From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756642Ab0CCVgX (ORCPT ); Wed, 3 Mar 2010 16:36:23 -0500 Received: from mail.openrapids.net ([64.15.138.104]:57589 "EHLO blackscsi.openrapids.net" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1753284Ab0CCVgQ (ORCPT ); Wed, 3 Mar 2010 16:36:16 -0500 Date: Wed, 3 Mar 2010 16:36:14 -0500 From: Mathieu Desnoyers To: Masami Hiramatsu Cc: Ingo Molnar , lkml , systemtap , DLE , Jason Baron Subject: Re: [PATCH -tip 2/2] x86: Issue at least one memory barrier in stop_machine_text_poke(). Message-ID: <20100303213614.GA29880@Krystal> References: <20100303212656.25645.48466.stgit@localhost6.localdomain6> <20100303212703.25645.50986.stgit@localhost6.localdomain6> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20100303212703.25645.50986.stgit@localhost6.localdomain6> X-Editor: vi X-Info: http://www.efficios.com X-Operating-System: Linux/2.6.26-2-686 (i686) X-Uptime: 16:33:02 up 40 days, 10 min, 6 users, load average: 1.29, 0.60, 0.64 User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Masami Hiramatsu (mhiramat@redhat.com) wrote: > Fix stop_machine_text_poke() to issue smp_mb() before exiting waiting > loop. Also, use ACCESS_ONCE() to check a flag according to Mathieu's > comment. > > Signed-off-by: Masami Hiramatsu > Cc: Mathieu Desnoyers > Cc: Ingo Molnar > Cc: Jason Baron > --- > > arch/x86/kernel/alternative.c | 6 +++--- > 1 files changed, 3 insertions(+), 3 deletions(-) > > diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c > index 635e4f4..3236a11 100644 > --- a/arch/x86/kernel/alternative.c > +++ b/arch/x86/kernel/alternative.c > @@ -596,9 +596,9 @@ static int __kprobes stop_machine_text_poke(void *data) > smp_wmb(); /* Make sure other cpus see that this has run */ > wrote_text = 1; > } else { > - while (!wrote_text) > - smp_rmb(); > - sync_core(); > + while (!ACCESS_ONCE(wrote_text)) Well, cpu_relax() has a "memory" clobber, so the access once is not required to ensure the variable is re-read. And, sorry to contradict my previous statement somewhat, but given that we don't care if the compiler fetches wrote_text in chunks or not, ACCESS_ONCE() is not required here. So rather than leaving people wondering why we put an ACCESS_ONCE() here, it's probably better to leave it out. Thanks, Mathieu > + cpu_relax(); > + smp_mb(); /* Load wrote_text before following execution */ > } > > flush_icache_range((unsigned long)tpp->addr, > > > -- > Masami Hiramatsu > > Software Engineer > Hitachi Computer Products (America), Inc. > Software Solutions Division > > e-mail: mhiramat@redhat.com -- Mathieu Desnoyers Operating System Efficiency Consultant EfficiOS Inc. http://www.efficios.com