Re: [PATCH V7 5/6] block: support PREEMPT_ONLY

Christoph Hellwig
2017-10-02T13:50:27Z

> +void blk_set_preempt_only(struct request_queue *q, bool preempt_only)
> +{
> + blk_mq_freeze_queue(q);
> + if (preempt_only)
> + queue_flag_set_unlocked(QUEUE_FLAG_PREEMPT_ONLY, q);
> + else
> + queue_flag_clear_unlocked(QUEUE_FLAG_PREEMPT_ONLY, q);
> + blk_mq_unfreeze_queue(q);
> +}
> +EXPORT_SYMBOL(blk_set_preempt_only);

What do you need the queue freeze for?

> + /*
> + * preempt_only flag has to be set after queue is frozen,
> + * so it can be checked here lockless and safely
> + */
> + if (blk_queue_preempt_only(q)) {

We can always check a single bit flag safely, so I really don't
understand that comment.

> + if (!(flags & BLK_REQ_PREEMPT))
> + goto slow_path;
> + }
> +
> if (percpu_ref_tryget_live(&q->q_usage_counter))
> return 0;
> -
> + slow_path:

Also this looks a very spaghetti, why not:

if (!blk_queue_preempt_only(q) || (flags & BLK_MQ_REQ_PREEMPT)) {
if (percpu_ref_tryget_live(&q->q_usage_counter))
return 0;
}