From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759515Ab1IIRfK (ORCPT ); Fri, 9 Sep 2011 13:35:10 -0400 Received: from e3.ny.us.ibm.com ([32.97.182.143]:54995 "EHLO e3.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759356Ab1IIRfI (ORCPT ); Fri, 9 Sep 2011 13:35:08 -0400 Date: Fri, 9 Sep 2011 10:34:55 -0700 From: "Paul E. McKenney" To: Olof Johansson Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca, josh@joshtriplett.org, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, eric.dumazet@gmail.com, darren@dvhart.com, patches@linaro.org, anton@samba.org, benh@kernel.crashing.org, paulus@samba.org, linuxppc-dev@ozlabs.org Subject: Re: [PATCH tip/core/rcu 48/55] powerpc: strengthen value-returning-atomics memory barriers Message-ID: <20110909173455.GH2435@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20110906180015.GA2560@linux.vnet.ibm.com> <1315332049-2604-48-git-send-email-paulmck@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.5.20 (2009-06-14) x-cbid: 11090917-8974-0000-0000-000000085258 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Sep 09, 2011 at 10:23:33AM -0700, Olof Johansson wrote: > [+linuxppc-dev] > > On Tue, Sep 6, 2011 at 11:00 AM, Paul E. McKenney > wrote: > > The trailing isync/lwsync in PowerPC value-returning atomics needs > > to be a sync in order to provide the required ordering properties. > > The leading lwsync/eieio can remain, as the remainder of the required > > ordering guarantees are provided by the atomic instructions: Any > > reordering will cause the stwcx to fail, which will result in a retry. > > Admittedly, my powerpc barrier memory is starting to fade, but isn't > isync sufficient here? It will make sure all instructions before it > have retired, and will restart any speculative/issued instructions > beyond it. > > lwsync not being sufficient makes sense since a load can overtake it. As I understand it, although isync waits for the prior stwcx to execute, it does not guarantee that the corresponding store is visible to all processors before any following loads. > > diff --git a/arch/powerpc/include/asm/synch.h b/arch/powerpc/include/asm/synch.h > > index d7cab44..4d97fbe 100644 > > --- a/arch/powerpc/include/asm/synch.h > > +++ b/arch/powerpc/include/asm/synch.h > > @@ -37,11 +37,7 @@ static inline void isync(void) > >  #endif > > > >  #ifdef CONFIG_SMP > > -#define __PPC_ACQUIRE_BARRIER                          \ > > -       START_LWSYNC_SECTION(97);                       \ > > -       isync;                                          \ > > -       MAKE_LWSYNC_SECTION_ENTRY(97, __lwsync_fixup); > > -#define PPC_ACQUIRE_BARRIER    "\n" stringify_in_c(__PPC_ACQUIRE_BARRIER) > > +#define PPC_ACQUIRE_BARRIER    "\n" stringify_in_c(sync;) > > This can just be done as "\n\tsync\n" instead of the stringify stuff. That does sound a bit more straightforward, now that you mention it. ;-) Thanx, Paul