block: fix drain_all condition in blk_drain_queue()

When trying to drain all requests, blk_drain_queue() checked only
q->rq.count[]; however, this only tracks REQ_ALLOCED requests.  This
patch updates blk_drain_queue() such that it looks at all the counters
and queues so that request_queue is actually empty on completion.
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-core.c

@@ -358,7 +358,8 @@ EXPORT_SYMBOL(blk_put_queue);
 void blk_drain_queue(struct request_queue *q, bool drain_all)
 {
 	while (true) {
-		int nr_rqs;
+		bool drain = false;
+		int i;
 
 		spin_lock_irq(q->queue_lock);
 
@@ -368,14 +369,25 @@ void blk_drain_queue(struct request_queue *q, bool drain_all)
 
 		__blk_run_queue(q);
 
-		if (drain_all)
-			nr_rqs = q->rq.count[0] + q->rq.count[1];
-		else
-			nr_rqs = q->rq.elvpriv;
+		drain |= q->rq.elvpriv;
+
+		/*
+		 * Unfortunately, requests are queued at and tracked from
+		 * multiple places and there's no single counter which can
+		 * be drained.  Check all the queues and counters.
+		 */
+		if (drain_all) {
+			drain |= !list_empty(&q->queue_head);
+			for (i = 0; i < 2; i++) {
+				drain |= q->rq.count[i];
+				drain |= q->in_flight[i];
+				drain |= !list_empty(&q->flush_queue[i]);
+			}
+		}
 
 		spin_unlock_irq(q->queue_lock);
 
-		if (!nr_rqs)
+		if (!drain)
 			break;
 		msleep(10);
 	}