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

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Date:   Wed, 13 Sep 2017 09:01:25 +0800
From:   "jianchao.wang" <jianchao.w.wang@...cle.com>
To:     Ming Lei <ming.lei@...hat.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

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.

Thanks
Jianchao