ia64/linux-2.6.18-xen.hg

view Documentation/filesystems/Locking @ 524:7f8b544237bf

netfront: Allow netfront in domain 0.

This is useful if your physical network device is in a utility domain.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
author Keir Fraser <keir.fraser@citrix.com>
date Tue Apr 15 15:18:58 2008 +0100 (2008-04-15)
parents 831230e53067
children
line source
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 *);
19 locking rules:
20 none have BKL
21 dcache_lock rename_lock ->d_lock may block
22 d_revalidate: no no no yes
23 d_hash 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
27 d_iput: no no no yes
29 --------------------------- inode_operations ---------------------------
30 prototypes:
31 int (*create) (struct inode *,struct dentry *,int, struct nameidata *);
32 struct dentry * (*lookup) (struct inode *,struct dentry *, struct nameid
33 ata *);
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 *);
53 locking rules:
54 all may block, none have BKL
55 i_sem(inode)
56 lookup: yes
57 create: yes
58 link: yes (both)
59 mknod: yes
60 symlink: yes
61 mkdir: yes
62 unlink: yes (both)
63 rmdir: yes (both) (see below)
64 rename: yes (all) (see below)
65 readlink: no
66 follow_link: no
67 truncate: yes (see below)
68 setattr: yes
69 permission: no
70 getattr: no
71 setxattr: yes
72 getxattr: no
73 listxattr: no
74 removexattr: yes
75 Additionally, ->rmdir(), ->unlink() and ->rename() have ->i_sem on
76 victim.
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
82 passed).
84 See Documentation/filesystems/directory-locking for more detailed discussion
85 of the locking scheme for directory operations.
87 --------------------------- super_operations ---------------------------
88 prototypes:
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 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 read_inode: no (see below)
116 dirty_inode: no (must not sleep)
117 write_inode: no
118 put_inode: no
119 drop_inode: no !!!inode_lock!!!
120 delete_inode: no
121 put_super: yes yes no
122 write_super: no yes read
123 sync_fs: no no read
124 write_super_lockfs: ?
125 unlockfs: ?
126 statfs: no no no
127 remount_fs: no yes maybe (see below)
128 clear_inode: no
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 ---------------------------
144 prototypes:
145 int (*get_sb) (struct file_system_type *, int,
146 const char *, void *, struct vfsmount *);
147 void (*kill_sb) (struct super_block *);
148 locking rules:
149 may block BKL
150 get_sb yes yes
151 kill_sb yes yes
153 ->get_sb() returns error or 0 with locked superblock attached to the vfsmount
154 (exclusive on ->s_umount).
155 ->kill_sb() takes a write-locked superblock, does all shutdown work on it,
156 unlocks and drops the reference.
158 --------------------------- address_space_operations --------------------------
159 prototypes:
160 int (*writepage)(struct page *page, struct writeback_control *wbc);
161 int (*readpage)(struct file *, struct page *);
162 int (*sync_page)(struct page *);
163 int (*writepages)(struct address_space *, struct writeback_control *);
164 int (*set_page_dirty)(struct page *page);
165 int (*readpages)(struct file *filp, struct address_space *mapping,
166 struct list_head *pages, unsigned nr_pages);
167 int (*prepare_write)(struct file *, struct page *, unsigned, unsigned);
168 int (*commit_write)(struct file *, struct page *, unsigned, unsigned);
169 sector_t (*bmap)(struct address_space *, sector_t);
170 int (*invalidatepage) (struct page *, unsigned long);
171 int (*releasepage) (struct page *, int);
172 int (*direct_IO)(int, struct kiocb *, const struct iovec *iov,
173 loff_t offset, unsigned long nr_segs);
175 locking rules:
176 All except set_page_dirty may block
178 BKL PageLocked(page)
179 writepage: no yes, unlocks (see below)
180 readpage: no yes, unlocks
181 sync_page: no maybe
182 writepages: no
183 set_page_dirty no no
184 readpages: no
185 prepare_write: no yes
186 commit_write: no yes
187 bmap: yes
188 invalidatepage: no yes
189 releasepage: no yes
190 direct_IO: no
192 ->prepare_write(), ->commit_write(), ->sync_page() and ->readpage()
193 may be called from the request handler (/dev/loop).
195 ->readpage() unlocks the page, either synchronously or via I/O
196 completion.
198 ->readpages() populates the pagecache with the passed pages and starts
199 I/O against them. They come unlocked upon I/O completion.
201 ->writepage() is used for two purposes: for "memory cleansing" and for
202 "sync". These are quite different operations and the behaviour may differ
203 depending upon the mode.
205 If writepage is called for sync (wbc->sync_mode != WBC_SYNC_NONE) then
206 it *must* start I/O against the page, even if that would involve
207 blocking on in-progress I/O.
209 If writepage is called for memory cleansing (sync_mode ==
210 WBC_SYNC_NONE) then its role is to get as much writeout underway as
211 possible. So writepage should try to avoid blocking against
212 currently-in-progress I/O.
