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

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Message-ID: <1390502313.20232.49.camel@bobble.lax.corp.google.com>
Date:	Thu, 23 Jan 2014 10:38:33 -0800
From:	Frank Mayhar <fmayhar@...gle.com>
To:	Tejun Heo <tj@...nel.org>
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 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.
-- 
Frank Mayhar
310-460-4042

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