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

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Message-ID: <4C3FE4A4.3000902@zytor.com>
Date:	Thu, 15 Jul 2010 21:48:36 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Zachary Amsden <zamsden@...hat.com>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>, Avi Kivity <avi@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Palfrader <peter@...frader.org>,
	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>,
	Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [patch 134/149] x86, paravirt: Add a global synchronization point
 	for pvclock

On 07/14/2010 01:45 PM, Zachary Amsden wrote:
> That's the kind of bug I think Linus is talking about.  We've been
> expecting volatile to work that way for over a decade, by my
> recollection, and if it doesn't, there is going to be a lot of broken code.
> 
> Shouldn't we at least get a compiler switch to force the volatile
> behavior?  I'd suggest it default to conservative.

At this point, it looks like there is no reason to be alarmed.  The
documentation actually contains a statement about volatiles not being
mutually reordered across sequence points, and since asm is a statement
(rather than an expression) it is always surrounded by sequence points.
 I have filed a gcc ticket to ask for clarification.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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