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

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Message-ID: <20180402213953.GG388343@devbig577.frc2.facebook.com>
Date:   Mon, 2 Apr 2018 14:39:53 -0700
From:   "tj@...nel.org" <tj@...nel.org>
To:     Bart Van Assche <Bart.VanAssche@....com>
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

Hello,

On Mon, Apr 02, 2018 at 09:31:34PM +0000, Bart Van Assche 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);
...
> 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?

I'm not following at all.  How would blk_mq_start_request() get called
on the request while it's still owned by the timeout handler?  That
gstate clearing is what transfers the ownership back to the
non-timeout path.

Thanks.

-- 
tejun