Locking scheme used for directory operations is based on two
-kinds of locks - per-inode (->i_sem) and per-filesystem (->s_vfs_rename_sem).
+kinds of locks - per-inode (->i_mutex) and per-filesystem
+(->s_vfs_rename_mutex).
+
+ When taking the i_mutex on multiple non-directory objects, we
+always acquire the locks in order by increasing address. We'll call
+that "inode pointer" order in the following.
For our purposes all operations fall in 5 classes:
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.
+the parent and finds source and target. If target already exists, lock
+it. If source is a non-directory, lock it. If that means we need to
+lock both, lock them in inode pointer order.
5) link creation. Locking rules:
* lock parent
fail with -ENOTEMPTY
* if new parent is equal to or is a descendent of source
fail with -ELOOP
- * if target exists - lock it.
+ * If target exists, lock it. If source is a non-directory, lock
+ it. In case that means we need to lock both source and target,
+ do so in inode pointer order.
* call the method.
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).
+(3) locks on non-directory objects are acquired only after locks on
+ directory objects, and are acquired in inode pointer order.
+ (Proof: all operations but renames take lock on at most one
+ non-directory object, except renames, which take locks on source and
+ target in inode pointer order in the case they are not directories.)
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.
+Thus all processes are blocked on ->i_mutex.
+
+ By (3), any process holding a non-directory lock can only be
+waiting on another non-directory lock with a larger address. Therefore
+the process holding the "largest" such lock can always make progress, and
+non-directory objects are not included in the set of contended locks.
- 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.
+ 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
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
+would again have an infinite set 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
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