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         char *(*d_dname)((struct dentry *dentry, char *buffer, int buflen);
  19
  20 locking rules:
  21         none have BKL
  22                 dcache_lock     rename_lock     ->d_lock        may block
  23 d_revalidate:   no              no              no              yes
  24 d_hash          no              no              no              yes
  25 d_compare:      no              yes             no              no
  26 d_delete:       yes             no              yes             no
  27 d_release:      no              no              no              yes
  28 d_iput:         no              no              no              yes
  29 d_dname:        no              no              no              no
  30
  31 --------------------------- inode_operations ---------------------------
  32 prototypes:
  33         int (*create) (struct inode *,struct dentry *,int, struct nameidata *);
  34         struct dentry * (*lookup) (struct inode *,struct dentry *, struct nameid
  35 ata *);
  36         int (*link) (struct dentry *,struct inode *,struct dentry *);
  37         int (*unlink) (struct inode *,struct dentry *);
  38         int (*symlink) (struct inode *,struct dentry *,const char *);
  39         int (*mkdir) (struct inode *,struct dentry *,int);
  40         int (*rmdir) (struct inode *,struct dentry *);
  41         int (*mknod) (struct inode *,struct dentry *,int,dev_t);
  42         int (*rename) (struct inode *, struct dentry *,
  43                         struct inode *, struct dentry *);
  44         int (*readlink) (struct dentry *, char __user *,int);
  45         int (*follow_link) (struct dentry *, struct nameidata *);
  46         void (*truncate) (struct inode *);
  47         int (*permission) (struct inode *, int, struct nameidata *);
  48         int (*setattr) (struct dentry *, struct iattr *);
  49         int (*getattr) (struct vfsmount *, struct dentry *, struct kstat *);
  50         int (*setxattr) (struct dentry *, const char *,const void *,size_t,int);
  51         ssize_t (*getxattr) (struct dentry *, const char *, void *, size_t);
  52         ssize_t (*listxattr) (struct dentry *, char *, size_t);
  53         int (*removexattr) (struct dentry *, const char *);
  54
  55 locking rules:
  56         all may block, none have BKL
  57                 i_mutex(inode)
  58 lookup:         yes
  59 create:         yes
  60 link:           yes (both)
  61 mknod:          yes
  62 symlink:        yes
  63 mkdir:          yes
  64 unlink:         yes (both)
  65 rmdir:          yes (both)      (see below)
  66 rename:         yes (all)       (see below)
  67 readlink:       no
  68 follow_link:    no
  69 truncate:       yes             (see below)
  70 setattr:        yes
  71 permission:     no
  72 getattr:        no
  73 setxattr:       yes
  74 getxattr:       no
  75 listxattr:      no
  76 removexattr:    yes
  77         Additionally, ->rmdir(), ->unlink() and ->rename() have ->i_mutex on
  78 victim.
  79         cross-directory ->rename() has (per-superblock) ->s_vfs_rename_sem.
  80         ->truncate() is never called directly - it's a callback, not a
  81 method. It's called by vmtruncate() - library function normally used by
  82 ->setattr(). Locking information above applies to that call (i.e. is
  83 inherited from ->setattr() - vmtruncate() is used when ATTR_SIZE had been
  84 passed).
  85
  86 See Documentation/filesystems/directory-locking for more detailed discussion
  87 of the locking scheme for directory operations.
  88
  89 --------------------------- super_operations ---------------------------
  90 prototypes:
  91         struct inode *(*alloc_inode)(struct super_block *sb);
  92         void (*destroy_inode)(struct inode *);
  93         void (*dirty_inode) (struct inode *);
  94         int (*write_inode) (struct inode *, int);
  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         int (*freeze_fs) (struct super_block *);
 101         int (*unfreeze_fs) (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 dirty_inode:            no                              (must not sleep)
 116 write_inode:            no
 117 drop_inode:             no                              !!!inode_lock!!!
 118 delete_inode:           no
 119 put_super:              yes     yes     no
 120 write_super:            no      yes     read
 121 sync_fs:                no      no      read
 122 freeze_fs:              ?
 123 unfreeze_fs:            ?
 124 statfs:                 no      no      no
 125 remount_fs:             yes     yes     maybe           (see below)
 126 clear_inode:            no
 127 umount_begin:           yes     no      no
 128 show_options:           no                              (vfsmount->sem)
 129 quota_read:             no      no      no              (see below)
 130 quota_write:            no      no      no              (see below)
 131
 132 ->remount_fs() will have the s_umount lock if it's already mounted.
