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Message-ID: <04EAB7311EE43145B2D3536183D1A8445499CCB8@GSjpTKYDCembx31.service.hitachi.net> 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(); > ...
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