Re: [PATCH 1/6] xen-blkfront: avoid to use start/stop queue

Bart Van Assche
2017-07-11T18:41:32Z

On Wed, 2017-07-12 at 02:20 +0800, Ming Lei wrote:
> This interfaces will be removed soon, so use quiesce and
> unquiesce instead, which should be more safe.
>=20
> The only one usage will be removed in the following
> congestion control patches.

Hello Ming,

The title of this patch is misleading since this patch does not touch the c=
alls
related to queue congestion (blk_mq_stop_hw_queue() and
blk_mq_start_stopped_hw_queues()). I assume that you meant that this patch =
avoids
that the xen-blkfront driver uses blk_mq_(start|stop)_hw_queues() (with que=
ues in
plural form)? Can you please reflect that in the subject of this and relate=
d
patches?

Additionally, it's probably a good idea that this is not just an interface =
change
but that this kind of patches fix a (hard to trigger?) race condition.

> static inline void kick_pending_request_queues_locked(struct blkfront_ri=
ng_info *rinfo)
> {
> - if (!RING_FULL(&rinfo->ring))
> + if (!RING_FULL(&rinfo->ring)) {
> blk_mq_start_stopped_hw_queues(rinfo->dev_info->rq, true);
> + blk_mq_kick_requeue_list(rinfo->dev_info->rq);
> + }
> }
> =20
> static void kick_pending_request_queues(struct blkfront_ring_info *rinfo=
)
> @@ -1225,7 +1227,8 @@ static void kick_pending_request_queues(struct blkf=
ront_ring_info *rinfo)
> unsigned long flags;
> =20
> spin_lock_irqsave(&rinfo->ring_lock, flags);
> - kick_pending_request_queues_locked(rinfo);
> + if (!RING_FULL(&rinfo->ring))
> + blk_mq_run_hw_queues(rinfo->dev_info->rq, true);
> spin_unlock_irqrestore(&rinfo->ring_lock, flags);
> }

Why do some kick_pending_request_queues_locked() kick the requeue list and =
why
has the above kick_pending_request_queues_locked() call been converted into=
a
blk_mq_run_hw_queues() call and thereby ignores the requeue list?

Thanks,

Bart.=