1	The text below describes the locking rules for VFS-related methods.
  2It is (believed to be) up-to-date. *Please*, if you change anything in
  3prototypes or locking protocols - update this file. And update the relevant
  4instances in the tree, don't leave that to maintainers of filesystems/devices/
  5etc. At the very least, put the list of dubious cases in the end of this file.
  6Don't turn it into log - maintainers of out-of-the-tree code are supposed to
  7be able to use diff(1).
  8	Thing currently missing here: socket operations. Alexey?
  9
 10--------------------------- dentry_operations --------------------------
 11prototypes:
 12	int (*d_revalidate)(struct dentry *, struct nameidata *);
 13	int (*d_hash)(const struct dentry *, const struct inode *,
 14			struct qstr *);
 15	int (*d_compare)(const struct dentry *, const struct inode *,
 16			const struct dentry *, const struct inode *,
 17			unsigned int, const char *, const struct qstr *);
 18	int (*d_delete)(struct dentry *);
 19	void (*d_release)(struct dentry *);
 20	void (*d_iput)(struct dentry *, struct inode *);
 21	char *(*d_dname)((struct dentry *dentry, char *buffer, int buflen);
 22	struct vfsmount *(*d_automount)(struct path *path);
 23	int (*d_manage)(struct dentry *, bool);
 24
 25locking rules:
 26		rename_lock	->d_lock	may block	rcu-walk
 27d_revalidate:	no		no		yes (ref-walk)	maybe
 28d_hash		no		no		no		maybe
 29d_compare:	yes		no		no		maybe
 30d_delete:	no		yes		no		no
 31d_release:	no		no		yes		no
 32d_prune:        no              yes             no              no
 33d_iput:		no		no		yes		no
 34d_dname:	no		no		no		no
 35d_automount:	no		no		yes		no
 36d_manage:	no		no		yes (ref-walk)	maybe
 37
 38--------------------------- inode_operations --------------------------- 
 39prototypes:
 40	int (*create) (struct inode *,struct dentry *,umode_t, struct nameidata *);
 41	struct dentry * (*lookup) (struct inode *,struct dentry *, struct nameid
 42ata *);
 43	int (*link) (struct dentry *,struct inode *,struct dentry *);
 44	int (*unlink) (struct inode *,struct dentry *);
 45	int (*symlink) (struct inode *,struct dentry *,const char *);
 46	int (*mkdir) (struct inode *,struct dentry *,umode_t);
 47	int (*rmdir) (struct inode *,struct dentry *);
 48	int (*mknod) (struct inode *,struct dentry *,umode_t,dev_t);
 49	int (*rename) (struct inode *, struct dentry *,
 50			struct inode *, struct dentry *);
 51	int (*readlink) (struct dentry *, char __user *,int);
 52	void * (*follow_link) (struct dentry *, struct nameidata *);
 53	void (*put_link) (struct dentry *, struct nameidata *, void *);
 54	void (*truncate) (struct inode *);
 55	int (*permission) (struct inode *, int, unsigned int);
 56	int (*get_acl)(struct inode *, int);
 57	int (*setattr) (struct dentry *, struct iattr *);
 58	int (*getattr) (struct vfsmount *, struct dentry *, struct kstat *);
 59	int (*setxattr) (struct dentry *, const char *,const void *,size_t,int);
 60	ssize_t (*getxattr) (struct dentry *, const char *, void *, size_t);
 61	ssize_t (*listxattr) (struct dentry *, char *, size_t);
 62	int (*removexattr) (struct dentry *, const char *);
 63	int (*fiemap)(struct inode *, struct fiemap_extent_info *, u64 start, u64 len);
 64	void (*update_time)(struct inode *, struct timespec *, int);
 65
 66locking rules:
 67	all may block
 68		i_mutex(inode)
 69lookup:		yes
 70create:		yes
 71link:		yes (both)
 72mknod:		yes
 73symlink:	yes
 74mkdir:		yes
 75unlink:		yes (both)
 76rmdir:		yes (both)	(see below)
 77rename:		yes (all)	(see below)
 78readlink:	no
 79follow_link:	no
 80put_link:	no
 81truncate:	yes		(see below)
 82setattr:	yes
 83permission:	no (may not block if called in rcu-walk mode)
 84get_acl:	no
 85getattr:	no
 86setxattr:	yes
 87getxattr:	no
 88listxattr:	no
 89removexattr:	yes
 90fiemap:		no
 91update_time:	no
 92
 93	Additionally, ->rmdir(), ->unlink() and ->rename() have ->i_mutex on
 94victim.
