linux-kernel - Re: v2.6.26-rc7: BUG: unable to handle kernel NULL pointer dereference

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Message-Id: <200806271316.42771.rusty@rustcorp.com.au>
Date:	Fri, 27 Jun 2008 13:16:42 +1000
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	ego@...ibm.com
Cc:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	Mike Travis <travis@....com>,
	Vegard Nossum <vegard.nossum@...il.com>,
	Adrian Bunk <bunk@...nel.org>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	linux-kernel@...r.kernel.org, "Rafael J. Wysocki" <rjw@...k.pl>,
	"Zhang, Yanmin" <yanmin.zhang@...el.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>
Subject: Re: v2.6.26-rc7: BUG: unable to handle kernel NULL pointer dereference

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@...ux.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.
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