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

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Message-ID: <19f34abd0806240137w31191b67t45e243a208cba933@mail.gmail.com>
Date:	Tue, 24 Jun 2008 10:37:46 +0200
From:	"Vegard Nossum" <vegard.nossum@...il.com>
To:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
Cc:	"Rusty Russell" <rusty@...tcorp.com.au>,
	"Mike Travis" <travis@....com>, "Adrian Bunk" <bunk@...nel.org>,
	"Srivatsa Vaddagiri" <vatsa@...ibm.com>,
	linux-kernel@...r.kernel.org, "Gautham R Shenoy" <ego@...ibm.com>,
	"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 Tue, Jun 24, 2008 at 10:06 AM, Zhang, Yanmin
<yanmin_zhang@...ux.intel.com> wrote:
> 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.

Ahhha! Well done!

(Whew, I have a lot to learn :-D)

>
> 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?

Sure, but it can take a while. 1) I have no idea why the processor
failed to initialize in the first place. So far, it only ever happened
this one time. 2) There seems to be a couple of other failure cases
related to cpu hotplug (usually the machine freezes hard), so it's not
certain that we hit this (failed to initialize) first. But I will try!

Thanks for solving the mystery.


Vegard

-- 
"The animistic metaphor of the bug that maliciously sneaked in while
the programmer was not looking is intellectually dishonest as it
disguises that the error is the programmer's own creation."
	-- E. W. Dijkstra, EWD1036