 133 When called from get_sb_single, it does NOT have the s_umount lock.
 134 ->quota_read() and ->quota_write() functions are both guaranteed to
 135 be the only ones operating on the quota file by the quota code (via
 136 dqio_sem) (unless an admin really wants to screw up something and
 137 writes to quota files with quotas on). For other details about locking
 138 see also dquot_operations section.
 139
 140 --------------------------- file_system_type ---------------------------
 141 prototypes:
 142         int (*get_sb) (struct file_system_type *, int,
 143                        const char *, void *, struct vfsmount *);
 144         void (*kill_sb) (struct super_block *);
 145 locking rules:
 146                 may block       BKL
 147 get_sb          yes             no
 148 kill_sb         yes             no
 149
 150 ->get_sb() returns error or 0 with locked superblock attached to the vfsmount
 151 (exclusive on ->s_umount).
 152 ->kill_sb() takes a write-locked superblock, does all shutdown work on it,
 153 unlocks and drops the reference.
 154
 155 --------------------------- address_space_operations --------------------------
 156 prototypes:
 157         int (*writepage)(struct page *page, struct writeback_control *wbc);
 158         int (*readpage)(struct file *, struct page *);
 159         int (*sync_page)(struct page *);
 160         int (*writepages)(struct address_space *, struct writeback_control *);
 161         int (*set_page_dirty)(struct page *page);
 162         int (*readpages)(struct file *filp, struct address_space *mapping,
 163                         struct list_head *pages, unsigned nr_pages);
 164         int (*write_begin)(struct file *, struct address_space *mapping,
 165                                 loff_t pos, unsigned len, unsigned flags,
 166                                 struct page **pagep, void **fsdata);
 167         int (*write_end)(struct file *, struct address_space *mapping,
 168                                 loff_t pos, unsigned len, unsigned copied,
 169                                 struct page *page, void *fsdata);
 170         sector_t (*bmap)(struct address_space *, sector_t);
 171         int (*invalidatepage) (struct page *, unsigned long);
 172         int (*releasepage) (struct page *, int);
 173         int (*direct_IO)(int, struct kiocb *, const struct iovec *iov,
 174                         loff_t offset, unsigned long nr_segs);
 175         int (*launder_page) (struct page *);
 176
 177 locking rules:
 178         All except set_page_dirty may block
 179
 180                         BKL     PageLocked(page)        i_sem
 181 writepage:              no      yes, unlocks (see below)
 182 readpage:               no      yes, unlocks
 183 sync_page:              no      maybe
 184 writepages:             no
 185 set_page_dirty          no      no
 186 readpages:              no
 187 write_begin:            no      locks the page          yes
 188 write_end:              no      yes, unlocks            yes
 189 perform_write:          no      n/a                     yes
 190 bmap:                   no
 191 invalidatepage:         no      yes
 192 releasepage:            no      yes
 193 direct_IO:              no
 194 launder_page:           no      yes
 195
 196         ->write_begin(), ->write_end(), ->sync_page() and ->readpage()
 197 may be called from the request handler (/dev/loop).
 198
 199         ->readpage() unlocks the page, either synchronously or via I/O
 200 completion.
 201
 202         ->readpages() populates the pagecache with the passed pages and starts
 203 I/O against them.  They come unlocked upon I/O completion.
 204
 205         ->writepage() is used for two purposes: for "memory cleansing" and for
 206 "sync".  These are quite different operations and the behaviour may differ
 207 depending upon the mode.
 208
 209 If writepage is called for sync (wbc->sync_mode != WBC_SYNC_NONE) then
 210 it *must* start I/O against the page, even if that would involve
 211 blocking on in-progress I/O.
 212
 213 If writepage is called for memory cleansing (sync_mode ==
 214 WBC_SYNC_NONE) then its role is to get as much writeout underway as
 215 possible.  So writepage should try to avoid blocking against
 216 currently-in-progress I/O.
 217
 218 If the filesystem is not called for "sync" and it determines that it
 219 would need to block against in-progress I/O to be able to start new I/O
 220 against the page the filesystem should redirty the page with
 221 redirty_page_for_writepage(), then unlock the page and return zero.
 222 This may also be done to avoid internal deadlocks, but rarely.
 223
 224 If the filesystem is called for sync then it must wait on any
 225 in-progress I/O and then start new I/O.
 226
 227 The filesystem should unlock the page synchronously, before returning to the
 228 caller, unless ->writepage() returns special WRITEPAGE_ACTIVATE
 229 value. WRITEPAGE_ACTIVATE means that page cannot really be written out
 230 currently, and VM should stop calling ->writepage() on this page for some
 231 time. VM does this by moving page to the head of the active list, hence the
 232 name.
