From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754497Ab0ALQ1s (ORCPT ); Tue, 12 Jan 2010 11:27:48 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754231Ab0ALQ1p (ORCPT ); Tue, 12 Jan 2010 11:27:45 -0500 Received: from hrndva-omtalb.mail.rr.com ([71.74.56.124]:38330 "EHLO hrndva-omtalb.mail.rr.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754160Ab0ALQ1n (ORCPT ); Tue, 12 Jan 2010 11:27:43 -0500 X-Authority-Analysis: v=1.0 c=1 a=r_nf4N-T2GkA:10 a=7U3hwN5JcxgA:10 a=eRsrllKB-jbIgT_ME3wA:9 a=q4jENQOr_ERuBBRAUweNf1WLQ1UA:4 X-Cloudmark-Score: 0 X-Originating-IP: 74.67.89.75 Subject: Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier (v3b) From: Steven Rostedt Reply-To: rostedt@goodmis.org To: Mathieu Desnoyers Cc: "Paul E. McKenney" , 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 In-Reply-To: <20100112153854.GA8122@Krystal> References: <20100110014456.GG25790@Krystal> <1263089578.2231.22.camel@frodo> <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> Content-Type: text/plain; charset="ISO-8859-15" Organization: Kihon Technologies Inc. Date: Tue, 12 Jan 2010 11:27:39 -0500 Message-ID: <1263313659.28171.3810.camel@gandalf.stny.rr.com> Mime-Version: 1.0 X-Mailer: Evolution 2.28.1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. -- Steve