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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 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



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