On Thu, 17 May 2012 07:06:52 +0400 Glauber Costa [off-list ref] wrote:

...

quoted

quoted

+	else if (val != RESOURCE_MAX) {
+		/*
+		 * ->activated needs to be written after the static_key update.
+		 *  This is what guarantees that the socket activation function
+		 *  is the last one to run. See sock_update_memcg() for details,
+		 *  and note that we don't mark any socket as belonging to this
+		 *  memcg until that flag is up.
+		 *
+		 *  We need to do this, because static_keys will span multiple
+		 *  sites, but we can't control their order. If we mark a socket
+		 *  as accounted, but the accounting functions are not patched in
+		 *  yet, we'll lose accounting.
+		 *
+		 *  We never race with the readers in sock_update_memcg(), because
+		 *  when this value change, the code to process it is not patched in
+		 *  yet.
+		 */
+		if (!cg_proto->activated) {
+			static_key_slow_inc(&memcg_socket_limit_enabled);
+			cg_proto->activated = true;
+		}

If two threads run this code concurrently, they can both see
cg_proto->activated==false and they will both run
static_key_slow_inc().

Hopefully there's some locking somewhere which prevents this, but it is
unobvious.  We should comment this, probably at the cg_proto.activated
definition site.  Or we should fix the bug ;)

If that happens, locking in static_key_slow_inc will prevent any damage.
My previous version had explicit code to prevent that, but we were 
pointed out that this is already part of the static_key expectations, so 
that was dropped.

This makes no sense.  If two threads run that code concurrently,
key->enabled gets incremented twice.  Nobody anywhere has a record that
this happened so it cannot be undone.  key->enabled is now in an
unknown state.