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:	Wed, 20 Apr 2011 19:33:19 +0200
From:	Jens Axboe <jaxboe@...ionio.com>
To:	Michal Hocko <mhocko@...e.cz>
CC:	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

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:
>>> Hi,
>>> I am not able to boot with the current git tree
>>> (2.6.39-rc4-00089-g2f666bc). 2.6.39-rc3 boots just fine.
>>>
>>> The backtrace [1] looks really messy.
>>>
>>> I have seen a similar backtrace at https://lkml.org/lkml/2011/3/25/169
>>> and tried the patch from https://lkml.org/lkml/2011/3/25/173 but it
>>> didn't help. The backtrace looks similar (much smaller) but the machine
>>> ends up dead as well with unbalanced preempt counter[2].
>>>
>>> The config is attached.
>>>
>>> Is this a known problem?
>>>
>>> ---
>>> * [1] backtrace with the current git tree (2f666bc)
>>>
>>> [   31.207283] BUG: unable to handle kernel NULL pointer dereference at 0000000c
>>> NULL pointer dereference[   31.207540] IP: at 0000000c
>>> [   31.207540] IP: [<c028dece>] cfq_insert_request+0x1d/0x3f5
>>> [   31.207720] *pde = 00000000  [<c028dece>] cfq_insert_request+0x1d/0x3f5
>>> NULL pointer dereference[   31.207720] *pde = 00000000
>>>  at 0000000c
>>> [   31.207888] Oops: 0000 [#1]
>>> PREEMPT [   31.207540] IP:[   31.207888] Oops: 0000 [#1] SMP PREEMPT DEBUG_PAGEALLOC [<c028dece>] cfq_insert_request+0x1d/0x3f5
>>> SMP DEBUG_PAGEALLOC[   31.207720] *pde = 00000000
>>> [   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?

-- 
Jens Axboe

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