 233
 234 Unless the filesystem is going to redirty_page_for_writepage(), unlock the page
 235 and return zero, writepage *must* run set_page_writeback() against the page,
 236 followed by unlocking it.  Once set_page_writeback() has been run against the
 237 page, write I/O can be submitted and the write I/O completion handler must run
 238 end_page_writeback() once the I/O is complete.  If no I/O is submitted, the
 239 filesystem must run end_page_writeback() against the page before returning from
 240 writepage.
 241
 242 That is: after 2.5.12, pages which are under writeout are *not* locked.  Note,
 243 if the filesystem needs the page to be locked during writeout, that is ok, too,
 244 the page is allowed to be unlocked at any point in time between the calls to
 245 set_page_writeback() and end_page_writeback().
 246
 247 Note, failure to run either redirty_page_for_writepage() or the combination of
 248 set_page_writeback()/end_page_writeback() on a page submitted to writepage
 249 will leave the page itself marked clean but it will be tagged as dirty in the
 250 radix tree.  This incoherency can lead to all sorts of hard-to-debug problems
 251 in the filesystem like having dirty inodes at umount and losing written data.
 252
 253         ->sync_page() locking rules are not well-defined - usually it is called
 254 with lock on page, but that is not guaranteed. Considering the currently
 255 existing instances of this method ->sync_page() itself doesn't look
 256 well-defined...
 257
 258         ->writepages() is used for periodic writeback and for syscall-initiated
 259 sync operations.  The address_space should start I/O against at least
 260 *nr_to_write pages.  *nr_to_write must be decremented for each page which is
 261 written.  The address_space implementation may write more (or less) pages
 262 than *nr_to_write asks for, but it should try to be reasonably close.  If
 263 nr_to_write is NULL, all dirty pages must be written.
 264
 265 writepages should _only_ write pages which are present on
 266 mapping->io_pages.
 267
 268         ->set_page_dirty() is called from various places in the kernel
 269 when the target page is marked as needing writeback.  It may be called
 270 under spinlock (it cannot block) and is sometimes called with the page
 271 not locked.
 272
 273         ->bmap() is currently used by legacy ioctl() (FIBMAP) provided by some
 274 filesystems and by the swapper. The latter will eventually go away. All
 275 instances do not actually need the BKL. Please, keep it that way and don't
 276 breed new callers.
 277
 278         ->invalidatepage() is called when the filesystem must attempt to drop
 279 some or all of the buffers from the page when it is being truncated.  It
 280 returns zero on success.  If ->invalidatepage is zero, the kernel uses
 281 block_invalidatepage() instead.
 282
 283         ->releasepage() is called when the kernel is about to try to drop the
 284 buffers from the page in preparation for freeing it.  It returns zero to
 285 indicate that the buffers are (or may be) freeable.  If ->releasepage is zero,
 286 the kernel assumes that the fs has no private interest in the buffers.
 287
 288         ->launder_page() may be called prior to releasing a page if
 289 it is still found to be dirty. It returns zero if the page was successfully
 290 cleaned, or an error value if not. Note that in order to prevent the page
 291 getting mapped back in and redirtied, it needs to be kept locked
 292 across the entire operation.
 293
 294         Note: currently almost all instances of address_space methods are
 295 using BKL for internal serialization and that's one of the worst sources
 296 of contention. Normally they are calling library functions (in fs/buffer.c)
 297 and pass foo_get_block() as a callback (on local block-based filesystems,
 298 indeed). BKL is not needed for library stuff and is usually taken by
 299 foo_get_block(). It's an overkill, since block bitmaps can be protected by
 300 internal fs locking and real critical areas are much smaller than the areas
 301 filesystems protect now.
 302
 303 ----------------------- file_lock_operations ------------------------------
 304 prototypes:
 305         void (*fl_insert)(struct file_lock *);  /* lock insertion callback */
 306         void (*fl_remove)(struct file_lock *);  /* lock removal callback */
 307         void (*fl_copy_lock)(struct file_lock *, struct file_lock *);
 308         void (*fl_release_private)(struct file_lock *);
 309
 310
 311 locking rules:
 312                         BKL     may block
 313 fl_insert:              yes     no
 314 fl_remove:              yes     no
 315 fl_copy_lock:           yes     no
 316 fl_release_private:     yes     yes
 317
 318 ----------------------- lock_manager_operations ---------------------------
 319 prototypes:
 320         int (*fl_compare_owner)(struct file_lock *, struct file_lock *);
 321         void (*fl_notify)(struct file_lock *);  /* unblock callback */
 322         void (*fl_copy_lock)(struct file_lock *, struct file_lock *);
 323         void (*fl_release_private)(struct file_lock *);
 324         void (*fl_break)(struct file_lock *); /* break_lease callback */
 325
 326 locking rules:
 327                         BKL     may block
 328 fl_compare_owner:       yes     no
 329 fl_notify:              yes     no
 330 fl_copy_lock:           yes     no
 331 fl_release_private:     yes     yes
 332 fl_break:               yes     no
 333
 334         Currently only NFSD and NLM provide instances of this class. None of the
 335 them block. If you have out-of-tree instances - please, show up. Locking
 336 in that area will change.
