linux-kernel - Re: 2.6.39-rc4 BUG: unable to handle kernel NULL pointer dereference at 0000000c IP: cfq_insert

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Date:	Thu, 21 Apr 2011 21:00:26 +0200
From:	Jens Axboe <jaxboe@...ionio.com>
To:	Michal Hocko <mhocko@...e.cz>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jens Axboe <axboe@...nel.dk>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: 2.6.39-rc4 BUG: unable to handle kernel NULL pointer  dereference
 at 0000000c IP: cfq_insert_request+0x1d/0x3f5

On 2011-04-21 20:51, Michal Hocko wrote:
> On Thu 21-04-11 07:38:57, Linus Torvalds wrote:
>> On Thu, Apr 21, 2011 at 12:25 AM, Jens Axboe <axboe@...nel.dk> wrote:
>>>>
>>>> I am going to bisect, let's see if I can find anything.
>>>
>>> Thanks, that would be great!
>>
>> I'd expect it to be very timing-dependent, and thus could easily be
>> triggered (or hidden) by unrelated changes.
>>
>> Just happening to have a request added to the elevator at _just_ the
>> same moment that another CPU is changing it and getting rid of the
>> data structures for the old one.
> 
> And it really looks like a timing issue. I have bisected down to
> e710d7d5a9cab1041b7a3cf9e655b75d92786857. I had to skip[1] some commits
> due to compile errors [2].
> At first it looked quite promising because I was able to boot after I
> reverted that patch but then I have tried to revert it on top of rc4
> (2f666bcf757cb72549f360ef6da02f03620a48b6) and saw the same problem
> again.
> 
> So I do not think that bisecting will help here.

It will be timing dependent. If there's no allocated IO requests when
the switch happens, it'll work.

But the commit that caused this regression is 5e84ea3a. If you revert
that, it should work fine. Or just apply the patch I sent (or update to
Linus' tree, it's in now) and it'll work as well.

-- 
Jens Axboe

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