linux-kernel - Re: [V5 PATCH 3/4] kexec: Fix race between panic() and crash

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Message-ID: <20151203122220.GA3213@pd.tnic>
Date:	Thu, 3 Dec 2015 13:22:20 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	河合英宏 / KAWAI，HIDEHIRO 
	<hidehiro.kawai.ez@...achi.com>
Cc:	Jonathan Corbet <corbet@....net>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Vivek Goyal <vgoyal@...hat.com>, Baoquan He <bhe@...hat.com>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	"kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Michal Hocko <mhocko@...nel.org>,
	平松雅巳 / HIRAMATU，MASAMI 
	<masami.hiramatsu.pt@...achi.com>
Subject: Re: [V5 PATCH 3/4] kexec: Fix race between panic() and crash_kexec()
 called directly

On Thu, Dec 03, 2015 at 11:29:21AM +0000, 河合英宏 / KAWAI，HIDEHIRO wrote:
> I was arguing about the case of oops_end --> crash_kexec
> --> return from crash_kexec because of !kexec_crash_image -->
> panic.

Aha.

> In the case of panic --> __crash_kexec, __crash_kexec is called
> only once, so we don't need to check the return value of __crash_kexec
> as you suggested.  So I thought you stated about crash_kexec --> panic
> case.

No, I meant the other way around.

> I also mentioned !kexec_crash_image case...

I must've missed it.

> No.  The first CPU calls panic, and then it calls __crash_kexec.
> Because of !kexec_crash_image, it returns from __crash_kexec and
> continues to the panic procedure.  At the same time, another CPU
> tries to call panic(), but it doesn't run the panic procedure;
> panic_cpu prevents the second CPU from running it.
>
> This means __crash_kexec is called only once even if we don't
> check the return value of __crash_kexec.

I think we're on the same page, even if we express it differently - the
other CPUs entering panic() will loop in panic_smp_self_stop() so they
won't reach __crash_kexec().

> (Please note that crash_kexec can be called multiple times in the
> case of oops_end() --> crash_kexec().)

Right, and that was the case that was bugging me - calling into
crash_kexec() on multiple CPUs but it is a trylock and a pointer test -
I guess that's diminishingly small overhead to care.

Thanks.

-- 
Regards/Gruss,
    Boris.

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