linux-kernel - Re: [PATCH 2/8] blk-mq: change the method of iterating busy tags of a request

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Date:   Mon, 18 Mar 2019 09:53:26 -0600
From:   Keith Busch <kbusch@...nel.org>
To:     Ming Lei <tom.leiming@...il.com>
Cc:     Jianchao Wang <jianchao.w.wang@...cle.com>,
        "axboe@...nel.dk" <axboe@...nel.dk>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "jsmart2021@...il.com" <jsmart2021@...il.com>,
        "bvanassche@....org" <bvanassche@....org>,
        "josef@...icpanda.com" <josef@...icpanda.com>,
        "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Busch, Keith" <keith.busch@...el.com>,
        "hare@...e.de" <hare@...e.de>,
        "jthumshirn@...e.de" <jthumshirn@...e.de>,
        "hch@....de" <hch@....de>, "sagi@...mberg.me" <sagi@...mberg.me>
Subject: Re: [PATCH 2/8] blk-mq: change the method of iterating busy tags of
 a request_queue

On Sat, Mar 16, 2019 at 11:50:27PM -0700, Ming Lei wrote:
> On Sat, Mar 16, 2019 at 12:15 AM Keith Busch <kbusch@...nel.org> wrote:
> > On Fri, Mar 15, 2019 at 01:57:38AM -0700, Jianchao Wang wrote:
> > > +     if (rq && rq->q == hctx->queue &&
> > > +         rq->mq_hctx == hctx &&
> > > +         (!iter_data->inflight ||
> > > +          blk_mq_rq_state(rq) == MQ_RQ_IN_FLIGHT))
> > > +             return iter_data->fn(rq, iter_data->data, reserved);
> >
> > There is still a window where the check may succeed, but the request is
> > being assigned to a completely different request_queue. The callback
> > then operates on a request it doesn't own.
> 
> The situation above is only related with 'none' io scheduler.
> 
> However, this patch doesn't change behavior for none given .rq[tag] is same with
> .static_rq[tag], so I guess your concern may not be related with this patch.

Right, it's not new, but queue iteration wasn't exported for drivers
before this.