Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758510AbZGCOGQ (ORCPT ); Fri, 3 Jul 2009 10:06:16 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1758466AbZGCOEE (ORCPT ); Fri, 3 Jul 2009 10:04:04 -0400 Received: from tomts13.bellnexxia.net ([209.226.175.34]:58996 "EHLO tomts13-srv.bellnexxia.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758554AbZGCOEB (ORCPT ); Fri, 3 Jul 2009 10:04:01 -0400 X-IronPort-Anti-Spam-Filtered: true X-IronPort-Anti-Spam-Result: AgoFAL+rTUpMQWU3/2dsb2JhbACBUcxIhBIFgTo Date: Fri, 3 Jul 2009 10:04:01 -0400 From: Mathieu Desnoyers To: Ingo Molnar Cc: Jiri Olsa , Peter Zijlstra , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, fbl@redhat.com, nhorman@redhat.com, davem@redhat.com, htejun@gmail.com, jarkao2@gmail.com, oleg@redhat.com, davidel@xmailserver.org, eric.dumazet@gmail.com, Paul McKenney Subject: Re: [PATCHv5 2/2] memory barrier: adding smp_mb__after_lock Message-ID: <20090703140401.GA10256@Krystal> References: <20090703081219.GE2902@jolsa.lab.eng.brq.redhat.com> <20090703081445.GG2902@jolsa.lab.eng.brq.redhat.com> <20090703090606.GA3902@elte.hu> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Content-Disposition: inline In-Reply-To: <20090703090606.GA3902@elte.hu> X-Editor: vi X-Info: http://krystal.dyndns.org:8080 X-Operating-System: Linux/2.6.21.3-grsec (i686) X-Uptime: 09:49:36 up 125 days, 10:15, 3 users, load average: 0.12, 0.40, 0.41 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 Content-Length: 1782 Lines: 60 * Ingo Molnar (mingo@elte.hu) wrote: > > * Jiri Olsa wrote: > > > +++ b/arch/x86/include/asm/spinlock.h > > @@ -302,4 +302,7 @@ static inline void __raw_write_unlock(raw_rwlock_t *rw) > > #define _raw_read_relax(lock) cpu_relax() > > #define _raw_write_relax(lock) cpu_relax() > > > > +/* The {read|write|spin}_lock() on x86 are full memory barriers. */ > > +#define smp_mb__after_lock() do { } while (0) > Hm. Looking at http://lkml.org/lkml/2009/6/23/192, a very basic question comes to my mind : Why don't we create a read_lock without acquire semantic instead (e.g. read_lock_nomb(), or something with a better name like __read_lock()) ? On architectures where memory barriers are needed to provide the acquire semantic, it would be faster to do : __read_lock(); smp_mb(); than : read_lock(); <- e.g. lwsync + isync or something like that smp_mb(); <- full sync. Second point : __add_wait_queue/waitqueue_active/wake_up_interruptible would probably benefit from adding comments about their combined use with other checks and how nice memory barriers are. Mathieu > Two small stylistic comments, please make this an inline function: > > static inline void smp_mb__after_lock(void) { } > #define smp_mb__after_lock > > (untested) > > > +/* The lock does not imply full memory barrier. */ > > +#ifndef smp_mb__after_lock > > +#define smp_mb__after_lock() smp_mb() > > +#endif > > ditto. > > Ingo -- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/