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    SubjectRe: v2.6.26-rc7: BUG: unable to handle kernel NULL pointer dereference
    On Tue, Jun 24, 2008 at 10:06 AM, Zhang, Yanmin
    <yanmin_zhang@linux.intel.com> wrote:
    > In function _cpu_up, the panic happens when calling __raw_notifier_call_chain
    > at the second time. Kernel doesn't panic when calling it at the first time. If
    > just say because of nr_cpu_ids, that's not right.
    >
    > By checking source codes, I find function do_boot_cpu is the culprit.
    > Consider below call chain:
    > _cpu_up=>__cpu_up=>smp_ops.cpu_up=>native_cpu_up=>do_boot_cpu.
    >
    > So do_boot_cpu is called in the end. In do_boot_cpu, if boot_error==true,
    > cpu_clear(cpu, cpu_possible_map) is executed. So later on, when _cpu_up
    > calls __raw_notifier_call_chain at the second time to report CPU_UP_CANCELED,
    > because this cpu is already cleared from cpu_possible_map, get_cpu_sysdev returns
    > NULL.
    >
    > Many resources are related to cpu_possible_map, so it's better not to change it.
    >
    > Below patch against 2.6.26-rc7 fixes it by removing the bit clearing in cpu_possible_map.
    >
    > Vegard, would you like to help test it?

    Yay! I just hit this again with your patch applied

    Inquiring remote APIC #1...
    ... APIC #1 ID: failed
    ... APIC #1 VERSION: failed
    ... APIC #1 SPIV: failed

    and it works correctly, no NULL pointer error this time :-)

    I know it's applied to mainline already, actually with this tag too,
    even though I wasn't able to confirm it before:

    Tested-by: Vegard Nossum <vegard.nossum@gmail.com>

    Thanks :-)


    Vegard

    --
    "The animistic metaphor of the bug that maliciously sneaked in while
    the programmer was not looking is intellectually dishonest as it
    disguises that the error is the programmer's own creation."
    -- E. W. Dijkstra, EWD1036
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