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

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Message-Id: <200710022214.55381.nickpiggin@yahoo.com.au>
Date:	Tue, 2 Oct 2007 22:14:54 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Chuck Ebbert <cebbert@...hat.com>
Cc:	Arjan van de Ven <arjan@...radead.org>,
	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 Wednesday 03 October 2007 04:27, Chuck Ebbert wrote:
> 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);
>
> ?

I don't see how, as write_cr3 clobbers memory.
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