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

Ming Lei
2017-09-13T01:24:53Z

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