Commit b5718d6c authored Jul 16, 2020 by Yufen Yu Committed by Jens Axboe Jul 17, 2020

block: defer flush request no matter whether we have elevator



Commit 7520872c ("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 923218f6 ("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>

parent 943c4d90

block/blk-flush.c

+2 −7

Original line number	Diff line number	Diff line
		@@ -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;

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