The text below describes the locking rules for VFS-related methods.
  It is (believed to be) up-to-date. *Please*, if you change anything in
  prototypes or locking protocols - update this file. And update the relevant
  instances in the tree, don't leave that to maintainers of filesystems/devices/
  etc. At the very least, put the list of dubious cases in the end of this file.
  Don't turn it into log - maintainers of out-of-the-tree code are supposed to
  be able to use diff(1).
  	Thing currently missing here: socket operations. Alexey?
  
  --------------------------- dentry_operations --------------------------
  prototypes:
  	int (*d_revalidate)(struct dentry *, int);
  	int (*d_hash) (struct dentry *, struct qstr *);
  	int (*d_compare) (struct dentry *, struct qstr *, struct qstr *);
  	int (*d_delete)(struct dentry *);
  	void (*d_release)(struct dentry *);
  	void (*d_iput)(struct dentry *, struct inode *);

  	char *(*d_dname)((struct dentry *dentry, char *buffer, int buflen);

  
  locking rules:
  	none have BKL
  		dcache_lock	rename_lock	->d_lock	may block
  d_revalidate:	no		no		no		yes
  d_hash		no		no		no		yes
  d_compare:	no		yes		no		no 
  d_delete:	yes		no		yes		no
  d_release:	no		no		no		yes
  d_iput:		no		no		no		yes

  d_dname:	no		no		no		no

  
  --------------------------- inode_operations --------------------------- 
  prototypes:
  	int (*create) (struct inode *,struct dentry *,int, struct nameidata *);
  	struct dentry * (*lookup) (struct inode *,struct dentry *, struct nameid
  ata *);
  	int (*link) (struct dentry *,struct inode *,struct dentry *);
  	int (*unlink) (struct inode *,struct dentry *);
  	int (*symlink) (struct inode *,struct dentry *,const char *);
  	int (*mkdir) (struct inode *,struct dentry *,int);
  	int (*rmdir) (struct inode *,struct dentry *);
  	int (*mknod) (struct inode *,struct dentry *,int,dev_t);
  	int (*rename) (struct inode *, struct dentry *,
  			struct inode *, struct dentry *);
  	int (*readlink) (struct dentry *, char __user *,int);
  	int (*follow_link) (struct dentry *, struct nameidata *);
  	void (*truncate) (struct inode *);
  	int (*permission) (struct inode *, int, struct nameidata *);
  	int (*setattr) (struct dentry *, struct iattr *);
  	int (*getattr) (struct vfsmount *, struct dentry *, struct kstat *);
  	int (*setxattr) (struct dentry *, const char *,const void *,size_t,int);
  	ssize_t (*getxattr) (struct dentry *, const char *, void *, size_t);
  	ssize_t (*listxattr) (struct dentry *, char *, size_t);
  	int (*removexattr) (struct dentry *, const char *);
  
  locking rules:
  	all may block, none have BKL

  		i_mutex(inode)

  lookup:		yes
  create:		yes
  link:		yes (both)
  mknod:		yes
  symlink:	yes
  mkdir:		yes
  unlink:		yes (both)
  rmdir:		yes (both)	(see below)
  rename:		yes (all)	(see below)
  readlink:	no
  follow_link:	no
  truncate:	yes		(see below)
  setattr:	yes
  permission:	no
  getattr:	no
  setxattr:	yes
  getxattr:	no
  listxattr:	no
  removexattr:	yes

  	Additionally, ->rmdir(), ->unlink() and ->rename() have ->i_mutex on

  victim.
  	cross-directory ->rename() has (per-superblock) ->s_vfs_rename_sem.
  	->truncate() is never called directly - it's a callback, not a
  method. It's called by vmtruncate() - library function normally used by
  ->setattr(). Locking information above applies to that call (i.e. is
  inherited from ->setattr() - vmtruncate() is used when ATTR_SIZE had been
  passed).
  
  See Documentation/filesystems/directory-locking for more detailed discussion
  of the locking scheme for directory operations.
  
