From owner-freebsd-current@FreeBSD.ORG  Tue May 17 20:16:25 2011
Return-Path: <owner-freebsd-current@FreeBSD.ORG>
Delivered-To: freebsd-current@FreeBSD.org
Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34])
	by hub.freebsd.org (Postfix) with ESMTP id E78C21065673;
	Tue, 17 May 2011 20:16:25 +0000 (UTC) (envelope-from avg@FreeBSD.org)
Received: from citadel.icyb.net.ua (citadel.icyb.net.ua [212.40.38.140])
	by mx1.freebsd.org (Postfix) with ESMTP id 89F4C8FC12;
	Tue, 17 May 2011 20:16:24 +0000 (UTC)
Received: from porto.topspin.kiev.ua (porto-e.starpoint.kiev.ua
	[212.40.38.100])
	by citadel.icyb.net.ua (8.8.8p3/ICyb-2.3exp) with ESMTP id XAA03539;
	Tue, 17 May 2011 23:16:22 +0300 (EEST)
	(envelope-from avg@FreeBSD.org)
Received: from localhost.topspin.kiev.ua ([127.0.0.1])
	by porto.topspin.kiev.ua with esmtp (Exim 4.34 (FreeBSD))
	id 1QMQgo-0007df-Gf; Tue, 17 May 2011 23:16:22 +0300
Message-ID: <4DD2D795.6020106@FreeBSD.org>
Date: Tue, 17 May 2011 23:16:21 +0300
From: Andriy Gapon <avg@FreeBSD.org>
User-Agent: Mozilla/5.0 (X11; U; FreeBSD amd64; en-US;
	rv:1.9.2.17) Gecko/20110503 Lightning/1.0b2 Thunderbird/3.1.10
MIME-Version: 1.0
To: John Baldwin <jhb@FreeBSD.org>
References: <4DCD357D.6000109@FreeBSD.org> <201105170958.16847.jhb@freebsd.org>
	<4DD28781.6050002@FreeBSD.org> <201105171151.18038.jhb@freebsd.org>
In-Reply-To: <201105171151.18038.jhb@freebsd.org>
X-Enigmail-Version: 1.1.2
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
Cc: Max Laier <max@love2party.net>,
	FreeBSD current <freebsd-current@FreeBSD.org>, neel@FreeBSD.org,
	Peter Grehan <grehan@FreeBSD.org>
Subject: Re: proposed smp_rendezvous change
X-BeenThere: freebsd-current@freebsd.org
X-Mailman-Version: 2.1.5
Precedence: list
List-Id: Discussions about the use of FreeBSD-current
	<freebsd-current.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-current>, 
	<mailto:freebsd-current-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-current>
List-Post: <mailto:freebsd-current@freebsd.org>
List-Help: <mailto:freebsd-current-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-current>,
	<mailto:freebsd-current-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Tue, 17 May 2011 20:16:26 -0000

on 17/05/2011 18:51 John Baldwin said the following:
> On Tuesday, May 17, 2011 10:34:41 am Andriy Gapon wrote:
>> on 17/05/2011 16:58 John Baldwin said the following:
>>> No, it doesn't quite work that way.  It wouldn't work on Alpha for example.
>>>
>>> All load_acq is a load with a memory barrier to order other loads after it.
>>> It is still free to load stale data.  Only a read-modify-write operation
>>> would actually block until it could access an up-to-date value.
>>
>> Hmm, ok.
>> How about atomic_add_acq_int(&smp_rv_waiters[0], 0) ? :-)
>> Or an equivalent MI action that doesn't actually change smp_rv_waiters[0] value,
>> if there could be any.
>> Maybe explicit atomic_cmpset_acq_int(&smp_rv_waiters[0], 0, 0) ?
>>
>> You see at what I am getting?
> 
> Yeah, either of those would work.  At this point just leaving the
> atomic_add_int() as-is would be the smallest diff. :)

Yes, I agree about the smallest diff.
But if we are going to remove that first spin-loop, then we would effectively be
wasting one volatile variable.  Only a tiny waste, but it could be confusing.

Whatever we decide, this was very educational for me.
-- 
Andriy Gapon