linux-kernel - RE: Re: [V4 PATCH 3/4] kexec: Fix race between panic() and crash

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Date:	Thu, 1 Oct 2015 02:04:55 +0000
From:	河合英宏 / KAWAI，HIDEHIRO 
	<hidehiro.kawai.ez@...achi.com>
To:	"'Peter Zijlstra'" <peterz@...radead.org>
CC:	"kbuild-all@...org" <kbuild-all@...org>,
	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: Re: [V4 PATCH 3/4] kexec: Fix race between panic() and
 crash_kexec() called directly

> 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.

I don't intend to use an explicit memory barrier.  There is no
memory ordering requirement here.  Also, atomic_set() which will be
used instead of atomic_xchg() is used as a RELEASE operation, so
I believe there is no problem.

Documentation/memory-barriers.txt:
> The following operations are potential problems as they do _not_ imply memory
> barriers, but might be used for implementing such things as RELEASE-class
> operations:
> 
>         atomic_set();
> ...