[dm-devel] [PATCH V3] blk-mq: introduce BLK_STS_DEV_RESOURCE

[Bart Van Assche]

On Sun, 2018-01-28 at 07:41 +0800, Ming Lei wrote:
> Not mention, the request isn't added to dispatch list yet in .queue_rq(),
> strictly speaking, it is not correct to call blk_mq_delay_run_hw_queue() in
> .queue_rq(), so the current block layer API can't handle it well enough.

I disagree that it is not correct to call blk_mq_delay_run_hw_queue() from
inside .queue_rq(). Additionally, I have already explained to you in
previous e-mails why it's fine to call that function from inside .queue_rq():
- Nobody has ever observed the race you described because the time after
  which a queue is rerun by blk_mq_delay_run_hw_queue() is much larger than
  the time during which that race exists.
- It is easy to fix this race inside the block layer, namely by using
  call_rcu() inside the blk_mq_delay_run_hw_queue() implementation to
  postpone the queue rerunning until after the request has been added back to
  the dispatch list.

> > - The patch at the start of this thread introduces a regression in the
> >   SCSI core, namely a queue stall if a request completion occurs concurrently
> >   with the newly added BLK_MQ_S_SCHED_RESTART test in the blk-mq core.
> 
> This patch only moves the blk_mq_delay_run_hw_queue() from scsi_queue_rq()
> to blk-mq, again, please explain it in detail how this patch V3 introduces this
> regression on SCSI.

Please reread the following message: https://marc.info/?l=dm-devel&m=151672480107560.

Thanks,

Bart.