linux-kernel - Re: Hang on "echo b > /proc/sysrq-trigger"

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Date:	Wed, 29 Feb 2012 10:07:00 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Keith Chew <keith.chew@...il.com>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Hang on "echo b > /proc/sysrq-trigger"

Keith Chew <keith.chew@...il.com> writes:

> Hi
>
> To test the reliability of a hardware, I have a script which reboots a
> machine every 15 minutes after boot up. This machine has a dual video
> output, VGA and DVI-D, both driven via an intel GM45 chipset (I am
> using kernel 2.6.39.24 kernel intel drivers).
>
> Some interesting results (which can be reproduced consistently):
> "echo b > /proc/sysrq-trigger" via DVI-D - after 2-3 days, it hangs
> (freezes) before reboot (dmesg only shows "Resetting...", nothing

My blind guess would be that it is the BIOS on the machine that is hung.

> after that, no panic, stack trace, etc)
> "echo b > /proc/sysrq-trigger" via VGA - runs > 1 week
> "reboot -fn" via VGA or DVI-D - runs > 1 week
> "reboot" via VGA or DVI-D - runs > 1 week
>
> I suspect that the intel graphics driver is not happy with the "echo b
>> /proc/sysrq-trigger" when it is still running.
>
> I would like to make the "echo b" successfully reboot the machine, but
> this would appear to be a hardware bug? Is there anything that can be
> done in the kernel to make the "echo b" successfully work 100%?

echo b > /proc/sysrq-trigger triggers the emergency_restart path which
tries but skips some steps so that it has a reasonable chance of working
when the kernel is wedged, it looks like some of those steps it skips
are needed on your hardware.

Eric
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