Documentation/filesystems/Locking

   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?
   9
  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
  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
  28
  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 *);
  52
  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).
  83
  84 See Documentation/filesystems/directory-locking for more detailed discussion
  85 of the locking scheme for directory operations.
  86
  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);
 109
 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:             yes     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)
 133
 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.
 142
 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
 152
 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.
 157
 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);
 174         int (*launder_page) (struct page *);
 175
 176 locking rules:
 177         All except set_page_dirty may block
 178
 179                         BKL     PageLocked(page)
 180 writepage:              no      yes, unlocks (see below)
 181 readpage:               no      yes, unlocks
 182 sync_page:              no      maybe
 183 writepages:             no
 184 set_page_dirty          no      no
 185 readpages:              no
 186 prepare_write:          no      yes
 187 commit_write:           no      yes
 188 bmap:                   yes
 189 invalidatepage:         no      yes
 190 releasepage:            no      yes
 191 direct_IO:              no
 192 launder_page:           no      yes
 193
 194         ->prepare_write(), ->commit_write(), ->sync_page() and ->readpage()
 195 may be called from the request handler (/dev/loop).
 196
 197         ->readpage() unlocks the page, either synchronously or via I/O
 198 completion.
 199
 200         ->readpages() populates the pagecache with the passed pages and starts
 201 I/O against them.  They come unlocked upon I/O completion.
 202
 203         ->writepage() is used for two purposes: for "memory cleansing" and for
 204 "sync".  These are quite different operations and the behaviour may differ
 205 depending upon the mode.
 206
 207 If writepage is called for sync (wbc->sync_mode != WBC_SYNC_NONE) then
 208 it *must* start I/O against the page, even if that would involve
 209 blocking on in-progress I/O.
 210
 211 If writepage is called for memory cleansing (sync_mode ==
 212 WBC_SYNC_NONE) then its role is to get as much writeout underway as
 213 possible.  So writepage should try to avoid blocking against
 214 currently-in-progress I/O.
 215
 216 If the filesystem is not called for "sync" and it determines that it
 217 would need to block against in-progress I/O to be able to start new I/O
 218 against the page the filesystem should redirty the page with
 219 redirty_page_for_writepage(), then unlock the page and return zero.
 220 This may also be done to avoid internal deadlocks, but rarely.
 221
 222 If the filesytem is called for sync then it must wait on any
 223 in-progress I/O and then start new I/O.
 224
 225 The filesystem should unlock the page synchronously, before returning to the
 226 caller, unless ->writepage() returns special WRITEPAGE_ACTIVATE
 227 value. WRITEPAGE_ACTIVATE means that page cannot really be written out
 228 currently, and VM should stop calling ->writepage() on this page for some
 229 time. VM does this by moving page to the head of the active list, hence the
 230 name.
 231
 232 Unless the filesystem is going to redirty_page_for_writepage(), unlock the page
 233 and return zero, writepage *must* run set_page_writeback() against the page,
 234 followed by unlocking it.  Once set_page_writeback() has been run against the
 235 page, write I/O can be submitted and the write I/O completion handler must run
 236 end_page_writeback() once the I/O is complete.  If no I/O is submitted, the
 237 filesystem must run end_page_writeback() against the page before returning from
 238 writepage.
 239
 240 That is: after 2.5.12, pages which are under writeout are *not* locked.  Note,
 241 if the filesystem needs the page to be locked during writeout, that is ok, too,
 242 the page is allowed to be unlocked at any point in time between the calls to
 243 set_page_writeback() and end_page_writeback().
 244
 245 Note, failure to run either redirty_page_for_writepage() or the combination of
 246 set_page_writeback()/end_page_writeback() on a page submitted to writepage
 247 will leave the page itself marked clean but it will be tagged as dirty in the
 248 radix tree.  This incoherency can lead to all sorts of hard-to-debug problems
 249 in the filesystem like having dirty inodes at umount and losing written data.
