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From: Tejun Heo <tj@kernel.org>
To: Ming Lei <ming.lei@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>,
	linux-block@vger.kernel.org,
	Bart Van Assche <bart.vanassche@wdc.com>,
	Christoph Hellwig <hch@lst.de>, Sagi Grimberg <sagi@grimberg.me>,
	Israel Rukshin <israelr@mellanox.com>,
	Max Gurtovoy <maxg@mellanox.com>,
	stable@vger.kernel.org
Subject: Re: [PATCH] blk-mq: fix race between complete and BLK_EH_RESET_TIMER
Date: Thu, 12 Apr 2018 06:57:12 -0700	[thread overview]
Message-ID: <20180412135712.GY793541@devbig577.frc2.facebook.com> (raw)
In-Reply-To: <20180411230504.GB31433@ming.t460p>

On Thu, Apr 12, 2018 at 07:05:13AM +0800, Ming Lei wrote:
> > Not really because aborted_gstate right now doesn't have any memory
> > barrier around it, so nothing ensures blk_add_timer() actually appears
> > before.  We can either add the matching barriers in aborted_gstate
> > update and when it's read in the normal completion path, or we can
> > wait for the update to be visible everywhere by waiting for rcu grace
> > period (because the reader is rcu protected).
> 
> Seems not necessary.
> 
> Suppose it is out of order, the only side-effect is that the new
> recycled request is timed out as a bit late, I think that is what
> we can survive, right?

It at least can mess up the timeout duration for the next recycle
instance because there can be two competing blk_add_timer() instances.
I'm not sure whether there can be other consequences.  When ownership
isn't clear, it becomes really difficult to reason about these things
and can lead to subtle failures.  I think it'd be best to always
establish who owns what.

Thanks.

-- 
tejun

  reply	other threads:[~2018-04-12 13:57 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-04-11 20:55 [PATCH] blk-mq: fix race between complete and BLK_EH_RESET_TIMER Ming Lei
2018-04-11 21:30 ` Tejun Heo
2018-04-11 22:43   ` Ming Lei
2018-04-11 22:47     ` Tejun Heo
2018-04-11 23:05       ` Ming Lei
2018-04-12 13:57         ` Tejun Heo [this message]
2018-04-13 15:38           ` Ming Lei
2018-04-11 22:49 ` Bart Van Assche
2018-04-11 22:49   ` Bart Van Assche
2018-04-11 23:06   ` Ming Lei

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