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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 2011-04-20 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.

    Haha, good point, it's the first time for me as well IO think.

    > Still want me to test the patch from the other email?

    No, that wont help you. I do wonder why this is a rc3 vs rc4 change.
    I'll dig around and send you something to try.

    --
    Jens Axboe



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