Re: [PATCH v3 3/9] blk-mq: use the introduced blk_mq_unquiesce_queue()

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

On Wed, May 31, 2017 at 03:21:41PM +0000, Bart Van Assche wrote:
> On Wed, 2017-05-31 at 20:37 +0800, Ming Lei wrote:
> > diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> > index 99e16ac479e3..ffcf05765e2b 100644
> > --- a/drivers/scsi/scsi_lib.c
> > +++ b/drivers/scsi/scsi_lib.c
> > @@ -3031,7 +3031,10 @@ scsi_internal_device_unblock(struct scsi_device *sdev,
> >  		return -EINVAL;
> >  
> >  	if (q->mq_ops) {
> > -		blk_mq_start_stopped_hw_queues(q, false);
> > +		if (blk_queue_quiesced(q))
> > +			blk_mq_unquiesce_queue(q);
> > +		else
> > +			blk_mq_start_stopped_hw_queues(q, false);
> >  	} else {
> >  		spin_lock_irqsave(q->queue_lock, flags);
> >  		blk_start_queue(q);
> 
> As I commented on v2, this change is really wrong. All what's needed here is
> a call to blk_mq_unquiesce_queue() and nothing else. Adding a call to
> blk_mq_start_stopped_hw_queues() is wrong because it makes it impossible to
> use the STOPPED flag in the SCSI core to make the block layer core stop calling
> .queue_rq() if a SCSI LLD returns "busy".

I am not sure if I understand your idea, could you explain a bit why it is wrong?

Let's see the function of scsi_internal_device_block():

	if (q->mq_ops) {
                if (wait)
                        blk_mq_quiesce_queue(q);
                else
                        blk_mq_stop_hw_queues(q);
	}

So the queue may be put into quiesced if 'wait' is true, or it is
stopped if 'wait' is false.

And this patch just makes the two SCSI APIs symmetrical.

Since we will not stop queue in blk_mq_quiesce_queue() later,
I have to unquiese one queue only if it is quiesced.

So suppose the queue is put into stopped in scsi_internal_device_block(),
do we expect not to restart it in scsi_internal_device_unblock() via
blk_mq_unquiesce_queue()?

Thanks,
Ming