From owner-cvs-all@FreeBSD.ORG  Wed Jun  9 16:05:42 2004
Return-Path: <owner-cvs-all@FreeBSD.ORG>
Delivered-To: cvs-all@freebsd.org
Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125])
	by hub.freebsd.org (Postfix) with ESMTP
	id 8C6DA16A4CE; Wed,  9 Jun 2004 16:05:42 +0000 (GMT)
Received: from harmony.village.org (rover.village.org [168.103.84.182])
	by mx1.FreeBSD.org (Postfix) with ESMTP
	id F0ADE43D1D; Wed,  9 Jun 2004 16:05:41 +0000 (GMT)
	(envelope-from imp@bsdimp.com)
Received: from localhost (warner@rover2.village.org [10.0.0.1])
	by harmony.village.org (8.12.11/8.12.11) with ESMTP id i59G3wlX043876;
	Wed, 9 Jun 2004 10:03:58 -0600 (MDT)
	(envelope-from imp@bsdimp.com)
Date: Wed, 09 Jun 2004 10:04:13 -0600 (MDT)
Message-Id: <20040609.100413.118633043.imp@bsdimp.com>
To: phk@phk.freebsd.dk
From: "M. Warner Losh" <imp@bsdimp.com>
In-Reply-To: <53418.1086773585@critter.freebsd.dk>
References: <200406090929.i599T8h6065944@repoman.freebsd.org>
	<53418.1086773585@critter.freebsd.dk>
X-Mailer: Mew version 3.3 on Emacs 21.3 / Mule 5.0 (SAKAKI)
Mime-Version: 1.0
Content-Type: Text/Plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
cc: cvs-src@FreeBSD.org
cc: src-committers@FreeBSD.org
cc: cvs-all@FreeBSD.org
Subject: Re: cvs commit: src/sys/kern kern_proc.c 
X-BeenThere: cvs-all@freebsd.org
X-Mailman-Version: 2.1.1
Precedence: list
List-Id: CVS commit messages for the entire tree <cvs-all.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/cvs-all>,
	<mailto:cvs-all-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/cvs-all>
List-Post: <mailto:cvs-all@freebsd.org>
List-Help: <mailto:cvs-all-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/cvs-all>,
	<mailto:cvs-all-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Wed, 09 Jun 2004 16:05:42 -0000

In message: <53418.1086773585@critter.freebsd.dk>
            "Poul-Henning Kamp" <phk@phk.freebsd.dk> writes:
: In message <200406090929.i599T8h6065944@repoman.freebsd.org>, Poul-Henning Kamp
:  writes:
: >
: >  Modified files:
: >    sys/kern             kern_proc.c 
: >  Log:
: >  Fix a race in destruction of sessions.
: 
: Not to pick on anybody, but this is a perfect example of getting locking
: almost right:
: 
: BAD:
: 
: 	LOCK(foo->lock)
: 	foo->refcount--;
: 	UNLOCK(foo->lock)
: 	if (foo->refcount == 0)
: 		destroy(foo);
: 
: GOOD:
: 
: 	LOCK(foo->lock)
: 	i = --foo->refcount;
: 	UNLOCK(foo->lock)
: 	if (i == 0)
: 		destroy(foo);
: 

Can you provide a couple of lines about why BAD is BAD and why GOOD
fixes that flaw?  That should help others from making this mistake in
the future.

Warner