Re: [V3 PATCH 3/4] kexec: Fix race between panic() and crash_kexec() called directly

From: Peter Zijlstra <peterz@infradead.org>
To: "河合英宏 / KAWAI，HIDEHIRO" <hidehiro.kawai.ez@hitachi.com>
Cc: "x86@kernel.org" <x86@kernel.org>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
	"kexec@lists.infradead.org" <kexec@lists.infradead.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"Michal Hocko" <mhocko@kernel.org>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"平松雅巳 / HIRAMATU，MASAMI" <masami.hiramatsu.pt@hitachi.com>,
	"Andrew Morton" <akpm@linux-foundation.org>,
	"Ingo Molnar" <mingo@kernel.org>,
	"Vivek Goyal" <vgoyal@redhat.com>
Subject: Re: [V3 PATCH 3/4] kexec: Fix race between panic() and crash_kexec() called directly
Date: Mon, 31 Aug 2015 11:07:33 +0200	[thread overview]
Message-ID: <20150831090733.GI19282@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <04EAB7311EE43145B2D3536183D1A8445495793E@GSjpTKYDCembx31.service.hitachi.net>

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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