From owner-freebsd-current@FreeBSD.ORG  Thu Apr  2 08:53:47 2009
Return-Path: <owner-freebsd-current@FreeBSD.ORG>
Delivered-To: current@freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34])
	by hub.freebsd.org (Postfix) with ESMTP id D7434106566B
	for <current@freebsd.org>; Thu,  2 Apr 2009 08:53:46 +0000 (UTC)
	(envelope-from luigi@onelab2.iet.unipi.it)
Received: from onelab2.iet.unipi.it (onelab2.iet.unipi.it [131.114.9.129])
	by mx1.freebsd.org (Postfix) with ESMTP id 9B19D8FC28
	for <current@freebsd.org>; Thu,  2 Apr 2009 08:53:46 +0000 (UTC)
	(envelope-from luigi@onelab2.iet.unipi.it)
Received: by onelab2.iet.unipi.it (Postfix, from userid 275)
	id E72B173098; Thu,  2 Apr 2009 10:58:54 +0200 (CEST)
Date: Thu, 2 Apr 2009 10:58:54 +0200
From: Luigi Rizzo <rizzo@iet.unipi.it>
To: Kostik Belousov <kostikbel@gmail.com>
Message-ID: <20090402085854.GA2237@onelab2.iet.unipi.it>
References: <20090402070605.GA96848@onelab2.iet.unipi.it>
	<20090402084833.GZ31897@deviant.kiev.zoral.com.ua>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20090402084833.GZ31897@deviant.kiev.zoral.com.ua>
User-Agent: Mutt/1.4.2.3i
Cc: current@freebsd.org
Subject: Re: cmpxchg / atomic_cmpset_int emulation for userland (i386)  ?
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: Thu, 02 Apr 2009 08:53:47 -0000

On Thu, Apr 02, 2009 at 11:48:33AM +0300, Kostik Belousov wrote:
> On Thu, Apr 02, 2009 at 09:06:05AM +0200, Luigi Rizzo wrote:
> > Hi,
> > I have some list manipulation algorithm that I would like to use
> > that relies rather centrally on atomic_cmpset_int().
> > 
> > This is an atomic instruction on 486+, but not available on 386
> > and maybe other platforms. i386/atomic.h has a replacement
> > but it uses "pushfl; cli; ... popfl;" so it cannot run in userland.
> > 
> > I was wondering if there is a good emulation for that instruction
> > on the i386 that is suitable for userland (other architectures
> > we support have a CPU instruction that does it, or in the case of ARM,
> > a usable emulation for userland).
> 
> FreeBSD cannot boot on anything < 486, i.e. cmpxchgl and xaddl may be
> considered always supported by the CPU.

It was a slightly more generic question -- this stuff is for userland
so while we can assume it works on modern FreeBSD versions, I would
like to see what constraints it has on older versions of FreeBSD.
Of course I can emulate the critical section with a pthread lock,
but that would be the worst case option.

	cheers
	luigi