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:	Fri, 22 Apr 2011 13:35:55 +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 2011-04-22 09:00, Michal Hocko wrote:
> On Thu 21-04-11 21:00:26, Jens Axboe wrote:
>> 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.
> 
> Great. I can boot just fine with the current Linus tree
> (2.6.39-rc4-00149-g91e8549).
> 
> Thanks a lot!

Great, with that confirmed from you, we have no pending bugs in this
area.

-- 
Jens Axboe

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