linux-kernel - Re: [RFC PATCH] x86: Use volatile semantics for atomic

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Date:	Tue, 23 Sep 2014 10:12:46 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Pranith Kumar <bobby.prani@...il.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"maintainer:X86 ARCHITECTURE..." <x86@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"open list:X86 ARCHITECTURE..." <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] x86: Use volatile semantics for atomic_set()

On Mon, Sep 22, 2014 at 05:42:16PM -0400, Pranith Kumar wrote:
> On Mon, Sep 22, 2014 at 4:08 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Mon, Sep 22, 2014 at 03:39:21PM -0400, Pranith Kumar wrote:
> >> All the atomic operations have volatile semantics to prevent compiler
> >> reordering. Enforce the same semantics for atomic_set() too.
> >
> > To what point? atomic_set() is typically not what you expect it to
> > anyhow.
> 
> This is the only atomic operation which allows compiler reordering. I
> am not sure why this exception was made. Any ideas as to why we do
> this?

Because it typically doesn't matter. And you should never use
atomic_set() where it does matter.
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