  --------------------------- super_operations ---------------------------
  prototypes:
  	struct inode *(*alloc_inode)(struct super_block *sb);
  	void (*destroy_inode)(struct inode *);

  	void (*dirty_inode) (struct inode *);
  	int (*write_inode) (struct inode *, int);

  	int (*drop_inode) (struct inode *);
  	void (*evict_inode) (struct inode *);

  	void (*put_super) (struct super_block *);
  	void (*write_super) (struct super_block *);
  	int (*sync_fs)(struct super_block *sb, int wait);

  	int (*freeze_fs) (struct super_block *);
  	int (*unfreeze_fs) (struct super_block *);

  	int (*statfs) (struct dentry *, struct kstatfs *);

  	int (*remount_fs) (struct super_block *, int *, char *);

  	void (*umount_begin) (struct super_block *);
  	int (*show_options)(struct seq_file *, struct vfsmount *);
  	ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
  	ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
  
  locking rules:

  	All may block [not true, see below]

  	None have BKL
  			s_umount
  alloc_inode:
  destroy_inode:
  dirty_inode:				(must not sleep)
  write_inode:
  drop_inode:				!!!inode_lock!!!

  evict_inode:

  put_super:		write
  write_super:		read
  sync_fs:		read
  freeze_fs:		read
  unfreeze_fs:		read

  statfs:			maybe(read)	(see below)
  remount_fs:		write

  umount_begin:		no
  show_options:		no		(namespace_sem)
  quota_read:		no		(see below)
  quota_write:		no		(see below)

  ->statfs() has s_umount (shared) when called by ustat(2) (native or
  compat), but that's an accident of bad API; s_umount is used to pin
  the superblock down when we only have dev_t given us by userland to
  identify the superblock.  Everything else (statfs(), fstatfs(), etc.)
  doesn't hold it when calling ->statfs() - superblock is pinned down
  by resolving the pathname passed to syscall.

  ->quota_read() and ->quota_write() functions are both guaranteed to
  be the only ones operating on the quota file by the quota code (via
  dqio_sem) (unless an admin really wants to screw up something and
  writes to quota files with quotas on). For other details about locking
  see also dquot_operations section.
  
  --------------------------- file_system_type ---------------------------
  prototypes:

  	int (*get_sb) (struct file_system_type *, int,
  		       const char *, void *, struct vfsmount *);

  	void (*kill_sb) (struct super_block *);
  locking rules:
  		may block	BKL

  get_sb		yes		no
  kill_sb		yes		no

  ->get_sb() returns error or 0 with locked superblock attached to the vfsmount
  (exclusive on ->s_umount).

  ->kill_sb() takes a write-locked superblock, does all shutdown work on it,
  unlocks and drops the reference.
  
  --------------------------- address_space_operations --------------------------
  prototypes:
  	int (*writepage)(struct page *page, struct writeback_control *wbc);
  	int (*readpage)(struct file *, struct page *);
  	int (*sync_page)(struct page *);
  	int (*writepages)(struct address_space *, struct writeback_control *);
  	int (*set_page_dirty)(struct page *page);
  	int (*readpages)(struct file *filp, struct address_space *mapping,
  			struct list_head *pages, unsigned nr_pages);

  	int (*write_begin)(struct file *, struct address_space *mapping,
  				loff_t pos, unsigned len, unsigned flags,
  				struct page **pagep, void **fsdata);
  	int (*write_end)(struct file *, struct address_space *mapping,
  				loff_t pos, unsigned len, unsigned copied,
  				struct page *page, void *fsdata);

  	sector_t (*bmap)(struct address_space *, sector_t);
  	int (*invalidatepage) (struct page *, unsigned long);
  	int (*releasepage) (struct page *, int);
  	int (*direct_IO)(int, struct kiocb *, const struct iovec *iov,
  			loff_t offset, unsigned long nr_segs);

  	int (*launder_page) (struct page *);

  
  locking rules:
  	All except set_page_dirty may block

  			BKL	PageLocked(page)	i_mutex

  writepage:		no	yes, unlocks (see below)
  readpage:		no	yes, unlocks
  sync_page:		no	maybe
  writepages:		no
  set_page_dirty		no	no
  readpages:		no

  write_begin:		no	locks the page		yes
  write_end:		no	yes, unlocks		yes
  perform_write:		no	n/a			yes

  bmap:			no

  invalidatepage:		no	yes
  releasepage:		no	yes
  direct_IO:		no

  launder_page:		no	yes

  	->write_begin(), ->write_end(), ->sync_page() and ->readpage()

  may be called from the request handler (/dev/loop).
  
  	->readpage() unlocks the page, either synchronously or via I/O
  completion.
  
  	->readpages() populates the pagecache with the passed pages and starts
  I/O against them.  They come unlocked upon I/O completion.
  
