Re: [PATCH v3 0/5] Avoid that scsi-mq queue processing stalls

[Ming Lei <ming.lei@redhat.com>]

On Fri, Apr 07, 2017 at 03:18:19PM +0000, Bart Van Assche wrote:
> On Fri, 2017-04-07 at 17:41 +0800, Ming Lei wrote:
> > On Thu, Apr 06, 2017 at 11:10:45AM -0700, Bart Van Assche wrote:
> > > Hello Jens,
> > > 
> > > The five patches in this patch series fix the queue lockup I reported
> > > recently on the linux-block mailing list. Please consider these patches
> > > for inclusion in the upstream kernel.
> > 
> > I read the commit log of the 5 patches, looks not found descriptions
> > about root cause of the queue lockup, so could you explain a bit about
> > the reason behind?
> 
> Hello Ming,
> 
> If a .queue_rq() function returns BLK_MQ_RQ_QUEUE_BUSY then the block
> driver that implements that function is responsible for rerunning the
> hardware queue once requests can be queued successfully again. That is
> not the case today for the SCSI core. Patch 5/5 ensures that hardware

The current .queue_rq() will call blk_mq_delay_queue() if QUEUE_BUSY is
returned, and once request is completed, the queue will be restarted
by blk_mq_start_stopped_hw_queues() in scsi_end_request(). This way
sounds OK in theory. And I just try to understand the specific reason
which causes the lockup, but still not get it.

Thanks,
Ming