From owner-cvs-src@FreeBSD.ORG  Wed Apr  2 14:53:19 2008
Return-Path: <owner-cvs-src@FreeBSD.ORG>
Delivered-To: cvs-src@FreeBSD.ORG
Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34])
	by hub.freebsd.org (Postfix) with ESMTP id CB822106564A;
	Wed,  2 Apr 2008 14:53:19 +0000 (UTC) (envelope-from das@FreeBSD.ORG)
Received: from zim.MIT.EDU (ZIM.MIT.EDU [18.95.3.101])
	by mx1.freebsd.org (Postfix) with ESMTP id 6CAB18FC2F;
	Wed,  2 Apr 2008 14:53:19 +0000 (UTC) (envelope-from das@FreeBSD.ORG)
Received: from zim.MIT.EDU (localhost [127.0.0.1])
	by zim.MIT.EDU (8.14.2/8.14.2) with ESMTP id m32EtKwP021559;
	Wed, 2 Apr 2008 10:55:20 -0400 (EDT) (envelope-from das@FreeBSD.ORG)
Received: (from das@localhost)
	by zim.MIT.EDU (8.14.2/8.14.2/Submit) id m32EtKVE021558;
	Wed, 2 Apr 2008 10:55:20 -0400 (EDT) (envelope-from das@FreeBSD.ORG)
Date: Wed, 2 Apr 2008 10:55:20 -0400
From: David Schultz <das@FreeBSD.ORG>
To: Jeff Roberson <jroberson@chesapeake.net>
Message-ID: <20080402145520.GA21462@zim.MIT.EDU>
Mail-Followup-To: Jeff Roberson <jroberson@chesapeake.net>,
	Max Laier <max@love2party.net>, Attilio Rao <attilio@FreeBSD.ORG>,
	src-committers@FreeBSD.ORG, cvs-src@FreeBSD.ORG,
	cvs-all@FreeBSD.ORG
References: <200804012031.m31KVtKs000176@repoman.freebsd.org>
	<200804020025.57684.max@love2party.net>
	<20080401123951.J72156@desktop>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20080401123951.J72156@desktop>
Cc: Attilio Rao <attilio@FreeBSD.ORG>, Max Laier <max@love2party.net>,
	src-committers@FreeBSD.ORG, cvs-all@FreeBSD.ORG, cvs-src@FreeBSD.ORG
Subject: Re: cvs commit: src/sys/kern kern_rwlock.c src/sys/sys rwlock.h
X-BeenThere: cvs-src@freebsd.org
X-Mailman-Version: 2.1.5
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, 02 Apr 2008 14:53:20 -0000

On Tue, Apr 01, 2008, Jeff Roberson wrote:
> On Wed, 2 Apr 2008, Max Laier wrote:
> 
> >On Tuesday 01 April 2008 22:31:55 Attilio Rao wrote:
> >>attilio     2008-04-01 20:31:55 UTC
> >>
> >>  FreeBSD src repository
> >>
> >>  Modified files:
> >>    sys/kern             kern_rwlock.c
> >>    sys/sys              rwlock.h
> >>  Log:
> >>  Add rw_try_rlock() and rw_try_wlock() to rwlocks.
> >>  These functions try the specified operation (rlocking and wlocking)
> >>and true is returned if the operation completes, false otherwise.
> >
> >hmmm ... I'm certainly missing something here, but what's a possible
> >usecase for these?  It seems there is not much you can do if you can't
> >obtain a rw_lock.  I can understand the need for sx_try_* where you want
> >to avoid sleeping, but I can't figure out the need for it on a locking
> >primitive that will only spin or wait (not 100% sure about the
> >terminology here).  This is especially strange for rw_try_wlock, unless
> >you plan to sleep manually on fail.  But then again you'd have a good
> >chance that you have to do it over and over again if timing is
> >unfortunate.
> 
> I asked for it.  We have a try operation for mtx already.  I was 
> experimenting with converting some code to use rwlocks from mtx and it 
> required it.  The specific case is in the softdep code where it uses 
> trylock to avoid deadlocking.  With trylock you can violate the lockorder.

One reason that many systems don't provide a rw_try_wlock()
primitive is that a constant stream of readers can easily starve
threads attempting to acquire the write lock without waiting.
There are probably situations where this isn't a problem, though,
e.g., if readers rarely hold the lock for a long time...