214 If the filesystem is not called for "sync" and it determines that it
215 would need to block against in-progress I/O to be able to start new I/O
216 against the page the filesystem should redirty the page with
217 redirty_page_for_writepage(), then unlock the page and return zero.
218 This may also be done to avoid internal deadlocks, but rarely.
220 If the filesytem is called for sync then it must wait on any
221 in-progress I/O and then start new I/O.
223 The filesystem should unlock the page synchronously, before returning to the
224 caller, unless ->writepage() returns special WRITEPAGE_ACTIVATE
225 value. WRITEPAGE_ACTIVATE means that page cannot really be written out
226 currently, and VM should stop calling ->writepage() on this page for some
227 time. VM does this by moving page to the head of the active list, hence the
228 name.
230 Unless the filesystem is going to redirty_page_for_writepage(), unlock the page
231 and return zero, writepage *must* run set_page_writeback() against the page,
232 followed by unlocking it. Once set_page_writeback() has been run against the
233 page, write I/O can be submitted and the write I/O completion handler must run
234 end_page_writeback() once the I/O is complete. If no I/O is submitted, the
235 filesystem must run end_page_writeback() against the page before returning from
236 writepage.
238 That is: after 2.5.12, pages which are under writeout are *not* locked. Note,
239 if the filesystem needs the page to be locked during writeout, that is ok, too,
240 the page is allowed to be unlocked at any point in time between the calls to
241 set_page_writeback() and end_page_writeback().
243 Note, failure to run either redirty_page_for_writepage() or the combination of
244 set_page_writeback()/end_page_writeback() on a page submitted to writepage
245 will leave the page itself marked clean but it will be tagged as dirty in the
246 radix tree. This incoherency can lead to all sorts of hard-to-debug problems
247 in the filesystem like having dirty inodes at umount and losing written data.
249 ->sync_page() locking rules are not well-defined - usually it is called
250 with lock on page, but that is not guaranteed. Considering the currently
251 existing instances of this method ->sync_page() itself doesn't look
252 well-defined...
254 ->writepages() is used for periodic writeback and for syscall-initiated
255 sync operations. The address_space should start I/O against at least
256 *nr_to_write pages. *nr_to_write must be decremented for each page which is
257 written. The address_space implementation may write more (or less) pages
258 than *nr_to_write asks for, but it should try to be reasonably close. If
259 nr_to_write is NULL, all dirty pages must be written.
261 writepages should _only_ write pages which are present on
262 mapping->io_pages.
264 ->set_page_dirty() is called from various places in the kernel
265 when the target page is marked as needing writeback. It may be called
266 under spinlock (it cannot block) and is sometimes called with the page
267 not locked.
269 ->bmap() is currently used by legacy ioctl() (FIBMAP) provided by some
270 filesystems and by the swapper. The latter will eventually go away. All
271 instances do not actually need the BKL. Please, keep it that way and don't
272 breed new callers.
274 ->invalidatepage() is called when the filesystem must attempt to drop
275 some or all of the buffers from the page when it is being truncated. It
276 returns zero on success. If ->invalidatepage is zero, the kernel uses
277 block_invalidatepage() instead.
279 ->releasepage() is called when the kernel is about to try to drop the
280 buffers from the page in preparation for freeing it. It returns zero to
281 indicate that the buffers are (or may be) freeable. If ->releasepage is zero,
282 the kernel assumes that the fs has no private interest in the buffers.
284 Note: currently almost all instances of address_space methods are
285 using BKL for internal serialization and that's one of the worst sources
286 of contention. Normally they are calling library functions (in fs/buffer.c)
287 and pass foo_get_block() as a callback (on local block-based filesystems,
288 indeed). BKL is not needed for library stuff and is usually taken by
289 foo_get_block(). It's an overkill, since block bitmaps can be protected by
290 internal fs locking and real critical areas are much smaller than the areas
291 filesystems protect now.
293 ----------------------- file_lock_operations ------------------------------
294 prototypes:
295 void (*fl_insert)(struct file_lock *); /* lock insertion callback */
296 void (*fl_remove)(struct file_lock *); /* lock removal callback */
297 void (*fl_copy_lock)(struct file_lock *, struct file_lock *);
298 void (*fl_release_private)(struct file_lock *);
301 locking rules:
302 BKL may block
303 fl_insert: yes no
304 fl_remove: yes no
305 fl_copy_lock: yes no
306 fl_release_private: yes yes
308 ----------------------- lock_manager_operations ---------------------------
309 prototypes:
310 int (*fl_compare_owner)(struct file_lock *, struct file_lock *);
311 void (*fl_notify)(struct file_lock *); /* unblock callback */
312 void (*fl_copy_lock)(struct file_lock *, struct file_lock *);
313 void (*fl_release_private)(struct file_lock *);
314 void (*fl_break)(struct file_lock *); /* break_lease callback */
316 locking rules:
317 BKL may block
318 fl_compare_owner: yes no
319 fl_notify: yes no
320 fl_copy_lock: yes no
321 fl_release_private: yes yes
322 fl_break: yes no
324 Currently only NFSD and NLM provide instances of this class. None of the
325 them block. If you have out-of-tree instances - please, show up. Locking
326 in that area will change.
