From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755075Ab0KWPDh (ORCPT ); Tue, 23 Nov 2010 10:03:37 -0500 Received: from mtagate3.uk.ibm.com ([194.196.100.163]:52748 "EHLO mtagate3.uk.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754132Ab0KWPDf (ORCPT ); Tue, 23 Nov 2010 10:03:35 -0500 Subject: Re: [PATCH] mutex: Introduce arch_mutex_cpu_relax() From: Gerald Schaefer Reply-To: gerald.schaefer@de.ibm.com To: Peter Zijlstra Cc: Andrew Morton , Ingo Molnar , Martin Schwidefsky , LKML , linux-s390 , Heiko Carstens In-Reply-To: <1290522000.2072.406.camel@laptop> References: <1290437256.7455.4.camel@thinkpad> <20101122121049.01c4690a.akpm@linux-foundation.org> <1290521556.16834.25.camel@thinkpad> <1290522000.2072.406.camel@laptop> Content-Type: text/plain; charset="UTF-8" Date: Tue, 23 Nov 2010 16:03:32 +0100 Message-ID: <1290524612.16834.69.camel@thinkpad> Mime-Version: 1.0 X-Mailer: Evolution 2.28.3 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Di, 2010-11-23 at 15:20 +0100, Peter Zijlstra wrote: > On Tue, 2010-11-23 at 15:12 +0100, Gerald Schaefer wrote: > > On Mo, 2010-11-22 at 12:10 -0800, Andrew Morton wrote: > > > On Mon, 22 Nov 2010 15:47:36 +0100 > > > Gerald Schaefer wrote: > > > > > > > From: Gerald Schaefer > > > > > > > > The spinning mutex implementation uses cpu_relax() in busy loops as a > > > > compiler barrier. Depending on the architecture, cpu_relax() may do more > > > > than needed in this specific mutex spin loops. On System z we also give > > > > up the time slice of the virtual cpu in cpu_relax(), which prevents > > > > effective spinning on the mutex. > > > > > > > > This patch replaces cpu_relax() in the spinning mutex code with > > > > arch_mutex_cpu_relax(), which can be defined by each architecture that > > > > selects HAVE_ARCH_MUTEX_CPU_RELAX. The default is still cpu_relax(), so > > > > this patch should not affect other architectures than System z for now. > > > > > > > > ... > > > > > > > > --- a/include/linux/mutex.h > > > > +++ b/include/linux/mutex.h > > > > @@ -160,4 +160,8 @@ extern int mutex_trylock(struct mutex *l > > > > extern void mutex_unlock(struct mutex *lock); > > > > extern int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock); > > > > > > > > +#ifndef CONFIG_HAVE_ARCH_MUTEX_CPU_RELAX > > > > +#define arch_mutex_cpu_relax() cpu_relax() > > > > +#endif > > > > > > A simpler way of doing this is to remove the CONFIG_ variable > > > altogether and do > > > > > > #ifndef arch_mutex_cpu_relax > > > #define arch_mutex_cpu_relax() cpu_relax() > > > #endif > > > > > > When doing this, one should be clear about _which_ arch file has the > > > responsibility of defining arch_mutex_cpu_relax, and make sure that > > > this arch file is reliably included in the .c file. > > > > Well, I've tried that with my last approach, defining arch_mutex_cpu_relax() > > in and including that from . This didn't work > > well because of ugly header file dependencies, and Peter also commented > > that "including "asm/mutex.h" isn't advised". The problem is the following > > code in kernel/mutex.c (after including ) when > > CONFIG_DEBUG_MUTEXES is set: > > > > #ifdef CONFIG_DEBUG_MUTEXES > > # include "mutex-debug.h" > > # include > > #else > > # include "mutex.h" > > # include > > #endif > > > > So I can only include from with an ugly > > "#ifndef CONFIG_DEBUG_MUTEXES" around it, or use a completely different > > or new arch header file (but seems like the right place > > for this). The CONFIG_ approach avoids all this header file dependency > > mess, or did I miss something (or maybe it's just me and it is not ugly > > at all)? > > Yeah, that all cause massive grief.. I've applied your patch as is, > assuming s390 already has the needed arch_mutex_cpu_relax() > implementation (otherwise I've just broken my s390 build). Thanks, my patch already includes the s390 implementation for arch_mutex_cpu_relax() in arch/s390/include/asm/mutex.h (we use a simple barrier() in this case). We still have HAVE_DEFAULT_NO_SPIN_MUTEXES selected though, this will be removed with one of Martins next patches, once this preparation patch is upstream.