 95	cross-directory ->rename() has (per-superblock) ->s_vfs_rename_sem.
 96	->truncate() is never called directly - it's a callback, not a
 97method. It's called by vmtruncate() - deprecated library function used by
 98->setattr(). Locking information above applies to that call (i.e. is
 99inherited from ->setattr() - vmtruncate() is used when ATTR_SIZE had been
100passed).
101
102See Documentation/filesystems/directory-locking for more detailed discussion
103of the locking scheme for directory operations.
104
105--------------------------- super_operations ---------------------------
106prototypes:
107	struct inode *(*alloc_inode)(struct super_block *sb);
108	void (*destroy_inode)(struct inode *);
109	void (*dirty_inode) (struct inode *, int flags);
110	int (*write_inode) (struct inode *, struct writeback_control *wbc);
111	int (*drop_inode) (struct inode *);
112	void (*evict_inode) (struct inode *);
113	void (*put_super) (struct super_block *);
114	void (*write_super) (struct super_block *);
115	int (*sync_fs)(struct super_block *sb, int wait);
116	int (*freeze_fs) (struct super_block *);
117	int (*unfreeze_fs) (struct super_block *);
118	int (*statfs) (struct dentry *, struct kstatfs *);
119	int (*remount_fs) (struct super_block *, int *, char *);
120	void (*umount_begin) (struct super_block *);
121	int (*show_options)(struct seq_file *, struct dentry *);
122	ssize_t (*quota_read)(struct super_block *, int, char *, size_t, loff_t);
123	ssize_t (*quota_write)(struct super_block *, int, const char *, size_t, loff_t);
124	int (*bdev_try_to_free_page)(struct super_block*, struct page*, gfp_t);
125
126locking rules:
127	All may block [not true, see below]
128			s_umount
129alloc_inode:
130destroy_inode:
131dirty_inode:
132write_inode:
133drop_inode:				!!!inode->i_lock!!!
134evict_inode:
135put_super:		write
136write_super:		read
137sync_fs:		read
138freeze_fs:		read
139unfreeze_fs:		read
140statfs:			maybe(read)	(see below)
141remount_fs:		write
142umount_begin:		no
143show_options:		no		(namespace_sem)
144quota_read:		no		(see below)
145quota_write:		no		(see below)
146bdev_try_to_free_page:	no		(see below)
147
148->statfs() has s_umount (shared) when called by ustat(2) (native or
149compat), but that's an accident of bad API; s_umount is used to pin
150the superblock down when we only have dev_t given us by userland to
151identify the superblock.  Everything else (statfs(), fstatfs(), etc.)
152doesn't hold it when calling ->statfs() - superblock is pinned down
153by resolving the pathname passed to syscall.
154->quota_read() and ->quota_write() functions are both guaranteed to
155be the only ones operating on the quota file by the quota code (via
156dqio_sem) (unless an admin really wants to screw up something and
157writes to quota files with quotas on). For other details about locking
158see also dquot_operations section.
159->bdev_try_to_free_page is called from the ->releasepage handler of
160the block device inode.  See there for more details.
