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Date: Mon, 2 Oct 2017 06:50:24 -0700
From: Christoph Hellwig <hch@infradead.org>
To: Ming Lei <ming.lei@redhat.com>
Cc: Jens Axboe <axboe@fb.com>, linux-block@vger.kernel.org,
        Christoph Hellwig <hch@infradead.org>, linux-scsi@vger.kernel.org,
        "Martin K . Petersen" <martin.petersen@oracle.com>,
        "James E . J . Bottomley" <jejb@linux.vnet.ibm.com>,
        Bart Van Assche <bart.vanassche@sandisk.com>,
        Oleksandr Natalenko <oleksandr@natalenko.name>,
        Johannes Thumshirn <jthumshirn@suse.de>,
        Cathy Avery <cavery@redhat.com>,
        Martin Steigerwald <martin@lichtvoll.de>, linux-kernel@vger.kernel.org,
        Hannes Reinecke <hare@suse.com>,
        Bart Van Assche <Bart.VanAssche@wdc.com>
Subject: Re: [PATCH V7 5/6] block: support PREEMPT_ONLY
Message-ID: <20171002135024.GE19119@infradead.org>
References: <20170930061214.10622-1-ming.lei@redhat.com>
 <20170930061214.10622-6-ming.lei@redhat.com>
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> +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;
	}