From: Nick Piggin <nickpiggin@yahoo.com.au>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Chuck Ebbert <cebbert@redhat.com>,
Arjan van de Ven <arjan@infradead.org>,
Kirill Korotaev <dev@openvz.org>, Andrew Morton <akpm@osdl.org>,
Andi Kleen <ak@suse.de>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
devel@openvz.org
Subject: Re: [PATCH] mark read_crX() asm code as volatile
Date: Wed, 3 Oct 2007 01:49:24 +1000 [thread overview]
Message-ID: <200710030149.24914.nickpiggin@yahoo.com.au> (raw)
In-Reply-To: <4703342E.3020101@zytor.com>
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?
next prev parent reply other threads:[~2007-10-03 8:40 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-10-02 14:08 [PATCH] mark read_crX() asm code as volatile Kirill Korotaev
2007-10-02 14:17 ` Andi Kleen
2007-10-02 15:28 ` Arjan van de Ven
2007-10-02 18:27 ` Chuck Ebbert
2007-10-02 12:14 ` Nick Piggin
2007-10-03 6:18 ` H. Peter Anvin
2007-10-02 15:49 ` Nick Piggin [this message]
2007-10-03 8:45 ` Andi Kleen
2007-10-02 19:21 ` H. Peter Anvin
2007-10-03 8:25 ` Kirill Korotaev
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