linux-kernel - Re: [V3 PATCH 3/4] kexec: Fix race between panic() and crash

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Message-ID: <20150831090733.GI19282@twins.programming.kicks-ass.net>
Date:	Mon, 31 Aug 2015 11:07:33 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	河合英宏 / KAWAI，HIDEHIRO 
	<hidehiro.kawai.ez@...achi.com>
Cc:	Jonathan Corbet <corbet@....net>, 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>,
	"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: [V3 PATCH 3/4] kexec: Fix race between panic() and crash_kexec()
 called directly

On Mon, Aug 31, 2015 at 08:53:11AM +0000, 河合英宏 / KAWAI，HIDEHIRO wrote:
> > I understand your question.  I don't intend to permit the recursive
> > call of crash_kexec() as for 'old_cpu != this_cpu' check.  That is
> > needed for the case of panic() --> crash_kexec().  Since panic_cpu has
> > already been set to this_cpu in panic() (please see PATCH 1/4), no one
> > can run crash_kexec() without 'old_cpu != this_cpu' check.
> > 
> > If you don't like this check, I would also be able to handle this case
> > like below:
> > 
> > crash_kexec()
> > {
> > 	old_cpu = atomic_cmpxchg(&panic_cpu, -1, this_cpu);
> > 	if (old_cpu != -1)
> > 		return;
> > 
> > 	__crash_kexec();
> > }
> > 
> > panic()
> > {
> > 	atomic_cmpxchg(&panic_cpu, -1, this_cpu);
> > 	__crash_kexec();
> > ...
> > 
> 
> Is that OK?

I suppose so, but I think me getting confused means comments can be
added/improved.
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