From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752915Ab3KDJ6G (ORCPT ); Mon, 4 Nov 2013 04:58:06 -0500 Received: from cam-admin0.cambridge.arm.com ([217.140.96.50]:46602 "EHLO cam-admin0.cambridge.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750822Ab3KDJ6E (ORCPT ); Mon, 4 Nov 2013 04:58:04 -0500 Date: Mon, 4 Nov 2013 09:57:17 +0000 From: Will Deacon To: "Paul E. McKenney" Cc: Peter Zijlstra , Victor Kaplansky , Oleg Nesterov , Anton Blanchard , Benjamin Herrenschmidt , Frederic Weisbecker , LKML Subject: Re: perf events ring buffer memory barrier on powerpc Message-ID: <20131104095717.GB8419@mudshark.cambridge.arm.com> References: <20131028132634.GO19466@laptop.lan> <20131028163418.GD4126@linux.vnet.ibm.com> <20131028201735.GA15629@redhat.com> <20131103144017.GA25118@linux.vnet.ibm.com> <20131103170759.GC6871@mudshark.cambridge.arm.com> <20131103224712.GH3947@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20131103224712.GH3947@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Paul, On Sun, Nov 03, 2013 at 10:47:12PM +0000, Paul E. McKenney wrote: > On Sun, Nov 03, 2013 at 05:07:59PM +0000, Will Deacon wrote: > > On Sun, Nov 03, 2013 at 02:40:17PM +0000, Paul E. McKenney wrote: > > > On Sat, Nov 02, 2013 at 10:32:39AM -0700, Paul E. McKenney wrote: > > > > On Fri, Nov 01, 2013 at 03:56:34PM +0100, Peter Zijlstra wrote: > > > > > On Wed, Oct 30, 2013 at 11:40:15PM -0700, Paul E. McKenney wrote: > > > > > > > Now the whole crux of the question is if we need barrier A at all, since > > > > > > > the STORES issued by the @buf writes are dependent on the ubuf->tail > > > > > > > read. > > > > > > > > > > > > The dependency you are talking about is via the "if" statement? > > > > > > Even C/C++11 is not required to respect control dependencies. > > > > > > > > > > > > This one is a bit annoying. The x86 TSO means that you really only > > > > > > need barrier(), ARM (recent ARM, anyway) and Power could use a weaker > > > > > > barrier, and so on -- but smp_mb() emits a full barrier. > > > > > > > > > > > > Perhaps a new smp_tmb() for TSO semantics, where reads are ordered > > > > > > before reads, writes before writes, and reads before writes, but not > > > > > > writes before reads? Another approach would be to define a per-arch > > > > > > barrier for this particular case. > > > > > > > > > > I suppose we can only introduce new barrier primitives if there's more > > > > > than 1 use-case. > > > > Which barrier did you have in mind when you refer to `recent ARM' above? It > > seems to me like you'd need a combination if dmb ishld and dmb ishst, since > > the former doesn't order writes before writes. > > I heard a rumor that ARM had recently added a new dmb variant that acted > similarly to PowerPC's lwsync, and it was on my list to follow up. > > Given your response, I am guessing that there is no truth to this rumor... I think you're talking about the -ld option to dmb, which was introduced in ARMv8. That option orders loads against loads and stores, but doesn't order writes against writes. So you could do: dmb ishld dmb ishst but it's questionable whether that performs better than a dmb ish. Will