On Thu, Jul 16, 2020 at 02:52:01AM -0400, Yufen Yu wrote:
> 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 <yuyufen@xxxxxxxxxx>
> ---
> 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 15ae0155ec07..24c208d21793 100644
> --- a/block/blk-flush.c
> +++ b/block/blk-flush.c
> @@ -286,13 +286,8 @@ 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;
> --
> 2.25.4
>
Reviewed-by: Ming Lei <ming.lei@xxxxxxxxxx>
--
Ming
