dm thin: fix a race in thin_dtr

As long as struct thin_c is in the list, anyone can grab a reference of
it.  Consequently, we must wait for the reference count to drop to zero
*after* we remove the structure from the list, not before.
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-thin.c

@@ -3622,14 +3622,14 @@ static void thin_dtr(struct dm_target *ti)
 	struct thin_c *tc = ti->private;
 	unsigned long flags;
 
-	thin_put(tc);
-	wait_for_completion(&tc->can_destroy);
-
 	spin_lock_irqsave(&tc->pool->lock, flags);
 	list_del_rcu(&tc->list);
 	spin_unlock_irqrestore(&tc->pool->lock, flags);
 	synchronize_rcu();
 
+	thin_put(tc);
+	wait_for_completion(&tc->can_destroy);
+
 	mutex_lock(&dm_thin_pool_table.mutex);
 
 	__pool_dec(tc->pool);