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update D/f/directory-locking

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Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Al Viro 2016-05-26 00:04:18 -04:00
parent 0985b65d3b
commit d42b386834
1 changed files with 20 additions and 12 deletions
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32
Documentation/filesystems/directory-locking
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@ -1,30 +1,37 @@
Locking scheme used for directory operations is based on two Locking scheme used for directory operations is based on two
kinds of locks - per-inode (->i_mutex) and per-filesystem kinds of locks - per-inode (->i_rwsem) and per-filesystem
(->s_vfs_rename_mutex). (->s_vfs_rename_mutex).
When taking the i_mutex on multiple non-directory objects, we When taking the i_rwsem on multiple non-directory objects, we
always acquire the locks in order by increasing address. We'll call always acquire the locks in order by increasing address. We'll call
that "inode pointer" order in the following. that "inode pointer" order in the following.
For our purposes all operations fall in 5 classes: For our purposes all operations fall in 5 classes:
1) read access. Locking rules: caller locks directory we are accessing. 1) read access. Locking rules: caller locks directory we are accessing.
The lock is taken shared.
2) object creation. Locking rules: same as above. 2) object creation. Locking rules: same as above, but the lock is taken
exclusive.
3) object removal. Locking rules: caller locks parent, finds victim, 3) object removal. Locking rules: caller locks parent, finds victim,
locks victim and calls the method. locks victim and calls the method. Locks are exclusive.
4) rename() that is _not_ cross-directory. Locking rules: caller locks 4) rename() that is _not_ cross-directory. Locking rules: caller locks
the parent and finds source and target. If target already exists, lock the parent and finds source and target. In case of exchange (with
it. If source is a non-directory, lock it. If that means we need to RENAME_EXCHANGE in rename2() flags argument) lock both. In any case,
lock both, lock them in inode pointer order. if the target already exists, lock it. If the source is a non-directory,
lock it. If we need to lock both, lock them in inode pointer order.
Then call the method. All locks are exclusive.
NB: we might get away with locking the the source (and target in exchange
case) shared.
5) link creation. Locking rules: 5) link creation. Locking rules:
* lock parent * lock parent
* check that source is not a directory * check that source is not a directory
* lock source * lock source
* call the method. * call the method.
All locks are exclusive.
6) cross-directory rename. The trickiest in the whole bunch. Locking 6) cross-directory rename. The trickiest in the whole bunch. Locking
rules: rules:
@ -35,11 +42,12 @@ rules:
fail with -ENOTEMPTY fail with -ENOTEMPTY
* if new parent is equal to or is a descendent of source * if new parent is equal to or is a descendent of source
fail with -ELOOP fail with -ELOOP
* If target exists, lock it. If source is a non-directory, lock * If it's an exchange, lock both the source and the target.
it. In case that means we need to lock both source and target, * If the target exists, lock it. If the source is a non-directory,
do so in inode pointer order. lock it. If we need to lock both, do so in inode pointer order.
* call the method. * call the method.
All ->i_rwsem are taken exclusive. Again, we might get away with locking
the the source (and target in exchange case) shared.
The rules above obviously guarantee that all directories that are going to be The rules above obviously guarantee that all directories that are going to be
read, modified or removed by method will be locked by caller. read, modified or removed by method will be locked by caller.
@ -73,7 +81,7 @@ objects - A < B iff A is an ancestor of B.
attempt to acquire some lock and already holds at least one lock. Let's attempt to acquire some lock and already holds at least one lock. Let's
consider the set of contended locks. First of all, filesystem lock is consider the set of contended locks. First of all, filesystem lock is
not contended, since any process blocked on it is not holding any locks. not contended, since any process blocked on it is not holding any locks.
Thus all processes are blocked on ->i_mutex. Thus all processes are blocked on ->i_rwsem.
By (3), any process holding a non-directory lock can only be By (3), any process holding a non-directory lock can only be
waiting on another non-directory lock with a larger address. Therefore waiting on another non-directory lock with a larger address. Therefore