On Wed, 2017-03-08 at 17:55 -0500, Tejun Heo wrote:

Hello,

On Wed, Mar 08, 2017 at 05:48:31PM +0100, Jan Kara wrote:

quoted

@@ -710,6 +710,11 @@ static void cgwb_bdi_destroy(struct

backing_dev_info *bdi)
 	 */
 	atomic_dec(&bdi->usage_cnt);
 	wait_event(cgwb_release_wait, !atomic_read(&bdi
->usage_cnt));
+	/*
+	 * Grab back our reference so that we hold it when @bdi
gets
+	 * re-registered.
+	 */
+	atomic_inc(&bdi->usage_cnt);

So, this is more re-initializing the ref to the initial state so that
it can be re-used, right?  Maybe ATOMIC_INIT() is a better choice 
here just to clarify what's going on?

Seconded.  Eventually this is going to get converted to a refcount_t
and it will dump a spurious warning on the 0->1 transition.  We can
avoid that by making this a proper initialization.

James