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

[Bart Van Assche <Bart.VanAssche@xxxxxxx>]

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.
> 
> 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 calls
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 queues 
in
plural form)? Can you please reflect that in the subject of this and related
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_ring_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);
> +     }
>  }
>  
>  static void kick_pending_request_queues(struct blkfront_ring_info *rinfo)
> @@ -1225,7 +1227,8 @@ static void kick_pending_request_queues(struct 
> blkfront_ring_info *rinfo)
>       unsigned long flags;
>  
>       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.
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