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

jianchao.wang
2017-09-13T01:39:44Z

On 09/13/2017 09:24 AM, Ming Lei wrote:
> 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);
>
It seems that we even could change it as following:
if (!list_empty(list))
blk_mq_put_driver_tag(nxt);

thanks
jianchao