From owner-freebsd-arch@FreeBSD.ORG  Mon Apr 14 08:30:21 2008
Return-Path: <owner-freebsd-arch@FreeBSD.ORG>
Delivered-To: arch@freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34])
	by hub.freebsd.org (Postfix) with ESMTP id E37F3106566B
	for <arch@freebsd.org>; Mon, 14 Apr 2008 08:30:21 +0000 (UTC)
	(envelope-from jroberson@jroberson.net)
Received: from wf-out-1314.google.com (wf-out-1314.google.com [209.85.200.171])
	by mx1.freebsd.org (Postfix) with ESMTP id C4D858FC15
	for <arch@freebsd.org>; Mon, 14 Apr 2008 08:30:21 +0000 (UTC)
	(envelope-from jroberson@jroberson.net)
Received: by wf-out-1314.google.com with SMTP id 25so1510794wfa.7
	for <arch@freebsd.org>; Mon, 14 Apr 2008 01:30:21 -0700 (PDT)
Received: by 10.142.131.18 with SMTP id e18mr1727023wfd.39.1208161821472;
	Mon, 14 Apr 2008 01:30:21 -0700 (PDT)
Received: from ?10.0.1.199? ( [24.94.72.120])
	by mx.google.com with ESMTPS id 30sm10742859wfa.2.2008.04.14.01.30.18
	(version=SSLv3 cipher=OTHER); Mon, 14 Apr 2008 01:30:20 -0700 (PDT)
Date: Sun, 13 Apr 2008 22:30:46 -1000 (HST)
From: Jeff Roberson <jroberson@jroberson.net>
X-X-Sender: jroberson@desktop
To: Poul-Henning Kamp <phk@phk.freebsd.dk>
In-Reply-To: <4993.1208156428@critter.freebsd.dk>
Message-ID: <20080413222803.D959@desktop>
References: <4993.1208156428@critter.freebsd.dk>
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed
Cc: arch@FreeBSD.ORG, David Schultz <das@FreeBSD.ORG>
Subject: Re: f_offset 
X-BeenThere: freebsd-arch@freebsd.org
X-Mailman-Version: 2.1.5
Precedence: list
List-Id: Discussion related to FreeBSD architecture <freebsd-arch.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-arch>,
	<mailto:freebsd-arch-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-arch>
List-Post: <mailto:freebsd-arch@freebsd.org>
List-Help: <mailto:freebsd-arch-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-arch>,
	<mailto:freebsd-arch-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Mon, 14 Apr 2008 08:30:22 -0000


On Mon, 14 Apr 2008, Poul-Henning Kamp wrote:

> In message <20080413131422.V959@desktop>, Jeff Roberson writes:
>
>> However, f_offset is fetched and set with a shared vnode lock for readers.
>
> "fetched AND set" with a shared lock ?
>
> How can that possibly work out sanely ?

It doesn't matter which reader wins the race to update offset as long as 
the resulting offset is entirely valid from one.  This is a userspace race 
otherwise.

There is a complication for 64bit offsets on 32bit machines that needs 
some MD support.  Otherwise the write happens in two instructions and can 
have mixed results.

Jeff

>
> -- 
> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> phk@FreeBSD.ORG         | TCP/IP since RFC 956
> FreeBSD committer       | BSD since 4.3-tahoe
> Never attribute to malice what can adequately be explained by incompetence.
>