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

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Message-ID: <20070930120545.GB7697@wotan.suse.de>
Date:	Sun, 30 Sep 2007 14:05:45 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Davide Libenzi <davidel@...ilserver.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Paul McKenney <paulmck@...ibm.com>,
	David Howells <dhowells@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andi Kleen <ak@...e.de>
Subject: Re: [rfc][patch] i386: remove comment about barriers

On Sat, Sep 29, 2007 at 12:12:52PM -0700, Davide Libenzi wrote:
> On Sat, 29 Sep 2007, Nick Piggin 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? ]
> 
> No, that can't be. rmb+wmb can't be considered a full mb.

Oh I realise it doesn't work with current implementations (powerpc at
least also would be broken I think). And I've always assumed no... But
the question was just whether the Linux memory model requires it (it
theoretically could require that smp_rmb is sequentially ordered WRT
smp_wmb without actually sequentially ordering stores around itself).

> There was a 
> recent discussion about this in the thread originated by peterz scalable 
> rw_mutex patches.

OK, good to hear it has been discussed. I didn't see anything explicit
in the documentation about it.  A lot of the literature often says that
"barrier instructions" have a sequential ordering, so it could be useful
to explicitly say this isn't the case for Linux for rmb vs wmb.

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