  	->writepage() is used for two purposes: for "memory cleansing" and for
  "sync".  These are quite different operations and the behaviour may differ
  depending upon the mode.
  
  If writepage is called for sync (wbc->sync_mode != WBC_SYNC_NONE) then
  it *must* start I/O against the page, even if that would involve
  blocking on in-progress I/O.
  
  If writepage is called for memory cleansing (sync_mode ==
  WBC_SYNC_NONE) then its role is to get as much writeout underway as
  possible.  So writepage should try to avoid blocking against
  currently-in-progress I/O.
  
  If the filesystem is not called for "sync" and it determines that it
  would need to block against in-progress I/O to be able to start new I/O
  against the page the filesystem should redirty the page with
  redirty_page_for_writepage(), then unlock the page and return zero.
  This may also be done to avoid internal deadlocks, but rarely.

  If the filesystem is called for sync then it must wait on any

  in-progress I/O and then start new I/O.

  The filesystem should unlock the page synchronously, before returning to the
  caller, unless ->writepage() returns special WRITEPAGE_ACTIVATE
  value. WRITEPAGE_ACTIVATE means that page cannot really be written out
  currently, and VM should stop calling ->writepage() on this page for some
  time. VM does this by moving page to the head of the active list, hence the
  name.

  
  Unless the filesystem is going to redirty_page_for_writepage(), unlock the page
  and return zero, writepage *must* run set_page_writeback() against the page,
  followed by unlocking it.  Once set_page_writeback() has been run against the
  page, write I/O can be submitted and the write I/O completion handler must run
  end_page_writeback() once the I/O is complete.  If no I/O is submitted, the
  filesystem must run end_page_writeback() against the page before returning from
  writepage.
  
  That is: after 2.5.12, pages which are under writeout are *not* locked.  Note,
  if the filesystem needs the page to be locked during writeout, that is ok, too,
  the page is allowed to be unlocked at any point in time between the calls to
  set_page_writeback() and end_page_writeback().
  
  Note, failure to run either redirty_page_for_writepage() or the combination of
  set_page_writeback()/end_page_writeback() on a page submitted to writepage
  will leave the page itself marked clean but it will be tagged as dirty in the
  radix tree.  This incoherency can lead to all sorts of hard-to-debug problems
  in the filesystem like having dirty inodes at umount and losing written data.
  
  	->sync_page() locking rules are not well-defined - usually it is called
  with lock on page, but that is not guaranteed. Considering the currently
  existing instances of this method ->sync_page() itself doesn't look
  well-defined...
  
  	->writepages() is used for periodic writeback and for syscall-initiated
  sync operations.  The address_space should start I/O against at least
  *nr_to_write pages.  *nr_to_write must be decremented for each page which is
  written.  The address_space implementation may write more (or less) pages
  than *nr_to_write asks for, but it should try to be reasonably close.  If
  nr_to_write is NULL, all dirty pages must be written.
  
  writepages should _only_ write pages which are present on
  mapping->io_pages.
  
  	->set_page_dirty() is called from various places in the kernel
  when the target page is marked as needing writeback.  It may be called
  under spinlock (it cannot block) and is sometimes called with the page
  not locked.
  
  	->bmap() is currently used by legacy ioctl() (FIBMAP) provided by some
  filesystems and by the swapper. The latter will eventually go away. All
  instances do not actually need the BKL. Please, keep it that way and don't
  breed new callers.
  
  	->invalidatepage() is called when the filesystem must attempt to drop
  some or all of the buffers from the page when it is being truncated.  It
  returns zero on success.  If ->invalidatepage is zero, the kernel uses
  block_invalidatepage() instead.
  
  	->releasepage() is called when the kernel is about to try to drop the
  buffers from the page in preparation for freeing it.  It returns zero to
  indicate that the buffers are (or may be) freeable.  If ->releasepage is zero,
  the kernel assumes that the fs has no private interest in the buffers.

  	->launder_page() may be called prior to releasing a page if
  it is still found to be dirty. It returns zero if the page was successfully
  cleaned, or an error value if not. Note that in order to prevent the page
  getting mapped back in and redirtied, it needs to be kept locked
  across the entire operation.

  	Note: currently almost all instances of address_space methods are
  using BKL for internal serialization and that's one of the worst sources
  of contention. Normally they are calling library functions (in fs/buffer.c)
  and pass foo_get_block() as a callback (on local block-based filesystems,
  indeed). BKL is not needed for library stuff and is usually taken by
  foo_get_block(). It's an overkill, since block bitmaps can be protected by
  internal fs locking and real critical areas are much smaller than the areas
  filesystems protect now.
  
