From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1762772AbXGNSmY (ORCPT ); Sat, 14 Jul 2007 14:42:24 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1759466AbXGNSmQ (ORCPT ); Sat, 14 Jul 2007 14:42:16 -0400 Received: from pentafluge.infradead.org ([213.146.154.40]:52965 "EHLO pentafluge.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759305AbXGNSmQ (ORCPT ); Sat, 14 Jul 2007 14:42:16 -0400 Subject: Re: [PATCH -rt 4/5] use migrate_disable for __local_begin From: Peter Zijlstra To: Mathieu Desnoyers Cc: linux-kernel@vger.kernel.org, Ingo Molnar , Thomas Gleixner , Oleg Nesterov , Steven Rostedt , Christoph Lameter In-Reply-To: <20070714183557.GG6975@Krystal> References: <20070714175733.194012000@chello.nl> <20070714175840.153584000@chello.nl> <20070714171644.GC6975@Krystal> <1184434342.5284.49.camel@lappy> <20070714183557.GG6975@Krystal> Content-Type: text/plain Date: Sat, 14 Jul 2007 20:41:40 +0200 Message-Id: <1184438500.5284.64.camel@lappy> Mime-Version: 1.0 X-Mailer: Evolution 2.10.1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Sat, 2007-07-14 at 14:35 -0400, Mathieu Desnoyers wrote: > * Peter Zijlstra (a.p.zijlstra@chello.nl) wrote: > > On Sat, 2007-07-14 at 13:16 -0400, Mathieu Desnoyers wrote: > > > * Peter Zijlstra (a.p.zijlstra@chello.nl) wrote: > > > > Signed-off-by: Peter Zijlstra > > > > --- > > > > include/asm-i386/local.h | 7 ++++--- > > > > include/asm-x86_64/local.h | 7 ++++--- > > > > 2 files changed, 8 insertions(+), 6 deletions(-) > > > > > > > > Index: linux-2.6/include/asm-i386/local.h > > > > =================================================================== > > > > --- linux-2.6.orig/include/asm-i386/local.h > > > > +++ linux-2.6/include/asm-i386/local.h > > > > @@ -197,11 +197,12 @@ static __inline__ long local_sub_return( > > > > #define __local_begin(__flags) \ > > > > { \ > > > > (__flags) = 0; \ > > > > - preempt_disable(); \ > > > > + migrate_disable(); \ > > > > > > Brrrr. That's wrong. Your non atomic __local*() updates only makes sense > > > when preempt_disable/enable() protects them from concurrent threads on > > > the same CPU, which is not the case of migrate_disable/enable(). This is > > > why I suggest that you use local_begin/end() mapped to > > > migrate_disable/enable() for normal local variables, and, if you really > > > want a __local_begin/end(), then it should be mapped to > > > preempt_disable/enable() and should state that it provides no protection > > > against interrupts. > > > > Sure, but on -rt it does suffice, this part of the patch is rather WIP. > > > > > > Hrm, how can it suffice, I wonder ? migrate_disable() does not protect > against other threads on the same CPU, so you could suffer from > concurrent updates to the same variables. How is it different in -rt ? I thought the idea was that all these local_* operation were atomic wrt to the local cpu. The only difference with -rt is that we generally don't care about interrupts. Anyway, I'm dropping all this local stuff, and just hard code it right into slub.c