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

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Date:   Mon, 2 Oct 2017 16:27:59 +0000
From:   Bart Van Assche <Bart.VanAssche@....com>
To:     "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "hch@...radead.org" <hch@...radead.org>,
        "jejb@...ux.vnet.ibm.com" <jejb@...ux.vnet.ibm.com>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "axboe@...com" <axboe@...com>,
        "ming.lei@...hat.com" <ming.lei@...hat.com>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>
CC:     Bart Van Assche <Bart.VanAssche@....com>,
        "martin@...htvoll.de" <martin@...htvoll.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jthumshirn@...e.de" <jthumshirn@...e.de>,
        "oleksandr@...alenko.name" <oleksandr@...alenko.name>,
        "hare@...e.com" <hare@...e.com>,
        "cavery@...hat.com" <cavery@...hat.com>
Subject: Re: [PATCH V7 5/6] block: support PREEMPT_ONLY

On Sat, 2017-09-30 at 14:12 +0800, Ming Lei wrote:
> +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);
> +
>  /**
>   * __blk_run_queue_uncond - run a queue whether or not it has been stopped
>   * @q:	The queue to run
> @@ -771,9 +782,18 @@ int blk_queue_enter(struct request_queue *q, unsigned flags)
>  	while (true) {
>  		int ret;
>  
> +		/*
> +		 * 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)) {
> +			if (!(flags & BLK_REQ_PREEMPT))
> +				goto slow_path;
> +		}
> +
>  		if (percpu_ref_tryget_live(&q->q_usage_counter))
>  			return 0;

Sorry but I don't think that it is possible with these changes to prevent
that a non-preempt request gets allocated after a (SCSI) queue has been
quiesced. If the CPU that calls blk_queue_enter() observes the set of the
PREEMPT_ONLY flag after the queue has been unfrozen and after the SCSI
device state has been changed into QUIESCED then blk_queue_enter() can
succeed for a non-preempt request. I think this is exactly the scenario
we want to avoid. This is why a synchronize_rcu() call is present in my
patch before the queue is unfrozen and also why in my patch the
percpu_ref_tryget_live() call occurs before the test of the PREEMPT_ONLY
flag.

Bart.