From b5718d6c00afe51ce5327cf2beae4019da0c8e74 Mon Sep 17 00:00:00 2001 From: Yufen Yu Date: Thu, 16 Jul 2020 02:52:01 -0400 Subject: [PATCH] block: defer flush request no matter whether we have elevator Commit 7520872c0cf4 ("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 923218f6166a ("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 Reviewed-by: Ming Lei Signed-off-by: Jens Axboe --- block/blk-flush.c | 9 ++------- 1 file changed, 2 insertions(+), 7 deletions(-) diff --git a/block/blk-flush.c b/block/blk-flush.c index 71c7f520e040..6e1543c10493 100644 --- a/block/blk-flush.c +++ b/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;