linux-kernel - Re: [rfc][patch] i386: remove comment about barriers

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Message-ID: <29721.1191244484@redhat.com>
Date:	Mon, 01 Oct 2007 14:14:44 +0100
From:	David Howells <dhowells@...hat.com>
To:	Nick Piggin <npiggin@...e.de>
Cc:	dhowells@...hat.com,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Paul McKenney <paulmck@...ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andi Kleen <ak@...e.de>
Subject: Re: [rfc][patch] i386: remove comment about barriers

Nick Piggin <npiggin@...e.de> wrote:

> [ This is true for x86's sfence/lfence, but raises a question about Linux's
> memory barriers. Does anybody expect that a sequence of smp_wmb and smp_rmb
> would ever provide a full smp_mb barrier? I've always assumed no, but I
> don't know if it is actually documented? ]

I think you have to assume that smp_wmb() only orders stores and write
barriers, and that smp_rmb() only orders reads and read barriers.

smp_mb() implies both smp_wmb() and smp_rmb(), but is greater than the
combination of the two.

David
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