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

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Message-ID: <BANLkTinsqMaKuUjTwByXopXb+4a83nfApw@mail.gmail.com>
Date:	Fri, 22 Apr 2011 13:30:42 -0700
From:	Hugh Dickins <hughd@...gle.com>
To:	Jens Axboe <jaxboe@...ionio.com>
Cc:	Michal Hocko <mhocko@...e.cz>,
	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 Fri, Apr 22, 2011 at 4:35 AM, Jens Axboe <jaxboe@...ionio.com> wrote:
> 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.

Yes, I booted up last night's git kernel on the PowerPC G5, and
confirm that CONFIG_IDE=y worked with that, whereas rc4 still had the
hang (even with CONFIG_SMP=y CONFIG_PREEMPT=y).

Hugh
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