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: <x49mxjjpppi.fsf@segfault.boston.devel.redhat.com>
Date:	Thu, 21 Apr 2011 11:51:53 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Jens Axboe <axboe@...nel.dk>, Michal Hocko <mhocko@...e.cz>,
	Jens Axboe <JAxboe@...ionio.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	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

Vivek Goyal <vgoyal@...hat.com> writes:

> On Thu, Apr 21, 2011 at 11:04:45AM -0400, Jeff Moyer wrote:
>> Jens Axboe <axboe@...nel.dk> writes:
>> 
>> > On 21/04/2011, at 09.16, Michal Hocko <mhocko@...e.cz> wrote:
>> >
>> >> On Wed 20-04-11 19:33:19, Jens Axboe wrote:
>> >>> On 2011-04-20 15:29, Michal Hocko wrote:
>> >>>> On Wed 20-04-11 15:13:15, Jens Axboe wrote:
>> >>>>> On 2011-04-20 14:58, Michal Hocko wrote:
>> >> [...]
>> >>>>>> [   31.207888] Oops: 0000 [#1] PREEMPT [   31.208186] last sysfs file: /sys/devices/pci0000:00/0000:00:1f.2/host0/target0:0:0/0:0:0:0/block/sda/queue/scheduler
>> >>>>> 
>> >>>>> Ahh hang on, this may be a good clue. Does your boot scripts change the
>> >>>>> IO scheduler?
>> >>>> 
>> >>>> Good one...
>> >>>> Yes, I have:
>> >>>> echo deadline > /sys/block/sda/queue/scheduler
>> >>>> in /etc/rc.local
>> >>>> 
>> >>>> I am able to boot after I remove it. This is the first time I have seen
>> >>>> "last sysfs file" being useful.
>> >>>> Still want me to test the patch from the other email?
>> >>> 
>> >>> Is this a new addition to your system? IOW, how certain are you that
>> >>> this is a regression that occured between rc3 and rc4?
>> >> 
>> >> No, I am setting the scheduler this way for quite some time. If I use it
>> >> rc4 explodes while rc3 boots just fine. I am wondering, can this be a
>> >> timing issue? I am able to set the scheduler after system settles down
>> >> after boot and kde starts.
>> >> 
>> >> I am going to bisect, let's see if I can find anything.
>> >
>> > Thanks, that would be great!
>> 
>> OK, this is a long shot, but in a derivative kernel, I saw what may be
>> the same issue.  Is this kernel built with CONFIG_BLK_CGROUP=n by
>> chance?  The exact problem I saw was a panic on boot in
>> cfq_insert_request+0x77, which mapped to this:
>> 
>> /usr/src/debug/kernel-2.6.32-135.el6/linux-2.6.32-135.el6.x86_64/block/cfq-iosched.c:1997
>> ffffffff8125c390:       49 8b 84 24 a8 00 00    mov    0xa8(%r12),%rax
>> ffffffff8125c397:       00                          <---------------------
>> ffffffff8125c398:       83 80 ec 02 00 00 01    addl   $0x1,0x2ec(%rax)
>> 
>> cfq-iosched.c:1997 looks like this:
>> 
>>         (RQ_CFQG(rq))->dispatched++;
>> 
>> Enabling CONFIG_BLK_CGROUP made the problem go away.  Again, not sure
>> it's the same thing, but I figured I'd speak up in case it helps.
>
> Jeff, 
>
> I think we had fixed this issue upstream with following commit.
>
> commit 50eaeb323a170e231263ccb433bb2f99bd9e27ac
> Author: Dmitry Monakhov <dmonakhov@...nvz.org>
> Date:   Wed Apr 28 19:50:33 2010 +0200
>
>     cfq-iosched: fix broken cfq_ref_get_cfqf() for CONFIG_BLK_CGROUP=y &&
> CFQ_GR
>     
>     We should return the cfq_group for this case, not NULL.
>     
>     Signed-off-by: Jens Axboe <jens.axboe@...cle.com>
>
>
> I just booted my system with 39-rc4 with BLK_CGROUP=n. So I doubt that's the
> issue here.

OK, thanks for checking.

Cheers,
Jeff
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