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 may block
22 d_revalidate: no yes
23 d_hash no yes
24 d_compare: yes no
25 d_delete: yes no
26 d_release: no yes
27 d_iput: no yes
28
29 --------------------------- inode_operations ---------------------------
30 prototypes:
31 int (*create) (struct inode *,struct dentry *,int);
32 struct dentry * (*lookup) (struct inode *,struct dentry *);
33 int (*link) (struct dentry *,struct inode *,struct dentry *);
34 int (*unlink) (struct inode *,struct dentry *);
35 int (*symlink) (struct inode *,struct dentry *,const char *);
36 int (*mkdir) (struct inode *,struct dentry *,int);
37 int (*rmdir) (struct inode *,struct dentry *);
38 int (*mknod) (struct inode *,struct dentry *,int,int);
39 int (*rename) (struct inode *, struct dentry *,
40 struct inode *, struct dentry *);
41 int (*readlink) (struct dentry *, char *,int);
42 int (*follow_link) (struct dentry *, struct nameidata *);
43 void (*truncate) (struct inode *);
44 int (*permission) (struct inode *, int);
45 int (*revalidate) (struct dentry *);
46 int (*setattr) (struct dentry *, struct iattr *);
47 int (*getattr) (struct dentry *, struct iattr *);
48
49 locking rules:
50 all may block
51 BKL i_sem(inode) i_zombie(inode)
52 lookup: yes yes no
53 create: yes yes yes
54 link: yes yes yes
55 mknod: yes yes yes
56 mkdir: yes yes yes
57 unlink: yes yes yes
58 rmdir: yes yes yes (see below)
59 rename: yes yes (both) yes (both) (see below)
60 readlink: no no no
61 follow_link: no no no
62 truncate: yes yes no (see below)
63 setattr: yes if ATTR_SIZE no
64 permssion: yes no no
65 getattr: (see below)
66 revalidate: no (see below)
67 Additionally, ->rmdir() has i_zombie on victim and so does ->rename()
68 in case when target exists and is a directory.
69 ->rename() on directories has (per-superblock) ->s_vfs_rename_sem.
70 ->revalidate(), it may be called both with and without the i_sem
71 on dentry->d_inode. VFS never calls it with i_zombie on dentry->d_inode,
72 but watch for other methods directly calling this one...
73 ->truncate() is never called directly - it's a callback, not a
74 method. It's called by vmtruncate() - library function normally used by
75 ->setattr(). Locking information above applies to that call (i.e. is
76 inherited from ->setattr() - vmtruncate() is used when ATTR_SIZE had been
77 passed).
78 ->getattr() is currently unused.
79
80 --------------------------- super_operations ---------------------------
81 prototypes:
82 void (*read_inode) (struct inode *);
83 void (*write_inode) (struct inode *, int);
84 void (*put_inode) (struct inode *);
85 void (*delete_inode) (struct inode *);
86 void (*put_super) (struct super_block *);
87 void (*write_super) (struct super_block *);
88 int (*statfs) (struct super_block *, struct statfs *);
89 int (*remount_fs) (struct super_block *, int *, char *);
90 void (*clear_inode) (struct inode *);
91 void (*umount_begin) (struct super_block *);
92
93 locking rules:
94 All may block.
95 BKL s_lock mount_sem
96 read_inode: yes (see below)
97 write_inode: no
98 put_inode: no
99 delete_inode: no
100 clear_inode: no
101 put_super: yes yes maybe (see below)
102 write_super: yes yes maybe (see below)
103 statfs: yes no no
104 remount_fs: yes yes maybe (see below)
105 umount_begin: yes no maybe (see below)
106
107 ->read_inode() is not a method - it's a callback used in iget()/iget4().
108 rules for mount_sem are not too nice - it is going to die and be replaced
109 by better scheme anyway.