 250
 251         ->sync_page() locking rules are not well-defined - usually it is called
 252 with lock on page, but that is not guaranteed. Considering the currently
 253 existing instances of this method ->sync_page() itself doesn't look
 254 well-defined...
 255
 256         ->writepages() is used for periodic writeback and for syscall-initiated
 257 sync operations.  The address_space should start I/O against at least
 258 *nr_to_write pages.  *nr_to_write must be decremented for each page which is
 259 written.  The address_space implementation may write more (or less) pages
 260 than *nr_to_write asks for, but it should try to be reasonably close.  If
 261 nr_to_write is NULL, all dirty pages must be written.
 262
 263 writepages should _only_ write pages which are present on
 264 mapping->io_pages.
 265
 266         ->set_page_dirty() is called from various places in the kernel
 267 when the target page is marked as needing writeback.  It may be called
 268 under spinlock (it cannot block) and is sometimes called with the page
 269 not locked.
 270
 271         ->bmap() is currently used by legacy ioctl() (FIBMAP) provided by some
 272 filesystems and by the swapper. The latter will eventually go away. All
 273 instances do not actually need the BKL. Please, keep it that way and don't
 274 breed new callers.
 275
 276         ->invalidatepage() is called when the filesystem must attempt to drop
 277 some or all of the buffers from the page when it is being truncated.  It
 278 returns zero on success.  If ->invalidatepage is zero, the kernel uses
 279 block_invalidatepage() instead.
 280
 281         ->releasepage() is called when the kernel is about to try to drop the
 282 buffers from the page in preparation for freeing it.  It returns zero to
 283 indicate that the buffers are (or may be) freeable.  If ->releasepage is zero,
 284 the kernel assumes that the fs has no private interest in the buffers.
 285
 286         ->launder_page() may be called prior to releasing a page if
 287 it is still found to be dirty. It returns zero if the page was successfully
 288 cleaned, or an error value if not. Note that in order to prevent the page
 289 getting mapped back in and redirtied, it needs to be kept locked
 290 across the entire operation.
 291
 292         Note: currently almost all instances of address_space methods are
 293 using BKL for internal serialization and that's one of the worst sources
 294 of contention. Normally they are calling library functions (in fs/buffer.c)
 295 and pass foo_get_block() as a callback (on local block-based filesystems,
 296 indeed). BKL is not needed for library stuff and is usually taken by
 297 foo_get_block(). It's an overkill, since block bitmaps can be protected by
 298 internal fs locking and real critical areas are much smaller than the areas
 299 filesystems protect now.
 300
 301 ----------------------- file_lock_operations ------------------------------
 302 prototypes:
 303         void (*fl_insert)(struct file_lock *);  /* lock insertion callback */
 304         void (*fl_remove)(struct file_lock *);  /* lock removal callback */
 305         void (*fl_copy_lock)(struct file_lock *, struct file_lock *);
 306         void (*fl_release_private)(struct file_lock *);
 307
 308
 309 locking rules:
 310                         BKL     may block
 311 fl_insert:              yes     no
 312 fl_remove:              yes     no
 313 fl_copy_lock:           yes     no
 314 fl_release_private:     yes     yes
 315
 316 ----------------------- lock_manager_operations ---------------------------
 317 prototypes:
 318         int (*fl_compare_owner)(struct file_lock *, struct file_lock *);
 319         void (*fl_notify)(struct file_lock *);  /* unblock callback */
 320         void (*fl_copy_lock)(struct file_lock *, struct file_lock *);
 321         void (*fl_release_private)(struct file_lock *);
 322         void (*fl_break)(struct file_lock *); /* break_lease callback */
 323
 324 locking rules:
 325                         BKL     may block
 326 fl_compare_owner:       yes     no
 327 fl_notify:              yes     no
 328 fl_copy_lock:           yes     no
 329 fl_release_private:     yes     yes
 330 fl_break:               yes     no
 331
 332         Currently only NFSD and NLM provide instances of this class. None of the
 333 them block. If you have out-of-tree instances - please, show up. Locking
 334 in that area will change.
