linux-kernel - Re: [PATCH V7 5/6] block: support PREEMPT

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Date:   Mon, 2 Oct 2017 06:50:24 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     Jens Axboe <axboe@...com>, linux-block@...r.kernel.org,
        Christoph Hellwig <hch@...radead.org>,
        linux-scsi@...r.kernel.org,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        "James E . J . Bottomley" <jejb@...ux.vnet.ibm.com>,
        Bart Van Assche <bart.vanassche@...disk.com>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        Cathy Avery <cavery@...hat.com>,
        Martin Steigerwald <martin@...htvoll.de>,
        linux-kernel@...r.kernel.org, Hannes Reinecke <hare@...e.com>,
        Bart Van Assche <Bart.VanAssche@....com>
Subject: Re: [PATCH V7 5/6] block: support PREEMPT_ONLY

> +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;
	}