161
162--------------------------- file_system_type ---------------------------
163prototypes:
164	int (*get_sb) (struct file_system_type *, int,
165		       const char *, void *, struct vfsmount *);
166	struct dentry *(*mount) (struct file_system_type *, int,
167		       const char *, void *);
168	void (*kill_sb) (struct super_block *);
169locking rules:
170		may block
171mount		yes
172kill_sb		yes
173
174->mount() returns ERR_PTR or the root dentry; its superblock should be locked
175on return.
176->kill_sb() takes a write-locked superblock, does all shutdown work on it,
177unlocks and drops the reference.
178
179--------------------------- address_space_operations --------------------------
180prototypes:
181	int (*writepage)(struct page *page, struct writeback_control *wbc);
182	int (*readpage)(struct file *, struct page *);
183	int (*sync_page)(struct page *);
184	int (*writepages)(struct address_space *, struct writeback_control *);
185	int (*set_page_dirty)(struct page *page);
186	int (*readpages)(struct file *filp, struct address_space *mapping,
187			struct list_head *pages, unsigned nr_pages);
188	int (*write_begin)(struct file *, struct address_space *mapping,
189				loff_t pos, unsigned len, unsigned flags,
190				struct page **pagep, void **fsdata);
191	int (*write_end)(struct file *, struct address_space *mapping,
192				loff_t pos, unsigned len, unsigned copied,
193				struct page *page, void *fsdata);
194	sector_t (*bmap)(struct address_space *, sector_t);
195	int (*invalidatepage) (struct page *, unsigned long);
196	int (*releasepage) (struct page *, int);
197	void (*freepage)(struct page *);
198	int (*direct_IO)(int, struct kiocb *, const struct iovec *iov,
199			loff_t offset, unsigned long nr_segs);
200	int (*get_xip_mem)(struct address_space *, pgoff_t, int, void **,
201				unsigned long *);
202	int (*migratepage)(struct address_space *, struct page *, struct page *);
203	int (*launder_page)(struct page *);
204	int (*is_partially_uptodate)(struct page *, read_descriptor_t *, unsigned long);
205	int (*error_remove_page)(struct address_space *, struct page *);
206
207locking rules:
208	All except set_page_dirty and freepage may block
209
210			PageLocked(page)	i_mutex
211writepage:		yes, unlocks (see below)
212readpage:		yes, unlocks
213sync_page:		maybe
214writepages:
215set_page_dirty		no
216readpages:
217write_begin:		locks the page		yes
218write_end:		yes, unlocks		yes
219bmap:
220invalidatepage:		yes
221releasepage:		yes
222freepage:		yes
223direct_IO:
224get_xip_mem:					maybe
225migratepage:		yes (both)
226launder_page:		yes
227is_partially_uptodate:	yes
228error_remove_page:	yes
229
230	->write_begin(), ->write_end(), ->sync_page() and ->readpage()
231may be called from the request handler (/dev/loop).
232
233	->readpage() unlocks the page, either synchronously or via I/O
234completion.
235
236	->readpages() populates the pagecache with the passed pages and starts
237I/O against them.  They come unlocked upon I/O completion.
238
239	->writepage() is used for two purposes: for "memory cleansing" and for
240"sync".  These are quite different operations and the behaviour may differ
241depending upon the mode.
242
243If writepage is called for sync (wbc->sync_mode != WBC_SYNC_NONE) then
244it *must* start I/O against the page, even if that would involve
245blocking on in-progress I/O.
246
247If writepage is called for memory cleansing (sync_mode ==
248WBC_SYNC_NONE) then its role is to get as much writeout underway as
249possible.  So writepage should try to avoid blocking against
250currently-in-progress I/O.
251
252If the filesystem is not called for "sync" and it determines that it
253would need to block against in-progress I/O to be able to start new I/O
254against the page the filesystem should redirty the page with
255redirty_page_for_writepage(), then unlock the page and return zero.
256This may also be done to avoid internal deadlocks, but rarely.
257
258If the filesystem is called for sync then it must wait on any
259in-progress I/O and then start new I/O.
