linux-kernel - Re: [PATCH 3/8] blk-mq: replace timeout synchronization with a RCU and generation based scheme

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Message-ID: <20180109154607.GJ3668920@devbig577.frc2.facebook.com>
Date:   Tue, 9 Jan 2018 07:46:07 -0800
From:   "tj@...nel.org" <tj@...nel.org>
To:     Bart Van Assche <Bart.VanAssche@....com>
Cc:     "jbacik@...com" <jbacik@...com>, "jack@...e.cz" <jack@...e.cz>,
        "clm@...com" <clm@...com>, "axboe@...nel.dk" <axboe@...nel.dk>,
        "kernel-team@...com" <kernel-team@...com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "linux-btrfs@...r.kernel.org" <linux-btrfs@...r.kernel.org>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "jianchao.w.wang@...cle.com" <jianchao.w.wang@...cle.com>,
        "hch@....de" <hch@....de>
Subject: Re: [PATCH 3/8] blk-mq: replace timeout synchronization with a RCU
 and generation based scheme

On Mon, Jan 08, 2018 at 09:06:55PM +0000, Bart Van Assche wrote:
> On Mon, 2018-01-08 at 11:15 -0800, Tejun Heo wrote:
> > +static void blk_mq_rq_update_aborted_gstate(struct request *rq, u64 gstate)
> > +{
> > +	unsigned long flags;
> > +
> > +	local_irq_save(flags);
> > +	u64_stats_update_begin(&rq->aborted_gstate_sync);
> > +	rq->aborted_gstate = gstate;
> > +	u64_stats_update_end(&rq->aborted_gstate_sync);
> > +	local_irq_restore(flags);
> > +}
> 
> Please add a comment that explains the purpose of local_irq_save() and
> local_irq_restore(). Please also explain why you chose to disable interrupts

Will do.

> instead of disabling preemption. I think that disabling preemption should be
> sufficient since this is the only code that updates rq->aborted_gstate and
> since this function is always called from thread context.

blk_mq_complete_request() can read it from the irq context.  If that
happens between update_begin and end, it'll end up looping infinitely.

> > @@ -801,6 +840,12 @@ void blk_mq_rq_timed_out(struct request *req, bool reserved)
> >  		__blk_mq_complete_request(req);
> >  		break;
> >  	case BLK_EH_RESET_TIMER:
> > +		/*
> > +		 * As nothing prevents from completion happening while
> > +		 * ->aborted_gstate is set, this may lead to ignored
> > +		 * completions and further spurious timeouts.
> > +		 */
> > +		blk_mq_rq_update_aborted_gstate(req, 0);
> >  		blk_add_timer(req);
> >  		blk_clear_rq_complete(req);
> >  		break;
> 
> Is the race that the comment refers to addressed by one of the later patches?

No, but it's not a new race.  It has always been there and I suppose
doesn't lead to practical problems - the race window is pretty small
and the effect isn't critical.  I'm just documenting the existing race
condition.  Will note that in the description.

Thanks.

-- 
tejun