Messages in this thread Patch in this message | | | Subject | Re: v2.6.26-rc7: BUG: unable to handle kernel NULL pointer dereference | From | "Zhang, Yanmin" <> | Date | Tue, 24 Jun 2008 16:06:23 +0800 |
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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>
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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; }
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