linux-kernel - Re: [PATCH] mark read_crX() asm code as volatile

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Message-ID: <47029A32.7010803@zytor.com>
Date:	Tue, 02 Oct 2007 12:21:22 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Arjan van de Ven <arjan@...radead.org>
CC:	Kirill Korotaev <dev@...nvz.org>, Andrew Morton <akpm@...l.org>,
	Andi Kleen <ak@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	devel@...nvz.org
Subject: Re: [PATCH] mark read_crX() asm code as volatile

Arjan van de Ven wrote:
> On Tue, 02 Oct 2007 18:08:32 +0400
> Kirill Korotaev <dev@...nvz.org> wrote:
> 
>> Some gcc versions (I checked at least 4.1.1 from RHEL5 & 4.1.2 from
>> gentoo) can generate incorrect code with read_crX()/write_crX()
>> functions mix up, due to cached results of read_crX().
>>
> 
> I'm not so sure volatile is the right answer, as compared to giving the
> asm more strict contraints....
> 
> asm volatile tends to mean something else than "the result has
> changed"....
> 

One of the aspect of volatility is "the result will change in ways you 
(gcc) just don't understand."

	-hpa
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