368 For many filesystems, it is probably safe to acquire the inode
369 semaphore. Note some filesystems (i.e. remote ones) provide no
370 protection for i_size so you will need to use the BKL.
372 ->open() locking is in-transit: big lock partially moved into the methods.
373 The only exception is ->open() in the instances of file_operations that never
374 end up in ->i_fop/->proc_fops, i.e. ones that belong to character devices
375 (chrdev_open() takes lock before replacing ->f_op and calling the secondary
376 method. As soon as we fix the handling of module reference counters all
377 instances of ->open() will be called without the BKL.
379 Note: ext2_release() was *the* source of contention on fs-intensive
380 loads and dropping BKL on ->release() helps to get rid of that (we still
381 grab BKL for cases when we close a file that had been opened r/w, but that
382 can and should be done using the internal locking with smaller critical areas).
383 Current worst offender is ext2_get_block()...
385 ->fasync() is a mess. This area needs a big cleanup and that will probably
388 ->readdir() and ->ioctl() on directories must be changed. Ideally we would
389 move ->readdir() to inode_operations and use a separate method for directory
390 ->ioctl() or kill the latter completely. One of the problems is that for
391 anything that resembles union-mount we won't have a struct file for all
392 components. And there are other reasons why the current interface is a mess...
394 ->read on directories probably must go away - we should just enforce -EISDIR
395 in sys_read() and friends.
397 ->fsync() has i_sem on inode.
399 --------------------------- dquot_operations -------------------------------
401 void (*initialize) (struct inode *, short);
402 void (*drop) (struct inode *);
403 int (*alloc_block) (const struct inode *, unsigned long, char);
404 int (*alloc_inode) (const struct inode *, unsigned long);
405 void (*free_block) (const struct inode *, unsigned long);
406 void (*free_inode) (const struct inode *, unsigned long);
407 int (*transfer) (struct dentry *, struct iattr *);
419 --------------------------- vm_operations_struct -----------------------------
421 void (*open)(struct vm_area_struct*);
422 void (*close)(struct vm_area_struct*);
423 struct page *(*nopage)(struct vm_area_struct*, unsigned long, int *);
431 ================================================================================
434 (if you break something or notice that it is broken and do not fix it yourself
435 - at least put it here)
437 ipc/shm.c::shm_delete() - may need BKL.
438 ->read() and ->write() in many drivers are (probably) missing BKL.
439 drivers/sgi/char/graphics.c::sgi_graphics_nopage() - may need BKL.