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

[Christoph Hellwig <hch@infradead.org>]

> +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;
	}