Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S935993AbXHNX3k (ORCPT ); Tue, 14 Aug 2007 19:29:40 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S936102AbXHNX2k (ORCPT ); Tue, 14 Aug 2007 19:28:40 -0400 Received: from e6.ny.us.ibm.com ([32.97.182.146]:59283 "EHLO e6.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S936067AbXHNX2i (ORCPT ); Tue, 14 Aug 2007 19:28:38 -0400 Date: Tue, 14 Aug 2007 16:28:35 -0700 From: "Paul E. McKenney" To: Christoph Lameter Cc: Chris Snook , linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, Linus Torvalds , akpm@linux-foundation.org, Segher Boessenkool , "Luck, Tony" , Chris Friesen , "Robert P. J. Day" Subject: Re: [PATCH 1/23] document preferred use of volatile with atomic_t Message-ID: <20070814232835.GH8243@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com 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 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.13 (2006-08-11) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1878 Lines: 39 On Tue, Aug 14, 2007 at 03:56:51PM -0700, Christoph Lameter wrote: > 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. The overhead due to volatile access is -way- small. Not like barrier(), which can flush out a fair fraction of the machine registers. Thanx, Paul - 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/