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SubjectRe: 2.6.39-rc4 BUG: unable to handle kernel NULL pointer dereference at 0000000c IP: cfq_insert_request+0x1d/0x3f5
On Thu, Apr 21, 2011 at 11:04:45AM -0400, 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.

Jeff,

I think we had fixed this issue upstream with following commit.

commit 50eaeb323a170e231263ccb433bb2f99bd9e27ac
Author: Dmitry Monakhov <dmonakhov@openvz.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@oracle.com>


I just booted my system with 39-rc4 with BLK_CGROUP=n. So I doubt that's the
issue here.

Thanks
Vivek


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