linux-kernel - Re: [patch 134/149] x86, paravirt: Add a global synchronization point for pvclock

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Message-ID: <4C3DF519.6030406@goop.org>
Date:	Wed, 14 Jul 2010 10:34:17 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Palfrader <peter@...frader.org>,
	Avi Kivity <avi@...hat.com>, Greg KH <gregkh@...e.de>,
	linux-kernel@...r.kernel.org, stable@...nel.org,
	stable-review@...nel.org, akpm@...ux-foundation.org,
	alan@...rguk.ukuu.org.uk, Glauber Costa <glommer@...hat.com>,
	Zachary Amsden <zamsden@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [patch 134/149] x86, paravirt: Add a global synchronization point
 	for pvclock

On 07/14/2010 10:30 AM, H. Peter Anvin wrote:
> If gcc ever starts reordering volatile operations, including "asm
> volatile", the kernel will break, and will be unfixable.  Just about
> every single driver will break.  All over the kernel we're explicitly or
> implicitly making the assumption that volatile operations are strictly
> ordered by the compiler with respect to each other.
>   

Can you give an example?  All the cases I've seen rely on the ordering
properties of "memory" clobbers, which is sound.  (And volatile
variables are a completely unrelated issue, of course.)

    J
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