From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from smtp107.mail.mud.yahoo.com ([209.191.85.217]:46251 "HELO smtp107.mail.mud.yahoo.com") by vger.kernel.org with SMTP id S1752115AbWCOBZY (ORCPT ); Tue, 14 Mar 2006 20:25:24 -0500 Message-ID: <44176CF9.90909@yahoo.com.au> Date: Wed, 15 Mar 2006 12:25:13 +1100 From: Nick Piggin MIME-Version: 1.0 Subject: Re: [PATCH] Document Linux's memory barriers [try #4] References: <17431.14867.211423.851470@cargo.ozlabs.ibm.com> <16835.1141936162@warthog.cambridge.redhat.com> <32068.1142371612@warthog.cambridge.redhat.com> <2301.1142380768@warthog.cambridge.redhat.com> In-Reply-To: Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-arch-owner@vger.kernel.org To: Linus Torvalds Cc: David Howells , Paul Mackerras , "Eric W. Biederman" , akpm@osdl.org, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, mingo@redhat.com, alan@redhat.com, linuxppc64-dev@ozlabs.org List-ID: Linus Torvalds wrote: > >On Tue, 14 Mar 2006, David Howells wrote: > >>But that doesn't make any sense! >> >>That would mean we that we'd've read b into d before having read the new value >>of p into q, and thus before having calculated the address from which to read d >>(ie: &b) - so how could we know we were supposed to read d from b and not from >>a without first having read p? >> >>Unless, of course, the smp_wmb() isn't effective, and the write to b happens >>after the write to p; or the Alpha's cache isn't fully coherent. >> > >The cache is fully coherent, but the coherency isn't _ordered_. > > This is what I was referring to when I said your (David's) idea of "memory" WRT memory consistency isn't correct -- cache coherency can be out of order. -- Send instant messages to your online friends http://au.messenger.yahoo.com