linux-kernel - Re: [PATCH] blk-mq: put the driver tag of nxt rq before first one is requeued

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Message-ID: <20170913012452.GA24642@ming.t460p>
Date:   Wed, 13 Sep 2017 09:24:53 +0800
From:   Ming Lei <ming.lei@...hat.com>
To:     "jianchao.wang" <jianchao.w.wang@...cle.com>
Cc:     Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] blk-mq: put the driver tag of nxt rq before first one is
 requeued

On Wed, Sep 13, 2017 at 09:01:25AM +0800, jianchao.wang wrote:
> Hi ming
> 
> On 09/12/2017 06:23 PM, Ming Lei wrote:
> >> @@ -1029,14 +1029,20 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list)
> >>  		if (list_empty(list))
> >>  			bd.last = true;
> >>  		else {
> >> -			struct request *nxt;
> >> -
> >>  			nxt = list_first_entry(list, struct request, queuelist);
> >>  			bd.last = !blk_mq_get_driver_tag(nxt, NULL, false);
> >>  		}
> >>  
> >>  		ret = q->mq_ops->queue_rq(hctx, &bd);
> >>  		if (ret == BLK_STS_RESOURCE) {
> >> +			/*
> >> +			 * If an I/O scheduler has been configured and we got a
> >> +			 * driver tag for the next request already, free it again.
> >> +			 */
> >> +			if (!list_empty(list)) {
> >> +				nxt = list_first_entry(list, struct request, queuelist);
> >> +				blk_mq_put_driver_tag(nxt);
> >> +			}
> > The following way might be more simple and clean:
> > 
> > 			if (nxt)
> > 				blk_mq_put_driver_tag(nxt);
> > 
> > meantime 'nxt' need to be cleared inside the 'if (list_empty(list))'
> > before .queue_rq().
> 
> I had ever thought about that, but to avoid add extra command in the 
> fast path, I made the patch above.

Got it, so how about changing to the following way simply:

 			if (nxt && !list_empty(list))
 				blk_mq_put_driver_tag(nxt);

-- 
Ming