block: defer flush request no matter whether we have elevator

Commit 7520872c0c ("block: don't defer flushes on blk-mq + scheduling")
tried to fix deadlock for cycled wait between flush requests and data
request into flush_data_in_flight. The former holded all driver tags
and wait for data request completion, but the latter can not complete
for waiting free driver tags.

After commit 923218f616 ("blk-mq: don't allocate driver tag upfront
for flush rq"), flush requests will not get driver tag before queuing
into flush queue.

* With elevator, flush request just get sched_tags before inserting
  flush queue. It will not get driver tag until issue them to driver.
  data request on list fq->flush_data_in_flight will complete in
  the end.

* Without elevator, each flush request will get a driver tag when
  allocate request. Then data request on fq->flush_data_in_flight
  don't worry about lacking driver tag.

In both of these cases, cycled wait cannot be true. So we may allow
to defer flush request.

Signed-off-by: Yufen Yu <yuyufen@huawei.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-flush.c

@@ -283,13 +283,8 @@ static void blk_kick_flush(struct request_queue *q, struct blk_flush_queue *fq,
 	if (fq->flush_pending_idx != fq->flush_running_idx || list_empty(pending))
 		return;
 
-	/* C2 and C3
-	 *
-	 * For blk-mq + scheduling, we can risk having all driver tags
-	 * assigned to empty flushes, and we deadlock if we are expecting
-	 * other requests to make progress. Don't defer for that case.
-	 */
-	if (!list_empty(&fq->flush_data_in_flight) && q->elevator &&
+	/* C2 and C3 */
+	if (!list_empty(&fq->flush_data_in_flight) &&
 	    time_before(jiffies,
 			fq->flush_pending_since + FLUSH_PENDING_TIMEOUT))
 		return;