From owner-cvs-all@FreeBSD.ORG  Tue Dec 28 03:17:07 2004
Return-Path: <owner-cvs-all@FreeBSD.ORG>
Delivered-To: cvs-all@freebsd.org
Received: by hub.freebsd.org (Postfix, from userid 680)
	id 2AE6816A4CF; Tue, 28 Dec 2004 03:17:07 +0000 (GMT)
Date: Tue, 28 Dec 2004 03:17:07 +0000
From: Darren Reed <darrenr@hub.freebsd.org>
To: Alan Cox <alc@cs.rice.edu>
Message-ID: <20041228031707.GA38011@hub.freebsd.org>
References: <20041226165927.GA18879@hub.freebsd.org>
	<Pine.BSF.4.53.0412261707210.68472@e0-0.zab2.int.zabbadoz.net>
	<20041226182537.GB20920@hub.freebsd.org>
	<20041226.222435.52824948.imp@bsdimp.com>
	<20041227054931.GC20920@hub.freebsd.org>
	<20041227204846.GC15591@noel.cs.rice.edu>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20041227204846.GC15591@noel.cs.rice.edu>
User-Agent: Mutt/1.4.2.1i
cc: src-committers@freebsd.org
cc: cvs-src@freebsd.org
cc: scottl@freebsd.org
cc: cvs-all@freebsd.org
cc: darrenr@freebsd.org
cc: bzeeb-lists@lists.zabbadoz.net
cc: "M. Warner Losh" <imp@bsdimp.com>
Subject: Re: cvs commit: src/sys/contrib/ipfilter/netinet ip_auth.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: Tue, 28 Dec 2004 03:17:07 -0000

On Mon, Dec 27, 2004 at 02:48:46PM -0600, Alan Cox wrote:
> Darren,
> 
> Based upon a quick look at the above URL, I would say that the problem
> is that you are acquiring an sx lock after a mutex is held.

Yes, that is what I thought the problem was too.

> That is not allowed.

Well, it should be.

> More generally, an sx lock is termed a "sleepable" lock
> and a mutex is not; all desired sleepable locks must be acquired
> before any non-sleepable locks are acquired.  Witness enforces this
> prohibition, but perhaps causes confusion by calling it a reversal.

What can I say that I haven't already ?
This is a nonsense rule and I'm quite happy that it isn't enforced at
all.  The assumption by someone that all of these scenarios lead to
deadlocks is false.

I imagine it is part of why things like the IFNET_*LOCK macros use mtx
rather than sx.  On systems where the interface list is of non-trivial
size, I imagine this would have a measurable impact.

Perhaps the most stupid realisation from this has been that IPFilter
code gets called with a UDP mutex of some sort held.  Like WTF ?

Darren