260
261The filesystem should unlock the page synchronously, before returning to the
262caller, unless ->writepage() returns special WRITEPAGE_ACTIVATE
263value. WRITEPAGE_ACTIVATE means that page cannot really be written out
264currently, and VM should stop calling ->writepage() on this page for some
265time. VM does this by moving page to the head of the active list, hence the
266name.
267
268Unless the filesystem is going to redirty_page_for_writepage(), unlock the page
269and return zero, writepage *must* run set_page_writeback() against the page,
270followed by unlocking it.  Once set_page_writeback() has been run against the
271page, write I/O can be submitted and the write I/O completion handler must run
272end_page_writeback() once the I/O is complete.  If no I/O is submitted, the
273filesystem must run end_page_writeback() against the page before returning from
274writepage.
275
276That is: after 2.5.12, pages which are under writeout are *not* locked.  Note,
277if the filesystem needs the page to be locked during writeout, that is ok, too,
278the page is allowed to be unlocked at any point in time between the calls to
279set_page_writeback() and end_page_writeback().
280
281Note, failure to run either redirty_page_for_writepage() or the combination of
282set_page_writeback()/end_page_writeback() on a page submitted to writepage
283will leave the page itself marked clean but it will be tagged as dirty in the
284radix tree.  This incoherency can lead to all sorts of hard-to-debug problems
285in the filesystem like having dirty inodes at umount and losing written data.
286
287	->sync_page() locking rules are not well-defined - usually it is called
288with lock on page, but that is not guaranteed. Considering the currently
289existing instances of this method ->sync_page() itself doesn't look
290well-defined...
291
292	->writepages() is used for periodic writeback and for syscall-initiated
293sync operations.  The address_space should start I/O against at least
294*nr_to_write pages.  *nr_to_write must be decremented for each page which is
295written.  The address_space implementation may write more (or less) pages
296than *nr_to_write asks for, but it should try to be reasonably close.  If
297nr_to_write is NULL, all dirty pages must be written.
298
299writepages should _only_ write pages which are present on
300mapping->io_pages.
301
302	->set_page_dirty() is called from various places in the kernel
303when the target page is marked as needing writeback.  It may be called
304under spinlock (it cannot block) and is sometimes called with the page
305not locked.
306
307	->bmap() is currently used by legacy ioctl() (FIBMAP) provided by some
308filesystems and by the swapper. The latter will eventually go away.  Please,
309keep it that way and don't breed new callers.
310
311	->invalidatepage() is called when the filesystem must attempt to drop
312some or all of the buffers from the page when it is being truncated.  It
313returns zero on success.  If ->invalidatepage is zero, the kernel uses
314block_invalidatepage() instead.
315
316	->releasepage() is called when the kernel is about to try to drop the
317buffers from the page in preparation for freeing it.  It returns zero to
318indicate that the buffers are (or may be) freeable.  If ->releasepage is zero,
319the kernel assumes that the fs has no private interest in the buffers.
320
321	->freepage() is called when the kernel is done dropping the page
322from the page cache.
323
324	->launder_page() may be called prior to releasing a page if
325it is still found to be dirty. It returns zero if the page was successfully
326cleaned, or an error value if not. Note that in order to prevent the page
327getting mapped back in and redirtied, it needs to be kept locked
328across the entire operation.
329
330----------------------- file_lock_operations ------------------------------
331prototypes:
332	void (*fl_copy_lock)(struct file_lock *, struct file_lock *);
333	void (*fl_release_private)(struct file_lock *);
334
335
336locking rules:
337			file_lock_lock	may block
338fl_copy_lock:		yes		no
339fl_release_private:	maybe		no
340
341----------------------- lock_manager_operations ---------------------------
342prototypes:
343	int (*lm_compare_owner)(struct file_lock *, struct file_lock *);
344	void (*lm_notify)(struct file_lock *);  /* unblock callback */
345	int (*lm_grant)(struct file_lock *, struct file_lock *, int);
346	void (*lm_release_private)(struct file_lock *);
347	void (*lm_break)(struct file_lock *); /* break_lease callback */
348	int (*lm_change)(struct file_lock **, int);
349
350locking rules:
351			file_lock_lock	may block
352lm_compare_owner:	yes		no
353lm_notify:		yes		no
354lm_grant:		no		no
355lm_release_private:	maybe		no
356lm_break:		yes		no
357lm_change		yes		no
358
359--------------------------- buffer_head -----------------------------------
360prototypes:
361	void (*b_end_io)(struct buffer_head *bh, int uptodate);
362
363locking rules:
364	called from interrupts. In other words, extreme care is needed here.
