From owner-cvs-src@FreeBSD.ORG  Thu Sep 16 22:25:18 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 0C10816A4CE; Thu, 16 Sep 2004 22:25:18 +0000 (GMT)
Received: from khavrinen.lcs.mit.edu (khavrinen.lcs.mit.edu [128.30.28.20])
	by mx1.FreeBSD.org (Postfix) with ESMTP
	id 8228543D31; Thu, 16 Sep 2004 22:25:17 +0000 (GMT)
	(envelope-from wollman@khavrinen.lcs.mit.edu)
Received: from khavrinen.lcs.mit.edu (localhost [IPv6:::1])
	by khavrinen.lcs.mit.edu (8.12.9/8.12.9) with ESMTP id i8GMPF8g010484
	(version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256
	verify=OK CN=khavrinen.lcs.mit.edu issuer=SSL+20Client+20CA);
	Thu, 16 Sep 2004 18:25:16 -0400 (EDT)
	(envelope-from wollman@khavrinen.lcs.mit.edu)
Received: (from wollman@localhost)
	by khavrinen.lcs.mit.edu (8.12.9/8.12.9/Submit) id i8GMPFFs010481;
	Thu, 16 Sep 2004 18:25:15 -0400 (EDT)
	(envelope-from wollman)
Date: Thu, 16 Sep 2004 18:25:15 -0400 (EDT)
From: Garrett Wollman <wollman@khavrinen.lcs.mit.edu>
Message-Id: <200409162225.i8GMPFFs010481@khavrinen.lcs.mit.edu>
To: Nate Lawson <nate@root.org>
In-Reply-To: <414A1073.8010404@root.org>
References: <20040916185923.2F92316A552@hub.freebsd.org>
	<4149EC27.9080200@root.org>
	<20040916204321.GE30151@darkness.comp.waw.pl>
	<414A1073.8010404@root.org>
X-Spam-Score: -19.8 ()
	IN_REP_TO,QUOTED_EMAIL_TEXT,REFERENCES,REPLY_WITH_QUOTES
X-Scanned-By: MIMEDefang 2.37
cc: cvs-src@FreeBSD.ORG
cc: src-committers@FreeBSD.ORG
cc: cvs-all@FreeBSD.ORG
Subject: Re: cvs commit: src/sys/dev/md md.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: Thu, 16 Sep 2004 22:25:18 -0000

<<On Thu, 16 Sep 2004 15:15:15 -0700, Nate Lawson <nate@root.org> said:

> You should be checking the work condition in thread 2 while holding the 
> mutex but before going to sleep.  Adding work to the queue happens in 
> thread 1 where you write "..." and that is done with the mutex held so 
> there is no race.  The full diagram with this detail included is:

Of course, getting this right is complicated enough that we have an
entire abstraction to assist.

> thread1               thread2
> -----------------------------
> mtx_lock(mtx)
> add work to queue
> mtx_unlock(mtx)
>                        mtx_lock(mtx)
> wakeup(ptr)
>                        check queue for work item
>                        if (!work item)
>                            msleep(ptr, mtx)
>                        else
>                            dequeue work item and loop

mtx_lock(mtx)
add work to queue
cv_signal(worktodo)
mtx_unlock(mtx)
			mtx_lock(mtx)
			for (;;) {
				check queue for work item
				if (!work item)
					cv_wait(cv, mtx)
				else {
					dequeue work item
					do work
				}
			}
			mtx_unlock(mtx)


-GAWollman