110
111 --------------------------- file_system_type ---------------------------
112 prototypes:
113 struct super_block *(*read_super) (struct super_block *, void *, int);
114 locking rules:
115 may block BKL ->s_lock mount_sem
116 yes yes yes maybe
117
118 --------------------------- address_space_operations --------------------------
119 prototypes:
120 int (*writepage)(struct file *, struct page *);
121 int (*readpage)(struct file *, struct page *);
122 int (*sync_page)(struct page *);
123 int (*prepare_write)(struct file *, struct page *, unsigned, unsigned);
124 int (*commit_write)(struct file *, struct page *, unsigned, unsigned);
125 int (*bmap)(struct address_space *, long);
126 locking rules:
127 All may block
128 BKL PageLocked(page)
129 writepage: no yes
130 readpage: no yes
131 sync_page: no maybe
132 prepare_write: no yes
133 commit_write: no yes
134 bmap: yes
135
136 ->prepare_write(), ->commit_write(), ->sync_page() and ->readpage()
137 may be called from the request handler (/dev/loop).
138 ->sync_page() locking rules are not well-defined - usually it is called
139 with lock on page, but that is not guaranteed. Considering the currently
140 existing instances of this method ->sync_page() itself doesn't look
141 well-defined...
142 ->bmap() is currently used by legacy ioctl() (FIBMAP) provided by some
143 filesystems and by the swapper. The latter will eventually go away. All
144 instances do not actually need the BKL. Please, keep it that way and don't
145 breed new callers.
146 Note: currently almost all instances of address_space methods are
147 using BKL for internal serialization and that's one of the worst sources
148 of contention. Normally they are calling library functions (in fs/buffer.c)
149 and pass foo_get_block() as a callback (on local block-based filesystems,
150 indeed). BKL is not needed for library stuff and is usually taken by
151 foo_get_block(). It's an overkill, since block bitmaps can be protected by
152 internal fs locking and real critical areas are much smaller than the areas
153 filesystems protect now.
154
155 --------------------------- file_lock ------------------------------------
156 prototypes:
157 void (*fl_notify)(struct file_lock *); /* unblock callback */
158 void (*fl_insert)(struct file_lock *); /* lock insertion callback */
159 void (*fl_remove)(struct file_lock *); /* lock removal callback */
160
161 locking rules:
162 BKL may block
163 fl_notify: yes no
164 fl_insert: yes maybe
165 fl_remove: yes maybe
166 Currently only NLM provides instances of this class. None of the
167 them block. If you have out-of-tree instances - please, show up. Locking
168 in that area will change.
169
170 --------------------------- buffer_head -----------------------------------
171 prototypes:
172 void (*b_end_io)(struct buffer_head *bh, int uptodate);
173
174 locking rules:
175 called from interrupts. In other words, extreme care is needed here.
176 bh is locked, but that's all warranties we have here. Currently only RAID1,
177 highmem and fs/buffer.c are providing these. Block devices call this method
178 upon the IO completion.
179
180 --------------------------- block_device_operations -----------------------
181 prototypes:
182 int (*open) (struct inode *, struct file *);
183 int (*release) (struct inode *, struct file *);
184 int (*ioctl) (struct inode *, struct file *, unsigned, unsigned long);
185 int (*check_media_change) (kdev_t);
186 int (*revalidate) (kdev_t);
187 locking rules:
188 BKL bd_sem
189 open: yes yes
190 release: yes yes
191 ioctl: yes no
192 check_media_change: yes no
193 revalidate: yes no
194
195 The last two are called only from check_disk_change(). Prototypes are very
196 bad - as soon as we'll get disk_struct they will change (and methods will
197 become per-disk instead of per-partition).
198
199 --------------------------- file_operations -------------------------------
200 prototypes:
201 loff_t (*llseek) (struct file *, loff_t, int);
202 ssize_t (*read) (struct file *, char *, size_t, loff_t *);
203 ssize_t (*write) (struct file *, const char *, size_t, loff_t *);
204 int (*readdir) (struct file *, void *, filldir_t);
205 unsigned int (*poll) (struct file *, struct poll_table_struct *);
206 int (*ioctl) (struct inode *, struct file *, unsigned int, unsigned long);
207 int (*mmap) (struct file *, struct vm_area_struct *);
208 int (*open) (struct inode *, struct file *);
209 int (*flush) (struct file *);
210 int (*release) (struct inode *, struct file *);
211 int (*fsync) (struct file *, struct dentry *, int datasync);
212 int (*fasync) (int, struct file *, int);
213 int (*lock) (struct file *, int, struct file_lock *);
214 ssize_t (*readv) (struct file *, const struct iovec *, unsigned long, loff_t *);
215 ssize_t (*writev) (struct file *, const struct iovec *, unsigned long, loff_t *);
216 };
217
218 locking rules:
219 All except ->poll() may block.
220 BKL
221 llseek: yes
222 read: no
223 write: no
224 readdir: yes (see below)
225 poll: no
226 ioctl: yes (see below)
227 mmap: no
228 open: maybe (see below)
229 flush: yes
230 release: no
231 fsync: yes (see below)
232 fasync: yes (see below)
233 lock: yes
234 readv: no
235 writev: no
236
237 ->open() locking is in-transit: big lock partially moved into the methods.
238 The only exception is ->open() in the instances of file_operations that never
239 end up in ->i_fop/->proc_fops, i.e. ones that belong to character devices
240 (chrdev_open() takes lock before replacing ->f_op and calling the secondary
241 method. As soon as we fix the handling of module reference counters all
242 instances of ->open() will be called without the BKL.
243
244 Note: ext2_release() was *the* source of contention on fs-intensive
245 loads and dropping BKL on ->release() helps to get rid of that (we still
246 grab BKL for cases when we close a file that had been opened r/w, but that
247 can and should be done using the internal locking with smaller critical areas).
248 Current worst offender is ext2_get_block()...
249
250 ->fasync() is a mess. This area needs a big cleanup and that will probably
251 affect locking.
252
253 ->readdir() and ->ioctl() on directories must be changed. Ideally we would
254 move ->readdir() to inode_operations and use a separate method for directory
255 ->ioctl() or kill the latter completely. One of the problems is that for
256 anything that resembles union-mount we won't have a struct file for all
257 components. And there are other reasons why the current interface is a mess...
258
259 ->read on directories probably must go away - we should just enforce -EISDIR
260 in sys_read() and friends.
261
262 ->fsync() has i_sem on inode.
263
264 --------------------------- dquot_operations -------------------------------
265 prototypes:
266 void (*initialize) (struct inode *, short);
267 void (*drop) (struct inode *);
268 int (*alloc_block) (const struct inode *, unsigned long, char);
269 int (*alloc_inode) (const struct inode *, unsigned long);
270 void (*free_block) (const struct inode *, unsigned long);
271 void (*free_inode) (const struct inode *, unsigned long);
272 int (*transfer) (struct dentry *, struct iattr *);
273
274 locking rules:
275 BKL
276 initialize: no
277 drop: no
278 alloc_block: yes
279 alloc_inode: yes
280 free_block: yes
281 free_inode: yes
282 transfer: no
283
284 --------------------------- vm_operations_struct -----------------------------
285 prototypes:
286 void (*open)(struct vm_area_struct*);
287 void (*close)(struct vm_area_struct*);
288 void (*unmap)(struct vm_area_struct*, unsigned long, size_t);
289 void (*protect)(struct vm_area_struct*, unsigned long, size_t, unsigned);
290 int (*sync)(struct vm_area_struct*, unsigned long, size_t, unsigned);
291 struct page *(*nopage)(struct vm_area_struct*, unsigned long, int);
292 struct page *(*wppage)(struct vm_area_struct*, unsigned long, struct page*);
293 int (*swapout)(struct page *, struct file *);
294
295 locking rules:
296 BKL mmap_sem
297 open: no yes
298 close: no yes
299 sync: no yes
300 unmap: no yes
301 nopage: no yes
302 swapout: yes yes
303 wpppage: (see below)
304 protect: (see below)
305
306 ->wppage() and ->protect() have no instances and nothing calls them; looks like
307 they must die...
308
309 ================================================================================
310 Dubious stuff
311
312 (if you break something or notice that it is broken and do not fix it yourself
313 - at least put it here)
314
315 ipc/shm.c::shm_delete() - may need BKL.
316 ->read() and ->write() in many drivers are (probably) missing BKL.
317 drivers/sgi/char/graphics.c::sgi_graphics_nopage() - may need BKL.
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