 337 --------------------------- buffer_head -----------------------------------
 338 prototypes:
 339         void (*b_end_io)(struct buffer_head *bh, int uptodate);
 340
 341 locking rules:
 342         called from interrupts. In other words, extreme care is needed here.
 343 bh is locked, but that's all warranties we have here. Currently only RAID1,
 344 highmem, fs/buffer.c, and fs/ntfs/aops.c are providing these. Block devices
 345 call this method upon the IO completion.
 346
 347 --------------------------- block_device_operations -----------------------
 348 prototypes:
 349         int (*open) (struct inode *, struct file *);
 350         int (*release) (struct inode *, struct file *);
 351         int (*ioctl) (struct inode *, struct file *, unsigned, unsigned long);
 352         int (*media_changed) (struct gendisk *);
 353         int (*revalidate_disk) (struct gendisk *);
 354
 355 locking rules:
 356                         BKL     bd_sem
 357 open:                   yes     yes
 358 release:                yes     yes
 359 ioctl:                  yes     no
 360 media_changed:          no      no
 361 revalidate_disk:        no      no
 362
 363 The last two are called only from check_disk_change().
 364
 365 --------------------------- file_operations -------------------------------
 366 prototypes:
 367         loff_t (*llseek) (struct file *, loff_t, int);
 368         ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
 369         ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
 370         ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
 371         ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
 372         int (*readdir) (struct file *, void *, filldir_t);
 373         unsigned int (*poll) (struct file *, struct poll_table_struct *);
 374         int (*ioctl) (struct inode *, struct file *, unsigned int,
 375                         unsigned long);
 376         long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
 377         long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
 378         int (*mmap) (struct file *, struct vm_area_struct *);
 379         int (*open) (struct inode *, struct file *);
 380         int (*flush) (struct file *);
 381         int (*release) (struct inode *, struct file *);
 382         int (*fsync) (struct file *, struct dentry *, int datasync);
 383         int (*aio_fsync) (struct kiocb *, int datasync);
 384         int (*fasync) (int, struct file *, int);
 385         int (*lock) (struct file *, int, struct file_lock *);
 386         ssize_t (*readv) (struct file *, const struct iovec *, unsigned long,
 387                         loff_t *);
 388         ssize_t (*writev) (struct file *, const struct iovec *, unsigned long,
 389                         loff_t *);
 390         ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t,
 391                         void __user *);
 392         ssize_t (*sendpage) (struct file *, struct page *, int, size_t,
 393                         loff_t *, int);
 394         unsigned long (*get_unmapped_area)(struct file *, unsigned long,
 395                         unsigned long, unsigned long, unsigned long);
 396         int (*check_flags)(int);
 397 };
 398
 399 locking rules:
 400         All may block.
 401                         BKL
 402 llseek:                 no      (see below)
 403 read:                   no
 404 aio_read:               no
 405 write:                  no
 406 aio_write:              no
 407 readdir:                no
 408 poll:                   no
 409 ioctl:                  yes     (see below)
 410 unlocked_ioctl:         no      (see below)
 411 compat_ioctl:           no
 412 mmap:                   no
 413 open:                   no
 414 flush:                  no
 415 release:                no
 416 fsync:                  no      (see below)
 417 aio_fsync:              no
 418 fasync:                 no
 419 lock:                   yes
 420 readv:                  no
 421 writev:                 no
 422 sendfile:               no
 423 sendpage:               no
 424 get_unmapped_area:      no
 425 check_flags:            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 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()...
 439
 440 ->fasync() is called without BKL protection, and is responsible for
 441 maintaining the FASYNC bit in filp->f_flags.  Most instances call
 442 fasync_helper(), which does that maintenance, so it's not normally
 443 something one needs to worry about.  Return values > 0 will be mapped to
 444 zero in the VFS layer.
 445
 446 ->readdir() and ->ioctl() on directories must be changed. Ideally we would
 447 move ->readdir() to inode_operations and use a separate method for directory
 448 ->ioctl() or kill the latter completely. One of the problems is that for
 449 anything that resembles union-mount we won't have a struct file for all
 450 components. And there are other reasons why the current interface is a mess...
