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

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Message-ID: <4C3E096B.8050505@zytor.com>
Date:	Wed, 14 Jul 2010 12:00:59 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"H.J. Lu" <hjl.tools@...il.com>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>,
	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 11:18 AM, H.J. Lu wrote:
> 
> There are some discussions on:
> 
> http://gcc.gnu.org/ml/gcc-patches/2010-06/msg02001.html
> http://gcc.gnu.org/ml/gcc-patches/2010-07/msg00001.html
> 
> Are they related?
> 

Not directly as far as I can tell.

The issue is if gcc can ever reorder, duplicate or elide a volatile
operation (either asm volatile or a volatile-annotated memory
reference.)  In my (and Linus') opinion, this would be an incredibly
serious bug.

The gcc 4.4 issue, which is separate from the definition issue, is that
it seems to assume that it can elide references to "const volatile"
objects.  "const volatile" should mean a value that could change at any
time but which is a bug to write to -- for example a readonly hardware
register.

	-hpa

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