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    SubjectRe: v2.6.26-rc7: BUG: unable to handle kernel NULL pointer dereference
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    On Tue, 2008-06-24 at 16:06 +0800, Zhang, Yanmin wrote:
    > On Tue, 2008-06-24 at 11:36 +1000, Rusty Russell wrote:
    > > On Tuesday 24 June 2008 02:58:44 Mike Travis wrote:
    > > > Rusty Russell wrote:
    > > > > On Monday 23 June 2008 02:29:07 Vegard Nossum wrote:
    > > > >> And the (cpu < nr_cpu_ids) fails because the CPU has just been
    > > > >> offlined (or failed to initialize, but it's the same thing), while
    > > > >> NR_CPUS is the value that was compiled in as CONFIG_NR_CPUS (so the
    > > > >> former check will always be true).
    > > > >>
    > > > >> I don't think it is valid to ask for a per_cpu() variable on a CPU
    > > > >> which does not exist, though
    > > > >
    > > > > Yes it is. As long as cpu_possible(cpu), per_cpu(cpu) is valid.
    > > > >
    > > > > The number check should be removed: checking cpu_possible() is
    > > > > sufficient.
    > > > >
    > > > > Hope that helps,
    > > > > Rusty.
    > > >
    > > > I don't see a check for index being out of range in cpu_possible().
    > >
    > > You're right. It assumes cpu is < NR_CPUS. Hmm, I have no idea what's going
    > > on. nr_cpu_ids (ignore that it's a horrible name for a bad idea) should be
    > > fine to test against.
    > >
    > > Vegard's analysis is flawed: just because cpu is offline, it still must be <
    > > nr_cpu_ids, which is based on possible cpus. Unless something crazy is
    > > happening, but a quick grep doesn't reveal anyone manipulating nr_cpu_ids.
    > >
    > > If changing this fixes the bug, something else is badly wrong...
    > > Rusty.
    >
    > 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?
    >
    > Signed-off-by: Zhang Yanmin <yanmin_zhang@linux.intel.com>
    >
    > ---
    >
    > diff -Nraup linux-2.6.26-rc7/arch/x86/kernel/smpboot.c linux-2.6.26-rc7_cpuhotplug/arch/x86/kernel/smpboot.c
    > --- linux-2.6.26-rc7/arch/x86/kernel/smpboot.c 2008-06-24 09:03:54.000000000 +0800
    > +++ linux-2.6.26-rc7_cpuhotplug/arch/x86/kernel/smpboot.c 2008-06-24 09:04:45.000000000 +0800
    > @@ -996,7 +996,6 @@ do_rest:
    > #endif
    > cpu_clear(cpu, cpu_callout_map); /* was set by do_boot_cpu() */
    > cpu_clear(cpu, cpu_initialized); /* was set by cpu_init() */
    > - cpu_clear(cpu, cpu_possible_map);
    > cpu_clear(cpu, cpu_present_map);
    > per_cpu(x86_cpu_to_apicid, cpu) = BAD_APICID;
    > }
    >
    Andrew,

    Would you like to pick up this patch? Rusty Russell <rusty@rustcorp.com.au> acked it.

    Thanks,
    yanmin


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