Re: 2.6.39-rc4 BUG: unable to handle kernel NULL pointer dereference at 0000000c IP: cfq_insert_request+0x1d/0x3f5

[Jens Axboe <jaxboe@fusionio.com>]

On 2011-04-21 17:04, Jeff Moyer wrote:
> Jens Axboe <axboe@kernel.dk> writes:
> 
>> On 21/04/2011, at 09.16, Michal Hocko <mhocko@suse.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.

I think this is just the canary in this particular case, I bet it does
not work reliably in general after the addition of the SORT_MERGE
insert.

-- 
Jens Axboe