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

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Date:	Wed, 3 Oct 2007 01:49:24 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Chuck Ebbert <cebbert@...hat.com>,
	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 16:18, H. Peter Anvin wrote:
> Nick Piggin wrote:
> >> 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.
>
> Because read_cr3() doesn't depend on memory, and b could be stored in a
> register.

How does the compiler know it doesn't depend on memory?

How do you say it depends on memory? You really need something
as heavy as volatile?
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