From owner-cvs-src@FreeBSD.ORG  Wed Jun  9 19:03:13 2004
Return-Path: <owner-cvs-src@FreeBSD.ORG>
Delivered-To: cvs-src@freebsd.org
Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125])
	by hub.freebsd.org (Postfix) with ESMTP
	id 75F3716A4D0; Wed,  9 Jun 2004 19:03:13 +0000 (GMT)
Received: from darkness.comp.waw.pl (darkness.comp.waw.pl [195.117.238.236])
	by mx1.FreeBSD.org (Postfix) with ESMTP
	id 2368A43D55; Wed,  9 Jun 2004 19:03:13 +0000 (GMT)
	(envelope-from pjd@darkness.comp.waw.pl)
Received: by darkness.comp.waw.pl (Postfix, from userid 1009)
	id 9E989AC995; Wed,  9 Jun 2004 21:02:58 +0200 (CEST)
Date: Wed, 9 Jun 2004 21:02:58 +0200
From: Pawel Jakub Dawidek <pjd@FreeBSD.org>
To: Julian Elischer <julian@elischer.org>
Message-ID: <20040609190258.GY12007@darkness.comp.waw.pl>
References: <20040609182041.GV12007@darkness.comp.waw.pl>
	<Pine.BSF.4.21.0406091151050.57882-100000@InterJet.elischer.org>
Mime-Version: 1.0
Content-Type: multipart/signed; micalg=pgp-sha1;
	protocol="application/pgp-signature"; boundary="s8ux8MQukyWAm3r7"
Content-Disposition: inline
In-Reply-To: <Pine.BSF.4.21.0406091151050.57882-100000@InterJet.elischer.org>
User-Agent: Mutt/1.4.2i
X-PGP-Key-URL: http://people.freebsd.org/~pjd/pjd.asc
X-OS: FreeBSD 5.2.1-RC2 i386
cc: src-committers@FreeBSD.org
cc: cvs-src@FreeBSD.org
cc: cvs-all@FreeBSD.org
cc: phk@phk.freebsd.dk
cc: Bosko Milekic <bmilekic@FreeBSD.org>
cc: Nate Lawson <nate@root.org>
cc: "M. Warner Losh" <imp@bsdimp.com>
Subject: Re: cvs commit: src/sys/kern kern_proc.c
X-BeenThere: cvs-src@freebsd.org
X-Mailman-Version: 2.1.1
Precedence: list
List-Id: CVS commit messages for the src tree <cvs-src.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/cvs-src>,
	<mailto:cvs-src-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/cvs-src>
List-Post: <mailto:cvs-src@freebsd.org>
List-Help: <mailto:cvs-src-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/cvs-src>,
	<mailto:cvs-src-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Wed, 09 Jun 2004 19:03:13 -0000


--s8ux8MQukyWAm3r7
Content-Type: text/plain; charset=iso-8859-2
Content-Disposition: inline
Content-Transfer-Encoding: quoted-printable

On Wed, Jun 09, 2004 at 11:54:06AM -0700, Julian Elischer wrote:
+> > Ok, I get it now. Even if there will be a race between two threads, i.=
e.:
+> >=20
+> > 	[ref_cnt is 2]
+> >=20
+> > 	thread1					thread2
+> >=20
+> > 	atomic_substract(obj, 1)
+> > 	[ref_cnt is 1]
+> > 						atomic_substract(obj, 1)
+> > 						[ref_cnt is 0]
+> > 	atomic_cmpset(obj, 0, 1) -> true
+> > 	[do clean up]
+> > 						atomic_cmpset(obj, 0, 1) -> false
+> > 						[skip cleanups]
+> >=20
+> > (thread2 sets ref_cnt to 0, but thread1 destroys objects) we free obje=
ct
+> > only once. Cool, I like it:)
+>=20
+> The method shown above has, however the disadvantage of being
+> less than obvious.  ALso thread 2 is likely to get a page fault, which
+> is a less than perfect result..
+>=20
+> the reference count must never be read or written after the [cleanup]
+> because  it may free the page on which teh counter is kept.

Right... Nice catch. So Bosko probebly have to rethink his model.

--=20
Pawel Jakub Dawidek                       http://www.FreeBSD.org
pjd@FreeBSD.org                           http://garage.freebsd.pl
FreeBSD committer                         Am I Evil? Yes, I Am!

--s8ux8MQukyWAm3r7
Content-Type: application/pgp-signature
Content-Disposition: inline

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.2.4 (FreeBSD)

iD8DBQFAx17iForvXbEpPzQRAtIfAKDxcQ5dQYTpabZt4Gxqzt6Pls2EDACeM2b1
jIg7lzU2AlEGNXL6MhNk2o0=
=99+x
-----END PGP SIGNATURE-----

--s8ux8MQukyWAm3r7--