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    SubjectRe: v2.6.26-rc7: BUG: unable to handle kernel NULL pointer dereference
    On Tue, Jun 24, 2008 at 11:14:51PM +1000, Rusty Russell wrote:
    > On Tuesday 24 June 2008 18:06:23 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);

    Nice catch.

    While we're at it, is the clearing of cpu from the cpu_present_map
    necessary if cpu_up failed for 'cpu' ?

    > > per_cpu(x86_cpu_to_apicid, cpu) = BAD_APICID;
    > > }
    >
    > Nice catch. Basically, cpu_possible_map should only be cleared at boot, and
    > probably not even then.
    >
    > Acked-by: Rusty Russell <rusty@rustcorp.com.au>
    >
    > Thanks,
    > Rusty.

    --
    Thanks and Regards
    gautham
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