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Date: Wed, 30 Sep 2015 13:53:47 +0200
From: Peter Zijlstra <peterz@infradead.org>
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	<hidehiro.kawai.ez@hitachi.com>
Cc: "kbuild-all@01.org" <kbuild-all@01.org>, Jonathan Corbet <corbet@lwn.net>,
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	<masami.hiramatsu.pt@hitachi.com>
Subject: Re: Re: [V4 PATCH 3/4] kexec: Fix race between panic() and
 crash_kexec() called directly
Message-ID: <20150930115347.GH2881@worktop.programming.kicks-ass.net>
References: <20150925112809.4258.39950.stgit@softrs>
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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.