Re: Re: [V4 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>,
	"kbuild-all@01.org" <kbuild-all@01.org>
Subject: Re: Re: [V4 PATCH 3/4] kexec: Fix race between panic() and crash_kexec() called directly
Date: Wed, 30 Sep 2015 13:53:47 +0200	[thread overview]
Message-ID: <20150930115347.GH2881@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <04EAB7311EE43145B2D3536183D1A84454994C26@GSjpTKYDCembx31.service.hitachi.net>

On Mon, Sep 28, 2015 at 07:08:19AM +0000, 河合英宏 / KAWAI，HIDEHIRO wrote:
> > >> kernel/kexec_core.c:899:3: note: in expansion of macro 'atomic_xchg'
> >       atomic_xchg(&panic_cpu, -1);
> >       ^
> 
> I changed to use atomic_xchg() instead of atomic_set() in V3
> because atomic_set() doesn't mean memory barrier.  However,
> I thought again and there is no need of barrier; there is no
> problem if a competitor sees old value of panic_cpu or new one.
> So, atomic_set() is sufficient and using it will remove this warning.
> 
> I will resend the fixed version later.

So if you rely on the memory barrier; you should have also put a comment
on explaining the ordering requirements.

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