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+	Locking scheme used for directory operations is based on two
+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:
+
+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 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
+	* 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.  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.
+
+
+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) 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_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.
+
+	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 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.