linux-kernel - kdump: crash_kexec()-smp_send

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Message-ID: <1319468137.3615.16.camel@br98xy6r>
Date:	Mon, 24 Oct 2011 16:55:37 +0200
From:	Michael Holzheu <holzheu@...ux.vnet.ibm.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	ebiederm@...ssion.com, akpm@...ux-foundation.org,
	schwidefsky@...ibm.com, heiko.carstens@...ibm.com,
	kexec@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: kdump: crash_kexec()-smp_send_stop() race in panic

Hello Vivek,

In our tests we ran into the following scenario:

Two CPUs have called panic at the same time. The first CPU called
crash_kexec() and the second CPU called smp_send_stop() in panic()
before crash_kexec() finished on the first CPU. So the second CPU
stopped the first CPU and therefore kdump failed.

1st CPU:
panic()->crash_kexec()->mutex_trylock(&kexec_mutex)-> do kdump

2nd CPU:
panic()->crash_kexec()->kexec_mutex already held by 1st CPU
       ->smp_send_stop()-> stop CPU 1 (stop kdump)

How should we fix this problem? One possibility could be to do
smp_send_stop() before we call crash_kexec().

What do you think?

Michael

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