365bh is locked, but that's all warranties we have here. Currently only RAID1,
366highmem, fs/buffer.c, and fs/ntfs/aops.c are providing these. Block devices
367call this method upon the IO completion.
368
369--------------------------- block_device_operations -----------------------
370prototypes:
371	int (*open) (struct block_device *, fmode_t);
372	int (*release) (struct gendisk *, fmode_t);
373	int (*ioctl) (struct block_device *, fmode_t, unsigned, unsigned long);
374	int (*compat_ioctl) (struct block_device *, fmode_t, unsigned, unsigned long);
375	int (*direct_access) (struct block_device *, sector_t, void **, unsigned long *);
376	int (*media_changed) (struct gendisk *);
377	void (*unlock_native_capacity) (struct gendisk *);
378	int (*revalidate_disk) (struct gendisk *);
379	int (*getgeo)(struct block_device *, struct hd_geometry *);
380	void (*swap_slot_free_notify) (struct block_device *, unsigned long);
381
382locking rules:
383			bd_mutex
384open:			yes
385release:		yes
386ioctl:			no
387compat_ioctl:		no
388direct_access:		no
389media_changed:		no
390unlock_native_capacity:	no
391revalidate_disk:	no
392getgeo:			no
393swap_slot_free_notify:	no	(see below)
394
395media_changed, unlock_native_capacity and revalidate_disk are called only from
396check_disk_change().
397
398swap_slot_free_notify is called with swap_lock and sometimes the page lock
399held.
400
401
402--------------------------- file_operations -------------------------------
403prototypes:
404	loff_t (*llseek) (struct file *, loff_t, int);
405	ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
406	ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
407	ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
408	ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
409	int (*readdir) (struct file *, void *, filldir_t);
410	unsigned int (*poll) (struct file *, struct poll_table_struct *);
411	long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
412	long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
413	int (*mmap) (struct file *, struct vm_area_struct *);
414	int (*open) (struct inode *, struct file *);
415	int (*flush) (struct file *);
416	int (*release) (struct inode *, struct file *);
417	int (*fsync) (struct file *, loff_t start, loff_t end, int datasync);
418	int (*aio_fsync) (struct kiocb *, int datasync);
419	int (*fasync) (int, struct file *, int);
420	int (*lock) (struct file *, int, struct file_lock *);
421	ssize_t (*readv) (struct file *, const struct iovec *, unsigned long,
422			loff_t *);
423	ssize_t (*writev) (struct file *, const struct iovec *, unsigned long,
424			loff_t *);
425	ssize_t (*sendfile) (struct file *, loff_t *, size_t, read_actor_t,
426			void __user *);
427	ssize_t (*sendpage) (struct file *, struct page *, int, size_t,
428			loff_t *, int);
429	unsigned long (*get_unmapped_area)(struct file *, unsigned long,
430			unsigned long, unsigned long, unsigned long);
431	int (*check_flags)(int);
432	int (*flock) (struct file *, int, struct file_lock *);
433	ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *,
434			size_t, unsigned int);
435	ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *,
436			size_t, unsigned int);
437	int (*setlease)(struct file *, long, struct file_lock **);
438	long (*fallocate)(struct file *, int, loff_t, loff_t);
439};
440
441locking rules:
442	All may block except for ->setlease.
443	No VFS locks held on entry except for ->setlease.
444
445->setlease has the file_list_lock held and must not sleep.
