From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754536Ab0ALQyT (ORCPT ); Tue, 12 Jan 2010 11:54:19 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754214Ab0ALQyR (ORCPT ); Tue, 12 Jan 2010 11:54:17 -0500 Received: from e2.ny.us.ibm.com ([32.97.182.142]:54185 "EHLO e2.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753786Ab0ALQyQ (ORCPT ); Tue, 12 Jan 2010 11:54:16 -0500 Date: Tue, 12 Jan 2010 08:54:13 -0800 From: "Paul E. McKenney" To: Steven Rostedt Cc: Mathieu Desnoyers , Oleg Nesterov , Peter Zijlstra , linux-kernel@vger.kernel.org, Ingo Molnar , akpm@linux-foundation.org, josh@joshtriplett.org, tglx@linutronix.de, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, laijs@cn.fujitsu.com, dipankar@in.ibm.com Subject: Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier (v3b) Message-ID: <20100112165413.GA6812@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20100110052508.GG9044@linux.vnet.ibm.com> <1263124209.28171.3798.camel@gandalf.stny.rr.com> <20100110174512.GH9044@linux.vnet.ibm.com> <20100110182423.GA22821@Krystal> <20100111011705.GJ9044@linux.vnet.ibm.com> <20100111042521.GB32213@Krystal> <20100111043016.GD32213@Krystal> <20100111224355.GH6632@linux.vnet.ibm.com> <20100112153854.GA8122@Krystal> <1263313659.28171.3810.camel@gandalf.stny.rr.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1263313659.28171.3810.camel@gandalf.stny.rr.com> User-Agent: Mutt/1.5.15+20070412 (2007-04-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jan 12, 2010 at 11:27:39AM -0500, Steven Rostedt wrote: > On Tue, 2010-01-12 at 10:38 -0500, Mathieu Desnoyers wrote: > > > > The UP-kernel case is handled by the #ifdef in sys_membarrier(), though > > > with a bit larger code footprint than the embedded guys would probably > > > prefer. (Or is the compiler smart enough to omit these function given no > > > calls to them? If not, recommend putting them under CONFIG_SMP #ifdef.) > > > > Hrm, that's a bit odd. I agree that UP systems could simply return > > -ENOSYS for sys_membarrier, but then I wonder how userland could > > distinguish between: > > > > - an old kernel not supporting sys_membarrier() > > -> in this case we need to use the smp_mb() fallback on the read-side > > and in synchronize_rcu(). > > - a recent kernel supporting sys_membarrier(), CONFIG_SMP > > -> can use the barrier() on read-side, call sys_membarrier upon > > update. > > - a recent kernel supporting sys_membarrier, !CONFIG_SMP > > -> calls to sys_membarrier() are not required, nor is barrier(). > > > > Or maybe we just postpone the userland smp_mb() question to another > > thread. This will eventually need to be addressed anyway. Maybe with a > > vgetmaxcpu() vsyscall. > > I think Paul means to wrap all your other functions under the #ifdef. > What you have for sys_membarrier() is fine (just return 0 on UP) but you > also need to wrap the helper function above it under #ifdef CONFIG_SMP. > Don't rely on the compiler to optimize them out. If anything, you'll > probably get a bunch of warnings about static functions unused. Yes -- much clearer statement of what I was getting at. ;-) Thanx, Paul