From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner+w=401wt.eu-S1755321AbZBRRF7@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1755321AbZBRRF7 (ORCPT <rfc822;w@1wt.eu>);
	Wed, 18 Feb 2009 12:05:59 -0500
Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753351AbZBRRFr
	(ORCPT <rfc822;linux-kernel-outgoing>);
	Wed, 18 Feb 2009 12:05:47 -0500
Received: from mx2.mail.elte.hu ([157.181.151.9]:44070 "EHLO mx2.mail.elte.hu"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1753122AbZBRRFq (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
	Wed, 18 Feb 2009 12:05:46 -0500
Date: Wed, 18 Feb 2009 18:05:17 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Suresh Siddha <suresh.b.siddha@intel.com>,
       "Pallipadi, Venkatesh" <venkatesh.pallipadi@intel.com>,
       Yinghai Lu <yinghai@kernel.org>, Nick Piggin <npiggin@suse.de>,
       "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
       Oleg Nesterov <oleg@redhat.com>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>,
       Jens Axboe <jens.axboe@oracle.com>,
       Rusty Russell <rusty@rustcorp.com.au>,
       Steven Rostedt <rostedt@goodmis.org>, linux-kernel@vger.kernel.org,
       linux-arch@vger.kernel.org
Subject: Re: Q: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove
	single ipi fallback for smp_call_function_many())
Message-ID: <20090218170517.GA9769@elte.hu>
References: <20090217112657.GE26402@wotan.suse.de> <20090217192810.GA4980@redhat.com> <20090217213256.GJ6761@linux.vnet.ibm.com> <20090217214518.GA13189@redhat.com> <20090217223910.GM6761@linux.vnet.ibm.com> <20090218135212.GB23125@wotan.suse.de> <alpine.LFD.2.00.0902180744520.21686@localhost.localdomain> <20090218162116.GC29863@elte.hu> <alpine.LFD.2.00.0902180828430.21686@localhost.localdomain> <20090218165808.GA9120@elte.hu>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20090218165808.GA9120@elte.hu>
User-Agent: Mutt/1.5.18 (2008-05-17)
X-ELTE-VirusStatus: clean
X-ELTE-SpamScore: -1.5
X-ELTE-SpamLevel: 
X-ELTE-SpamCheck: no
X-ELTE-SpamVersion: ELTE 2.0 
X-ELTE-SpamCheck-Details: score=-1.5 required=5.9 tests=BAYES_00 autolearn=no SpamAssassin version=3.2.3
	-1.5 BAYES_00               BODY: Bayesian spam probability is 0 to 1%
	[score: 0.0000]
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org


an off-list comment pointed out this piece of information as 
well:

  http://www.sandpile.org/ia32/coherent.htm

  A WRMSR to one of the x2APIC MSRs (0000_0800h...0000_0BFFh) is 
  not guaranteed to be serializing.

So i suspect we should just enclose it in smp_mb() pairs to make 
sure it's a full barrier in both directions?

Although since it's an explicitly async mechanism with no 
ordering expectations whatsoever, just doing an smp_mb() before 
it should be enough, to make sure the IPI can never arrive to 
another CPU/core faster than the event-horizon/visibility of 
preceding operations.

	Ingo