327 --------------------------- buffer_head -----------------------------------
328 prototypes:
329 void (*b_end_io)(struct buffer_head *bh, int uptodate);
331 locking rules:
332 called from interrupts. In other words, extreme care is needed here.
333 bh is locked, but that's all warranties we have here. Currently only RAID1,
334 highmem, fs/buffer.c, and fs/ntfs/aops.c are providing these. Block devices
335 call this method upon the IO completion.
337 --------------------------- block_device_operations -----------------------
338 prototypes:
339 int (*open) (struct inode *, struct file *);
340 int (*release) (struct inode *, struct file *);
341 int (*ioctl) (struct inode *, struct file *, unsigned, unsigned long);
342 int (*media_changed) (struct gendisk *);
343 int (*revalidate_disk) (struct gendisk *);
345 locking rules:
346 BKL bd_sem
347 open: yes yes
348 release: yes yes
349 ioctl: yes no
350 media_changed: no no
351 revalidate_disk: no no
353 The last two are called only from check_disk_change().
355 --------------------------- file_operations -------------------------------
356 prototypes:
357 loff_t (*llseek) (struct file *, loff_t, int);
358 ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
359 ssize_t (*aio_read) (struct kiocb *, char __user *, size_t, loff_t);
360 ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
361 ssize_t (*aio_write) (struct kiocb *, const char __user *, size_t,
362 loff_t);
363 int (*readdir) (struct file *, void *, filldir_t);
364 unsigned int (*poll) (struct file *, struct poll_table_struct *);
365 int (*ioctl) (struct inode *, struct file *, unsigned int,
366 unsigned long);
367 long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
368 long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
369 int (*mmap) (struct file *, struct vm_area_struct *);
370 int (*open) (struct inode *, struct file *);
371 int (*flush) (struct file *);
372 int (*release) (struct inode *, struct file *);
373 int (*fsync) (struct file *, struct dentry *, int datasync);
374 int (*aio_fsync) (struct kiocb *, int datasync);
375 int (*fasync) (int, struct file *, int);
376 int (*lock) (struct file *, int, struct file_lock *);
377 ssize_t (*readv) (struct file *, const struct iovec *, unsigned long,
378 loff_t *);
379 ssize_t (*writev) (struct file *, const struct iovec *, unsigned long,
380 loff_t *);
381 ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t,
382 void __user *);
383 ssize_t (*sendpage) (struct file *, struct page *, int, size_t,
384 loff_t *, int);
385 unsigned long (*get_unmapped_area)(struct file *, unsigned long,
386 unsigned long, unsigned long, unsigned long);
387 int (*check_flags)(int);
388 int (*dir_notify)(struct file *, unsigned long);
389 };
391 locking rules:
392 All except ->poll() may block.
393 BKL
394 llseek: no (see below)
395 read: no
396 aio_read: no
397 write: no
398 aio_write: no
399 readdir: no
400 poll: no
401 ioctl: yes (see below)
402 unlocked_ioctl: no (see below)
403 compat_ioctl: no
404 mmap: no
405 open: maybe (see below)
406 flush: no
407 release: no
408 fsync: no (see below)
409 aio_fsync: no
410 fasync: yes (see below)
411 lock: yes
412 readv: no
413 writev: no
414 sendfile: no
415 sendpage: no
416 get_unmapped_area: no
417 check_flags: no
418 dir_notify: no
420 ->llseek() locking has moved from llseek to the individual llseek
421 implementations. If your fs is not using generic_file_llseek, you
422 need to acquire and release the appropriate locks in your ->llseek().
423 For many filesystems, it is probably safe to acquire the inode
424 semaphore. Note some filesystems (i.e. remote ones) provide no
425 protection for i_size so you will need to use the BKL.
427 ->open() locking is in-transit: big lock partially moved into the methods.
428 The only exception is ->open() in the instances of file_operations that never
429 end up in ->i_fop/->proc_fops, i.e. ones that belong to character devices
430 (chrdev_open() takes lock before replacing ->f_op and calling the secondary
431 method. As soon as we fix the handling of module reference counters all
432 instances of ->open() will be called without 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_sem 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 struct page *(*nopage)(struct vm_area_struct*, unsigned long, int *);
506 locking rules:
507 BKL mmap_sem
508 open: no yes
509 close: no yes
510 nopage: no yes
512 ================================================================================
513 Dubious stuff
515 (if you break something or notice that it is broken and do not fix it yourself
516 - at least put it here)
518 ipc/shm.c::shm_delete() - may need BKL.
519 ->read() and ->write() in many drivers are (probably) missing BKL.
520 drivers/sgi/char/graphics.c::sgi_graphics_nopage() - may need BKL.