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

[Christoph Hellwig]

> +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;
        }