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

Ming Lei
2017-09-13T01:52:27Z

On Wed, Sep 13, 2017 at 09:39:44AM +0800, jianchao.wang wrote:
>
>
> 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);

Yeah, that is definitely more clean, :-)

Feel free to add "Reviewed-by: Ming Lei <ming.lei@redhat.com>"
in your next post.

--
Ming