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

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Message-ID: <1390405618.20232.42.camel@bobble.lax.corp.google.com>
Date:	Wed, 22 Jan 2014 07:46:58 -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 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.
-- 
Frank Mayhar
310-460-4042

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