 451
 452 ->ioctl() on regular files is superceded by the ->unlocked_ioctl() that
 453 doesn't take the BKL.
 454
 455 ->read on directories probably must go away - we should just enforce -EISDIR
 456 in sys_read() and friends.
 457
 458 ->fsync() has i_mutex on inode.
 459
 460 --------------------------- dquot_operations -------------------------------
 461 prototypes:
 462         int (*initialize) (struct inode *, int);
 463         int (*drop) (struct inode *);
 464         int (*alloc_space) (struct inode *, qsize_t, int);
 465         int (*alloc_inode) (const struct inode *, unsigned long);
 466         int (*free_space) (struct inode *, qsize_t);
 467         int (*free_inode) (const struct inode *, unsigned long);
 468         int (*transfer) (struct inode *, struct iattr *);
 469         int (*write_dquot) (struct dquot *);
 470         int (*acquire_dquot) (struct dquot *);
 471         int (*release_dquot) (struct dquot *);
 472         int (*mark_dirty) (struct dquot *);
 473         int (*write_info) (struct super_block *, int);
 474
 475 These operations are intended to be more or less wrapping functions that ensure
 476 a proper locking wrt the filesystem and call the generic quota operations.
 477
 478 What filesystem should expect from the generic quota functions:
 479
 480                 FS recursion    Held locks when called
 481 initialize:     yes             maybe dqonoff_sem
 482 drop:           yes             -
 483 alloc_space:    ->mark_dirty()  -
 484 alloc_inode:    ->mark_dirty()  -
 485 free_space:     ->mark_dirty()  -
 486 free_inode:     ->mark_dirty()  -
 487 transfer:       yes             -
 488 write_dquot:    yes             dqonoff_sem or dqptr_sem
 489 acquire_dquot:  yes             dqonoff_sem or dqptr_sem
 490 release_dquot:  yes             dqonoff_sem or dqptr_sem
 491 mark_dirty:     no              -
 492 write_info:     yes             dqonoff_sem
 493
 494 FS recursion means calling ->quota_read() and ->quota_write() from superblock
 495 operations.
 496
 497 ->alloc_space(), ->alloc_inode(), ->free_space(), ->free_inode() are called
 498 only directly by the filesystem and do not call any fs functions only
 499 the ->mark_dirty() operation.
 500
 501 More details about quota locking can be found in fs/dquot.c.
 502
 503 --------------------------- vm_operations_struct -----------------------------
 504 prototypes:
 505         void (*open)(struct vm_area_struct*);
 506         void (*close)(struct vm_area_struct*);
 507         int (*fault)(struct vm_area_struct*, struct vm_fault *);
 508         int (*page_mkwrite)(struct vm_area_struct *, struct vm_fault *);
 509         int (*access)(struct vm_area_struct *, unsigned long, void*, int, int);
 510
 511 locking rules:
 512                 BKL     mmap_sem        PageLocked(page)
 513 open:           no      yes
 514 close:          no      yes
 515 fault:          no      yes             can return with page locked
 516 page_mkwrite:   no      yes             can return with page locked
 517 access:         no      yes
 518
 519         ->fault() is called when a previously not present pte is about
 520 to be faulted in. The filesystem must find and return the page associated
 521 with the passed in "pgoff" in the vm_fault structure. If it is possible that
 522 the page may be truncated and/or invalidated, then the filesystem must lock
 523 the page, then ensure it is not already truncated (the page lock will block
 524 subsequent truncate), and then return with VM_FAULT_LOCKED, and the page
 525 locked. The VM will unlock the page.
 526
 527         ->page_mkwrite() is called when a previously read-only pte is
 528 about to become writeable. The filesystem again must ensure that there are
 529 no truncate/invalidate races, and then return with the page locked. If
 530 the page has been truncated, the filesystem should not look up a new page
 531 like the ->fault() handler, but simply return with VM_FAULT_NOPAGE, which
 532 will cause the VM to retry the fault.
 533
 534         ->access() is called when get_user_pages() fails in
 535 acces_process_vm(), typically used to debug a process through
 536 /proc/pid/mem or ptrace.  This function is needed only for
 537 VM_IO | VM_PFNMAP VMAs.
 538
 539 ================================================================================
 540                         Dubious stuff
 541
 542 (if you break something or notice that it is broken and do not fix it yourself
 543 - at least put it here)
 544
 545 ipc/shm.c::shm_delete() - may need BKL.
 546 ->read() and ->write() in many drivers are (probably) missing BKL.