linux-kernel - Re: [PATCH 2/2] blk-mq: Fix request handover from timeout path to normal execution

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Date:   Mon, 2 Apr 2018 21:31:34 +0000
From:   Bart Van Assche <Bart.VanAssche@....com>
To:     "tj@...nel.org" <tj@...nel.org>
CC:     "kernel-team@...com" <kernel-team@...com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "axboe@...nel.dk" <axboe@...nel.dk>
Subject: Re: [PATCH 2/2] blk-mq: Fix request handover from timeout path to
 normal execution

On Mon, 2018-04-02 at 14:10 -0700, Tejun Heo wrote:
> On Mon, Apr 02, 2018 at 02:08:37PM -0700, Bart Van Assche wrote:
> > On 04/02/18 12:01, Tejun Heo wrote:
> > > +	 * As nothing prevents from completion happening while
> > > +	 * ->aborted_gstate is set, this may lead to ignored completions
> > > +	 * and further spurious timeouts.
> > > +	 */
> > > +	if (rq->rq_flags & RQF_MQ_TIMEOUT_RESET)
> > > +		blk_mq_rq_update_aborted_gstate(rq, 0);
> > 
> > Hello Tejun,
> > 
> > Since this patch fixes one race but introduces another race, is this
> > patch really an improvement?
> 
> Oh, that's not a new race.  That's the same non-critical race which
> always existed.  It's just being documented.

Hello Tejun,

I think it can happen that the above code sees that (rq->rq_flags &
RQF_MQ_TIMEOUT_RESET) != 0, that blk_mq_start_request() executes the
following code:

	blk_mq_rq_update_state(rq, MQ_RQ_IN_FLIGHT);
	blk_add_timer(rq);

and that subsequently blk_mq_rq_update_aborted_gstate(rq, 0) is called,
which will cause the next completion to be lost. Is fixing one occurrence
of a race and reintroducing it in another code path really an improvement?

Thanks,

Bart.