 335 --------------------------- buffer_head -----------------------------------
 336 prototypes:
 337         void (*b_end_io)(struct buffer_head *bh, int uptodate);
 338
 339 locking rules:
 340         called from interrupts. In other words, extreme care is needed here.
 341 bh is locked, but that's all warranties we have here. Currently only RAID1,
 342 highmem, fs/buffer.c, and fs/ntfs/aops.c are providing these. Block devices
 343 call this method upon the IO completion.
 344
 345 --------------------------- block_device_operations -----------------------
 346 prototypes:
 347         int (*open) (struct inode *, struct file *);
 348         int (*release) (struct inode *, struct file *);
 349         int (*ioctl) (struct inode *, struct file *, unsigned, unsigned long);
 350         int (*media_changed) (struct gendisk *);
 351         int (*revalidate_disk) (struct gendisk *);
 352
 353 locking rules:
 354                         BKL     bd_sem
 355 open:                   yes     yes
 356 release:                yes     yes
 357 ioctl:                  yes     no
 358 media_changed:          no      no
 359 revalidate_disk:        no      no
 360
 361 The last two are called only from check_disk_change().
 362
 363 --------------------------- file_operations -------------------------------
 364 prototypes:
 365         loff_t (*llseek) (struct file *, loff_t, int);
 366         ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
 367         ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
 368         ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
 369         ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
 370         int (*readdir) (struct file *, void *, filldir_t);
 371         unsigned int (*poll) (struct file *, struct poll_table_struct *);
 372         int (*ioctl) (struct inode *, struct file *, unsigned int,
 373                         unsigned long);
 374         long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
 375         long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
 376         int (*mmap) (struct file *, struct vm_area_struct *);
 377         int (*open) (struct inode *, struct file *);
 378         int (*flush) (struct file *);
 379         int (*release) (struct inode *, struct file *);
 380         int (*fsync) (struct file *, struct dentry *, int datasync);
 381         int (*aio_fsync) (struct kiocb *, int datasync);
 382         int (*fasync) (int, struct file *, int);
 383         int (*lock) (struct file *, int, struct file_lock *);
 384         ssize_t (*readv) (struct file *, const struct iovec *, unsigned long,
 385                         loff_t *);
 386         ssize_t (*writev) (struct file *, const struct iovec *, unsigned long,
 387                         loff_t *);
 388         ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t,
 389                         void __user *);
 390         ssize_t (*sendpage) (struct file *, struct page *, int, size_t,
 391                         loff_t *, int);
 392         unsigned long (*get_unmapped_area)(struct file *, unsigned long,
 393                         unsigned long, unsigned long, unsigned long);
 394         int (*check_flags)(int);
 395         int (*dir_notify)(struct file *, unsigned long);
 396 };
 397
 398 locking rules:
 399         All except ->poll() may block.
 400                         BKL
 401 llseek:                 no      (see below)
 402 read:                   no
 403 aio_read:               no
 404 write:                  no
 405 aio_write:              no
 406 readdir:                no
 407 poll:                   no
 408 ioctl:                  yes     (see below)
 409 unlocked_ioctl:         no      (see below)
 410 compat_ioctl:           no
 411 mmap:                   no
 412 open:                   maybe   (see below)
 413 flush:                  no
 414 release:                no
 415 fsync:                  no      (see below)
 416 aio_fsync:              no
 417 fasync:                 yes     (see below)
 418 lock:                   yes
 419 readv:                  no
 420 writev:                 no
 421 sendfile:               no
 422 sendpage:               no
 423 get_unmapped_area:      no
 424 check_flags:            no
 425 dir_notify:             no
 426
 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.
 433
 434 ->open() locking is in-transit: big lock partially moved into the methods.
 435 The only exception is ->open() in the instances of file_operations that never
 436 end up in ->i_fop/->proc_fops, i.e. ones that belong to character devices
 437 (chrdev_open() takes lock before replacing ->f_op and calling the secondary
 438 method. As soon as we fix the handling of module reference counters all
 439 instances of ->open() will be called without the BKL.
