linux-kernel - Re: [PATCH 1/2] x86: mce: kdump: use under_crashdumping to turn off MCE in all CPUs together

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Date:	Mon, 23 Feb 2015 10:27:39 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Cc:	Tony Luck <tony.luck@...el.com>, Vivek Goyal <vgoyal@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Junichi Nomura <j-nomura@...jp.nec.com>,
	Kiyoshi Ueda <k-ueda@...jp.nec.com>
Subject: Re: [PATCH 1/2] x86: mce: kdump: use under_crashdumping to turn off
 MCE in all CPUs together

On Mon, Feb 23, 2015 at 09:12:29AM +0000, Naoya Horiguchi wrote:
> kexec disables (or "shoots down") all CPUs other than a crashing CPU before
> entering the 2nd kernel. This disablement is done via NMI, and the crashing
> CPU wait for the completions by spinning at most for 1 second.
> However, there is a race window if this NMI handling doesn't complete within
> the 1 second on some CPU, which cause the fragile situation where only a
> portion of online CPUs are responsive to MCE interrupt. If MCE happens during
> this race window, MCE synchronization always timeouts and results in kernel
> panic. So the user-visible effect of this bug is kdump failure.
> 
> Note that this race window did exist when current MCE handler was implemented
> around 2.6.32, and recently commit 716079f66eac ("mce: Panic when a core has
> reached a timeout") made it more visible by changing the default behavior of
> the synchronization timeout from "ignore" to "panic".

Let me guess: you could raise the tolerance level to 3 temporarily from
native_machine_crash_shutdown() and not touch the #MC handler at all,
right?

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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