  ----------------------- file_lock_operations ------------------------------
  prototypes:
  	void (*fl_insert)(struct file_lock *);	/* lock insertion callback */
  	void (*fl_remove)(struct file_lock *);	/* lock removal callback */
  	void (*fl_copy_lock)(struct file_lock *, struct file_lock *);
  	void (*fl_release_private)(struct file_lock *);
  
  
  locking rules:
  			BKL	may block
  fl_insert:		yes	no
  fl_remove:		yes	no
  fl_copy_lock:		yes	no
  fl_release_private:	yes	yes
  
  ----------------------- lock_manager_operations ---------------------------
  prototypes:
  	int (*fl_compare_owner)(struct file_lock *, struct file_lock *);
  	void (*fl_notify)(struct file_lock *);  /* unblock callback */
  	void (*fl_copy_lock)(struct file_lock *, struct file_lock *);
  	void (*fl_release_private)(struct file_lock *);
  	void (*fl_break)(struct file_lock *); /* break_lease callback */
  
  locking rules:
  			BKL	may block
  fl_compare_owner:	yes	no
  fl_notify:		yes	no
  fl_copy_lock:		yes	no
  fl_release_private:	yes	yes
  fl_break:		yes	no
  
  	Currently only NFSD and NLM provide instances of this class. None of the
  them block. If you have out-of-tree instances - please, show up. Locking
  in that area will change.
  --------------------------- buffer_head -----------------------------------
  prototypes:
  	void (*b_end_io)(struct buffer_head *bh, int uptodate);
  
  locking rules:
  	called from interrupts. In other words, extreme care is needed here.
  bh is locked, but that's all warranties we have here. Currently only RAID1,
  highmem, fs/buffer.c, and fs/ntfs/aops.c are providing these. Block devices
  call this method upon the IO completion.
  
  --------------------------- block_device_operations -----------------------
  prototypes:
  	int (*open) (struct inode *, struct file *);
  	int (*release) (struct inode *, struct file *);
  	int (*ioctl) (struct inode *, struct file *, unsigned, unsigned long);
  	int (*media_changed) (struct gendisk *);
  	int (*revalidate_disk) (struct gendisk *);
  
  locking rules:
  			BKL	bd_sem
  open:			yes	yes
  release:		yes	yes
  ioctl:			yes	no
  media_changed:		no	no
  revalidate_disk:	no	no
  
  The last two are called only from check_disk_change().
  
  --------------------------- file_operations -------------------------------
  prototypes:
  	loff_t (*llseek) (struct file *, loff_t, int);
  	ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);

  	ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);

  	ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
  	ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);

  	int (*readdir) (struct file *, void *, filldir_t);
  	unsigned int (*poll) (struct file *, struct poll_table_struct *);

  	long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
  	long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
  	int (*mmap) (struct file *, struct vm_area_struct *);
  	int (*open) (struct inode *, struct file *);
  	int (*flush) (struct file *);
  	int (*release) (struct inode *, struct file *);

  	int (*fsync) (struct file *, int datasync);

  	int (*aio_fsync) (struct kiocb *, int datasync);
  	int (*fasync) (int, struct file *, int);
  	int (*lock) (struct file *, int, struct file_lock *);
  	ssize_t (*readv) (struct file *, const struct iovec *, unsigned long,
  			loff_t *);
  	ssize_t (*writev) (struct file *, const struct iovec *, unsigned long,
  			loff_t *);
  	ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t,
  			void __user *);
  	ssize_t (*sendpage) (struct file *, struct page *, int, size_t,
  			loff_t *, int);
  	unsigned long (*get_unmapped_area)(struct file *, unsigned long,
  			unsigned long, unsigned long, unsigned long);
  	int (*check_flags)(int);

  };
  
  locking rules:

  	All may block.

  			BKL
  llseek:			no	(see below)
  read:			no
  aio_read:		no
  write:			no
  aio_write:		no
  readdir: 		no
  poll:			no

  unlocked_ioctl:		no

  compat_ioctl:		no
  mmap:			no

  open:			no

  flush:			no
  release:		no
  fsync:			no	(see below)
  aio_fsync:		no

  fasync:			no

  lock:			yes
  readv:			no
  writev:			no
  sendfile:		no
  sendpage:		no
  get_unmapped_area:	no
  check_flags:		no

  
  ->llseek() locking has moved from llseek to the individual llseek
  implementations.  If your fs is not using generic_file_llseek, you
  need to acquire and release the appropriate locks in your ->llseek().
  For many filesystems, it is probably safe to acquire the inode

  mutex or just to use i_size_read() instead.
  Note: this does not protect the file->f_pos against concurrent modifications
  since this is something the userspace has to take care about.

