Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S936978AbXHNW73 (ORCPT ); Tue, 14 Aug 2007 18:59:29 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S936662AbXHNW4y (ORCPT ); Tue, 14 Aug 2007 18:56:54 -0400 Received: from netops-testserver-4-out.sgi.com ([192.48.171.29]:40014 "EHLO relay.sgi.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S936651AbXHNW4w (ORCPT ); Tue, 14 Aug 2007 18:56:52 -0400 Date: Tue, 14 Aug 2007 15:56:51 -0700 (PDT) From: Christoph Lameter X-X-Sender: clameter@schroedinger.engr.sgi.com To: Chris Snook cc: linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, Linus Torvalds , akpm@linux-foundation.org, paulmck@linux.vnet.ibm.com, Segher Boessenkool , "Luck, Tony" , Chris Friesen , "Robert P. J. Day" Subject: Re: [PATCH 1/23] document preferred use of volatile with atomic_t In-Reply-To: <46C2325C.8040900@redhat.com> Message-ID: References: <46C03885.7000109@redhat.com> <20070813110415.GA24018@shell.boston.redhat.com> <46C2325C.8040900@redhat.com> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1602 Lines: 33 On Tue, 14 Aug 2007, Chris Snook wrote: > > volatile means that there is some vague notion of "read it now". But that > > really does not exist. Instead we control visibility via barriers (smp_wmb, > > smp_rmb). Would it not be best to not have volatile at all in atomic > > operations and let the barriers do the work? > > From my reply in the other thread... > > But barriers force a flush of *everything* in scope, which we generally don't > want. On the other hand, we pretty much always want to flush atomic_* > operations. One way or another, we should be restricting the volatile > behavior to the thing that needs it. On most architectures, this patch set > just moves that from the declaration, where it is considered harmful, to the > use, where it is considered an occasional necessary evil. > > If you really, *really* distrust the compiler that much, you shouldn't be > using barrier, since that uses volatile under the hood too. You should just > go ahead and implement the atomic operations in assembler, like Segher > Boessenkool did for powerpc in response to my previous patchset. >From my reply on the other thread: Maybe we need two read functions? One volatile, one not? The atomic_read()s that I have in slub really do not care about when the variables are read. And if volatile creates overhead then I rather not have it. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/