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Date:   Wed, 24 Apr 2019 18:18:34 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Ming Lei <ming.lei@redhat.com>
Cc:     Jens Axboe <axboe@kernel.dk>, linux-block@vger.kernel.org,
        Hannes Reinecke <hare@suse.com>,
        Keith Busch <keith.busch@intel.com>,
        linux-nvme@lists.infradead.org, Sagi Grimberg <sagi@grimberg.me>,
        James Smart <james.smart@broadcom.com>,
        Dongli Zhang <dongli.zhang@oracle.com>,
        Bart Van Assche <bart.vanassche@wdc.com>,
        linux-scsi@vger.kernel.org,
        "Martin K . Petersen" <martin.petersen@oracle.com>,
        Christoph Hellwig <hch@lst.de>,
        "James E . J . Bottomley" <jejb@linux.vnet.ibm.com>
Subject: Re: [PATCH V7 1/9] blk-mq: grab .q_usage_counter when queuing
 request from plug code path
Message-ID: <20190424161834.GA23854@lst.de>
References: <20190424110221.17435-1-ming.lei@redhat.com> <20190424110221.17435-2-ming.lei@redhat.com>
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On Wed, Apr 24, 2019 at 07:02:13PM +0800, Ming Lei wrote:
>  		if (rq->mq_hctx != this_hctx || rq->mq_ctx != this_ctx) {
>  			if (this_hctx) {
>  				trace_block_unplug(this_q, depth, !from_schedule);
> +
> +				percpu_ref_get(&this_q->q_usage_counter);
>  				blk_mq_sched_insert_requests(this_hctx, this_ctx,
>  								&rq_list,
>  								from_schedule);
> +				percpu_ref_put(&this_q->q_usage_counter);
>  			}
>  
>  			this_q = rq->q;
> @@ -1757,8 +1760,11 @@ void blk_mq_flush_plug_list(struct blk_plug *plug, bool from_schedule)
>  	 */
>  	if (this_hctx) {
>  		trace_block_unplug(this_q, depth, !from_schedule);
> +
> +		percpu_ref_get(&this_q->q_usage_counter);
>  		blk_mq_sched_insert_requests(this_hctx, this_ctx, &rq_list,
>  						from_schedule);
> +		percpu_ref_put(&this_q->q_usage_counter);

Why don't we push this into blk_mq_sched_insert_requests?  Yes, it
would need a request_queue argument, but that still seems saner
than duplicating it in both callers.