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

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Message-ID: <47028D99.8020100@redhat.com>
Date:	Tue, 02 Oct 2007 14:27:37 -0400
From:	Chuck Ebbert <cebbert@...hat.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

On 10/02/2007 11:28 AM, 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"....

It means "don't eliminate this code if it's reachable" which should be
just enough for this case. But it could still be reordered in some cases
that could break, I think.

This should work because the result gets used before reading again:

read_cr3(a);
write_cr3(a | 1);
read_cr3(a);

But this might be reordered so that b gets read before the write:

read_cr3(a);
write_cr3(a | 1);
read_cr3(b);

?
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