From owner-freebsd-current@FreeBSD.ORG  Wed Oct 14 11:59:01 2009
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 35149106568B
	for <freebsd-current@FreeBSD.org>; Wed, 14 Oct 2009 11:59:01 +0000 (UTC)
	(envelope-from avg@icyb.net.ua)
Received: from citadel.icyb.net.ua (citadel.icyb.net.ua [212.40.38.140])
	by mx1.freebsd.org (Postfix) with ESMTP id 7A84B8FC17
	for <freebsd-current@FreeBSD.org>; Wed, 14 Oct 2009 11:59:00 +0000 (UTC)
Received: from odyssey.starpoint.kiev.ua (alpha-e.starpoint.kiev.ua
	[212.40.38.101])
	by citadel.icyb.net.ua (8.8.8p3/ICyb-2.3exp) with ESMTP id OAA01491;
	Wed, 14 Oct 2009 14:58:56 +0300 (EEST)
	(envelope-from avg@icyb.net.ua)
Message-ID: <4AD5BD00.4050700@icyb.net.ua>
Date: Wed, 14 Oct 2009 14:58:56 +0300
From: Andriy Gapon <avg@icyb.net.ua>
User-Agent: Thunderbird 2.0.0.23 (X11/20090825)
MIME-Version: 1.0
To: Alexander Best <alexbestms@math.uni-muenster.de>
References: <permail-2009101411215080e26a0b00001bea-a_best01@message-id.uni-muenster.de>
In-Reply-To: <permail-2009101411215080e26a0b00001bea-a_best01@message-id.uni-muenster.de>
X-Enigmail-Version: 0.95.7
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
Cc: freebsd-current@FreeBSD.org
Subject: Re: panic when mounting device >= 2 times
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: Wed, 14 Oct 2009 11:59:01 -0000

on 14/10/2009 14:21 Alexander Best said the following:
> could somebody have a look at this PR please:
> http://www.freebsd.org/cgi/query-pr.cgi?pr=kern/108968
> 
> this still exists in -CURRENT.

This is because of serious and long-standing brokenness (probably from time of
introduction of GEOM) in g_vfs_open/g_vfs_close.
The essence:
1. we allow multiple concurrent read-only mounts of a device
2. but even a read-only mount mangles bufobj of device vnode
3. Bug!
Trivial fix with POLA violation - prohibit concurrent RO mounts.
Non-trivial fix - _somehow_ avoid changing or depending on device vnode bufobj.

See this for even more evil example of this problem:
http://lists.freebsd.org/pipermail/freebsd-fs/2009-February/005676.html

-- 
Andriy Gapon