 440
 441 Note: ext2_release() was *the* source of contention on fs-intensive
 442 loads and dropping BKL on ->release() helps to get rid of that (we still
 443 grab BKL for cases when we close a file that had been opened r/w, but that
 444 can and should be done using the internal locking with smaller critical areas).
 445 Current worst offender is ext2_get_block()...
 446
 447 ->fasync() is a mess. This area needs a big cleanup and that will probably
 448 affect locking.
 449
 450 ->readdir() and ->ioctl() on directories must be changed. Ideally we would
 451 move ->readdir() to inode_operations and use a separate method for directory
 452 ->ioctl() or kill the latter completely. One of the problems is that for
 453 anything that resembles union-mount we won't have a struct file for all
 454 components. And there are other reasons why the current interface is a mess...
 455
 456 ->ioctl() on regular files is superceded by the ->unlocked_ioctl() that
 457 doesn't take the BKL.
 458
 459 ->read on directories probably must go away - we should just enforce -EISDIR
 460 in sys_read() and friends.
 461
 462 ->fsync() has i_sem on inode.
 463
 464 --------------------------- dquot_operations -------------------------------
 465 prototypes:
 466         int (*initialize) (struct inode *, int);
 467         int (*drop) (struct inode *);
 468         int (*alloc_space) (struct inode *, qsize_t, int);
 469         int (*alloc_inode) (const struct inode *, unsigned long);
 470         int (*free_space) (struct inode *, qsize_t);
 471         int (*free_inode) (const struct inode *, unsigned long);
 472         int (*transfer) (struct inode *, struct iattr *);
 473         int (*write_dquot) (struct dquot *);
 474         int (*acquire_dquot) (struct dquot *);
 475         int (*release_dquot) (struct dquot *);
 476         int (*mark_dirty) (struct dquot *);
 477         int (*write_info) (struct super_block *, int);
 478
 479 These operations are intended to be more or less wrapping functions that ensure
 480 a proper locking wrt the filesystem and call the generic quota operations.
 481
 482 What filesystem should expect from the generic quota functions:
 483
 484                 FS recursion    Held locks when called
 485 initialize:     yes             maybe dqonoff_sem
 486 drop:           yes             -
 487 alloc_space:    ->mark_dirty()  -
 488 alloc_inode:    ->mark_dirty()  -
 489 free_space:     ->mark_dirty()  -
 490 free_inode:     ->mark_dirty()  -
 491 transfer:       yes             -
 492 write_dquot:    yes             dqonoff_sem or dqptr_sem
 493 acquire_dquot:  yes             dqonoff_sem or dqptr_sem
 494 release_dquot:  yes             dqonoff_sem or dqptr_sem
 495 mark_dirty:     no              -
 496 write_info:     yes             dqonoff_sem
 497
 498 FS recursion means calling ->quota_read() and ->quota_write() from superblock
 499 operations.
 500
 501 ->alloc_space(), ->alloc_inode(), ->free_space(), ->free_inode() are called
 502 only directly by the filesystem and do not call any fs functions only
 503 the ->mark_dirty() operation.
 504
 505 More details about quota locking can be found in fs/dquot.c.
 506
 507 --------------------------- vm_operations_struct -----------------------------
 508 prototypes:
 509         void (*open)(struct vm_area_struct*);
 510         void (*close)(struct vm_area_struct*);
 511         struct page *(*nopage)(struct vm_area_struct*, unsigned long, int *);
 512
 513 locking rules:
 514                 BKL     mmap_sem
 515 open:           no      yes
 516 close:          no      yes
 517 nopage:         no      yes
 518
 519 ================================================================================
 520                         Dubious stuff
 521
 522 (if you break something or notice that it is broken and do not fix it yourself
 523 - at least put it here)
 524
 525 ipc/shm.c::shm_delete() - may need BKL.
 526 ->read() and ->write() in many drivers are (probably) missing BKL.
 527 drivers/sgi/char/graphics.c::sgi_graphics_nopage() - may need BKL.