446
447->llseek() locking has moved from llseek to the individual llseek
448implementations.  If your fs is not using generic_file_llseek, you
449need to acquire and release the appropriate locks in your ->llseek().
450For many filesystems, it is probably safe to acquire the inode
451mutex or just to use i_size_read() instead.
452Note: this does not protect the file->f_pos against concurrent modifications
453since this is something the userspace has to take care about.
454
455->fasync() is responsible for maintaining the FASYNC bit in filp->f_flags.
456Most instances call fasync_helper(), which does that maintenance, so it's
457not normally something one needs to worry about.  Return values > 0 will be
458mapped to zero in the VFS layer.
459
460->readdir() and ->ioctl() on directories must be changed. Ideally we would
461move ->readdir() to inode_operations and use a separate method for directory
462->ioctl() or kill the latter completely. One of the problems is that for
463anything that resembles union-mount we won't have a struct file for all
464components. And there are other reasons why the current interface is a mess...
465
466->read on directories probably must go away - we should just enforce -EISDIR
467in sys_read() and friends.
468
469--------------------------- dquot_operations -------------------------------
470prototypes:
471	int (*write_dquot) (struct dquot *);
472	int (*acquire_dquot) (struct dquot *);
473	int (*release_dquot) (struct dquot *);
474	int (*mark_dirty) (struct dquot *);
475	int (*write_info) (struct super_block *, int);
476
477These operations are intended to be more or less wrapping functions that ensure
478a proper locking wrt the filesystem and call the generic quota operations.
479
480What filesystem should expect from the generic quota functions:
481
482		FS recursion	Held locks when called
483write_dquot:	yes		dqonoff_sem or dqptr_sem
484acquire_dquot:	yes		dqonoff_sem or dqptr_sem
485release_dquot:	yes		dqonoff_sem or dqptr_sem
486mark_dirty:	no		-
487write_info:	yes		dqonoff_sem
488
489FS recursion means calling ->quota_read() and ->quota_write() from superblock
490operations.
491
492More details about quota locking can be found in fs/dquot.c.
493
494--------------------------- vm_operations_struct -----------------------------
495prototypes:
496	void (*open)(struct vm_area_struct*);
497	void (*close)(struct vm_area_struct*);
498	int (*fault)(struct vm_area_struct*, struct vm_fault *);
499	int (*page_mkwrite)(struct vm_area_struct *, struct vm_fault *);
500	int (*access)(struct vm_area_struct *, unsigned long, void*, int, int);
501
502locking rules:
503		mmap_sem	PageLocked(page)
504open:		yes
505close:		yes
506fault:		yes		can return with page locked
507page_mkwrite:	yes		can return with page locked
508access:		yes
509
510	->fault() is called when a previously not present pte is about
511to be faulted in. The filesystem must find and return the page associated
512with the passed in "pgoff" in the vm_fault structure. If it is possible that
513the page may be truncated and/or invalidated, then the filesystem must lock
514the page, then ensure it is not already truncated (the page lock will block
515subsequent truncate), and then return with VM_FAULT_LOCKED, and the page
516locked. The VM will unlock the page.
517
518	->page_mkwrite() is called when a previously read-only pte is
519about to become writeable. The filesystem again must ensure that there are
520no truncate/invalidate races, and then return with the page locked. If
521the page has been truncated, the filesystem should not look up a new page
522like the ->fault() handler, but simply return with VM_FAULT_NOPAGE, which
523will cause the VM to retry the fault.
524
525	->access() is called when get_user_pages() fails in
526acces_process_vm(), typically used to debug a process through
527/proc/pid/mem or ptrace.  This function is needed only for
528VM_IO | VM_PFNMAP VMAs.
529
530================================================================================
531			Dubious stuff
532
533(if you break something or notice that it is broken and do not fix it yourself
534- at least put it here)