linux-kernel - Re: Crash in elevator_dispatch_fn() (e.g. deadline

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Message-ID: <20140123185642.GA29712@htj.dyndns.org>
Date:	Thu, 23 Jan 2014 13:56:42 -0500
From:	Tejun Heo <tj@...nel.org>
To:	Frank Mayhar <fmayhar@...gle.com>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	Jens Axboe <axboe@...nel.dk>
Subject: Re: Crash in elevator_dispatch_fn() (e.g. deadline_dispatch()) when
 changing elevators.

On Thu, Jan 23, 2014 at 10:38:33AM -0800, Frank Mayhar wrote:
> On Wed, 2014-01-22 at 07:46 -0800, Frank Mayhar wrote:
> > On Tue, 2014-01-21 at 07:58 -0800, Frank Mayhar wrote:
> > > Replacing?  Or adding to?  Is BYPASS always set when DYING is set?  (My
> > > guess is not but I haven't done an exhaustive analysis.)  So the
> > > relevant code snippet in __elv_next_request() would be:
> > > 		if (unlikely(blk_queue_dying(q)) ||
> > > 		    unlikely(blk_queue_bypass(q)) ||
> > > 		    !q->elevator->type->ops.elevator_dispatch_fn(q, 0))
> > > 			return NULL;
> > 
> > FYI, I've made this change and tested it.  I can't say for certain that
> > it fixes the crash (since it's one of those races that's difficult to
> > reproduce), but it does seem to pass all the tests I've thrown at it so
> > far.
> 
> Um, does anyone care about this?  Tejun?  Jens?  Anyone?
> 
> This is a real crash; it would be nice if someone would weigh in.

Yeah, we're gonna fix this and I *think* replacing dying with bypass
is the right thing to do as a queue is always bypassing when killed.
It's probably just that we're in the earlier part of the merge window
and I have some other things on my plate.  Will post a patch in a
couple days.

Thanks.

-- 
tejun
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