lkml.org 
[lkml]   [2008]   [Jun]   [26]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    From
    SubjectRe: v2.6.26-rc7: BUG: unable to handle kernel NULL pointer dereference
    Date
    On Thursday 26 June 2008 22:58:20 Gautham R Shenoy wrote:
    > 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' ?

    It's never necessary, but there there are not many places which cpu_present is
    examined. It just prevents it from being hot added again, AFAICT.

    Rusty.
    --
    To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    the body of a message to majordomo@vger.kernel.org
    More majordomo info at http://vger.kernel.org/majordomo-info.html
    Please read the FAQ at http://www.tux.org/lkml/

    \
     
     \ /
      Last update: 2008-06-27 05:21    [W:0.033 / U:2.824 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site