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Diffstat (limited to 'Documentation/filesystems/directory-locking')
| -rw-r--r-- | Documentation/filesystems/directory-locking | 113 |
1 files changed, 113 insertions, 0 deletions
diff --git a/Documentation/filesystems/directory-locking b/Documentation/filesystems/directory-locking new file mode 100644 index 00000000000..34380d4fbce --- /dev/null +++ b/Documentation/filesystems/directory-locking @@ -0,0 +1,113 @@ + Locking scheme used for directory operations is based on two +kinds of locks - per-inode (->i_sem) and per-filesystem (->s_vfs_rename_sem). + + For our purposes all operations fall in 5 classes: + +1) read access. Locking rules: caller locks directory we are accessing. + +2) object creation. Locking rules: same as above. + +3) object removal. Locking rules: caller locks parent, finds victim, +locks victim and calls the method. + +4) rename() that is _not_ cross-directory. Locking rules: caller locks +the parent, finds source and target, if target already exists - locks it +and then calls the method. + +5) link creation. Locking rules: + * lock parent + * check that source is not a directory + * lock source + * call the method. + +6) cross-directory rename. The trickiest in the whole bunch. Locking +rules: + * lock the filesystem + * lock parents in "ancestors first" order. + * find source and target. + * if old parent is equal to or is a descendent of target + fail with -ENOTEMPTY + * if new parent is equal to or is a descendent of source + fail with -ELOOP + * if target exists - lock it. + * call the method. + + +The rules above obviously guarantee that all directories that are going to be +read, modified or removed by method will be locked by caller. + + +If no directory is its own ancestor, the scheme above is deadlock-free. +Proof: + + First of all, at any moment we have a partial ordering of the +objects - A < B iff A is an ancestor of B. + + That ordering can change. However, the following is true: + +(1) if object removal or non-cross-directory rename holds lock on A and + attempts to acquire lock on B, A will remain the parent of B until we + acquire the lock on B. (Proof: only cross-directory rename can change + the parent of object and it would have to lock the parent). + +(2) if cross-directory rename holds the lock on filesystem, order will not + change until rename acquires all locks. (Proof: other cross-directory + renames will be blocked on filesystem lock and we don't start changing + the order until we had acquired all locks). + +(3) any operation holds at most one lock on non-directory object and + that lock is acquired after all other locks. (Proof: see descriptions + of operations). + + Now consider the minimal deadlock. Each process is blocked on +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 +not contended, since any process blocked on it is not holding any locks. +Thus all processes are blocked on ->i_sem. + + Non-directory objects are not contended due to (3). Thus link +creation can't be a part of deadlock - it can't be blocked on source +and it means that it doesn't hold any locks. + + Any contended object is either held by cross-directory rename or +has a child that is also contended. Indeed, suppose that it is held by +operation other than cross-directory rename. Then the lock this operation +is blocked on belongs to child of that object due to (1). + + It means that one of the operations is cross-directory rename. +Otherwise the set of contended objects would be infinite - each of them +would have a contended child and we had assumed that no object is its +own descendent. Moreover, there is exactly one cross-directory rename +(see above). + + Consider the object blocking the cross-directory rename. One +of its descendents is locked by cross-directory rename (otherwise we +would again have an infinite set of of contended objects). But that +means that cross-directory rename is taking locks out of order. Due +to (2) the order hadn't changed since we had acquired filesystem lock. +But locking rules for cross-directory rename guarantee that we do not +try to acquire lock on descendent before the lock on ancestor. +Contradiction. I.e. deadlock is impossible. Q.E.D. + + + These operations are guaranteed to avoid loop creation. Indeed, +the only operation that could introduce loops is cross-directory rename. +Since the only new (parent, child) pair added by rename() is (new parent, +source), such loop would have to contain these objects and the rest of it +would have to exist before rename(). I.e. at the moment of loop creation +rename() responsible for that would be holding filesystem lock and new parent +would have to be equal to or a descendent of source. But that means that +new parent had been equal to or a descendent of source since the moment when +we had acquired filesystem lock and rename() would fail with -ELOOP in that +case. + + While this locking scheme works for arbitrary DAGs, it relies on +ability to check that directory is a descendent of another object. Current +implementation assumes that directory graph is a tree. This assumption is +also preserved by all operations (cross-directory rename on a tree that would +not introduce a cycle will leave it a tree and link() fails for directories). + + Notice that "directory" in the above == "anything that might have +children", so if we are going to introduce hybrid objects we will need +either to make sure that link(2) doesn't work for them or to make changes +in is_subdir() that would make it work even in presence of such beasts. |