linux-kernel - Re: [PATCH] x86: cmpxchg_double: Add missing memory clobber

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Message-ID: <CAJhHMCBBWi+Qggg3t=Egp+HSBGL0bVHY8yhj5F2h+L7cdAQZSA@mail.gmail.com>
Date:	Tue, 6 Oct 2015 16:29:12 -0400
From:	Pranith Kumar <bobby.prani@...il.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
	"open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: cmpxchg_double: Add missing memory clobber

On Tue, Oct 6, 2015 at 4:16 PM, H. Peter Anvin <hpa@...or.com> wrote:
>
> NAK.  We already have the "+m" for exactly this reason; adding an
> explicit memory clobber should only be used to prevent movement of
> *other* memory operations around this one (i.e. a barrier).
>

OK. If that is so, can you please explain why we need it in the
__raw_cmpxchg() case? I think it is a good idea to make cmpxchg() and
cmpxchg_double() have similar barrier semantics.

Thanks!
-- 
Pranith
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