  Note: ext2_release() was *the* source of contention on fs-intensive
  loads and dropping BKL on ->release() helps to get rid of that (we still
  grab BKL for cases when we close a file that had been opened r/w, but that
  can and should be done using the internal locking with smaller critical areas).
  Current worst offender is ext2_get_block()...

  ->fasync() is called without BKL protection, and is responsible for
  maintaining the FASYNC bit in filp->f_flags.  Most instances call
  fasync_helper(), which does that maintenance, so it's not normally
  something one needs to worry about.  Return values > 0 will be mapped to
  zero in the VFS layer.

  
  ->readdir() and ->ioctl() on directories must be changed. Ideally we would
  move ->readdir() to inode_operations and use a separate method for directory
  ->ioctl() or kill the latter completely. One of the problems is that for
  anything that resembles union-mount we won't have a struct file for all
  components. And there are other reasons why the current interface is a mess...

  ->read on directories probably must go away - we should just enforce -EISDIR
  in sys_read() and friends.

  ->fsync() has i_mutex on inode.

  
  --------------------------- dquot_operations -------------------------------
  prototypes:

  	int (*write_dquot) (struct dquot *);
  	int (*acquire_dquot) (struct dquot *);
  	int (*release_dquot) (struct dquot *);
  	int (*mark_dirty) (struct dquot *);
  	int (*write_info) (struct super_block *, int);
  
  These operations are intended to be more or less wrapping functions that ensure
  a proper locking wrt the filesystem and call the generic quota operations.
  
  What filesystem should expect from the generic quota functions:
  
  		FS recursion	Held locks when called

  write_dquot:	yes		dqonoff_sem or dqptr_sem
  acquire_dquot:	yes		dqonoff_sem or dqptr_sem
  release_dquot:	yes		dqonoff_sem or dqptr_sem
  mark_dirty:	no		-
  write_info:	yes		dqonoff_sem
  
  FS recursion means calling ->quota_read() and ->quota_write() from superblock
  operations.

  More details about quota locking can be found in fs/dquot.c.
  
  --------------------------- vm_operations_struct -----------------------------
  prototypes:
  	void (*open)(struct vm_area_struct*);
  	void (*close)(struct vm_area_struct*);

  	int (*fault)(struct vm_area_struct*, struct vm_fault *);

  	int (*page_mkwrite)(struct vm_area_struct *, struct vm_fault *);

  	int (*access)(struct vm_area_struct *, unsigned long, void*, int, int);

  
  locking rules:

  		BKL	mmap_sem	PageLocked(page)

  open:		no	yes
  close:		no	yes

  fault:		no	yes		can return with page locked
  page_mkwrite:	no	yes		can return with page locked

  access:		no	yes

  	->fault() is called when a previously not present pte is about
  to be faulted in. The filesystem must find and return the page associated
  with the passed in "pgoff" in the vm_fault structure. If it is possible that
  the page may be truncated and/or invalidated, then the filesystem must lock
  the page, then ensure it is not already truncated (the page lock will block
  subsequent truncate), and then return with VM_FAULT_LOCKED, and the page
  locked. The VM will unlock the page.
  
  	->page_mkwrite() is called when a previously read-only pte is
  about to become writeable. The filesystem again must ensure that there are
  no truncate/invalidate races, and then return with the page locked. If
  the page has been truncated, the filesystem should not look up a new page
  like the ->fault() handler, but simply return with VM_FAULT_NOPAGE, which
  will cause the VM to retry the fault.

  	->access() is called when get_user_pages() fails in
  acces_process_vm(), typically used to debug a process through
  /proc/pid/mem or ptrace.  This function is needed only for
  VM_IO | VM_PFNMAP VMAs.

  ================================================================================
  			Dubious stuff
  
  (if you break something or notice that it is broken and do not fix it yourself
  - at least put it here)
  
  ipc/shm.c::shm_delete() - may need BKL.
  ->read() and ->write() in many drivers are (probably) missing BKL.