From owner-freebsd-net@FreeBSD.ORG  Thu Aug 29 16:02:20 2013
Return-Path: <owner-freebsd-net@FreeBSD.ORG>
Delivered-To: freebsd-net@freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org [8.8.178.115])
 (using TLSv1 with cipher ADH-AES256-SHA (256/256 bits))
 (No client certificate requested)
 by hub.freebsd.org (Postfix) with ESMTP id E36A8E26;
 Thu, 29 Aug 2013 16:02:20 +0000 (UTC) (envelope-from jhb@freebsd.org)
Received: from bigwig.baldwin.cx (bigwig.baldwin.cx [IPv6:2001:470:1f11:75::1])
 (using TLSv1 with cipher ADH-CAMELLIA256-SHA (256/256 bits))
 (No client certificate requested)
 by mx1.freebsd.org (Postfix) with ESMTPS id B1B0E23FE;
 Thu, 29 Aug 2013 16:02:20 +0000 (UTC)
Received: from jhbbsd.localnet (unknown [38.105.238.108])
 by bigwig.baldwin.cx (Postfix) with ESMTPSA id 7183BB924;
 Thu, 29 Aug 2013 12:02:19 -0400 (EDT)
From: John Baldwin <jhb@freebsd.org>
To: Scott Long <scott4long@yahoo.com>
Subject: Re: [rfc] migrate lagg to an rmlock
Date: Thu, 29 Aug 2013 12:01:03 -0400
User-Agent: KMail/1.13.5 (FreeBSD/8.2-CBSD-20110714-p28; KDE/4.5.5; amd64; ; )
References: <CAJ-Vmo=VKVDEmmPrTbob6Ft+7FWypodNoL36Og=7p_CXBSfktg@mail.gmail.com>
 <201308291042.13282.jhb@freebsd.org>
 <41614148-3900-4FE0-88AC-40F10DAE2030@yahoo.com>
In-Reply-To: <41614148-3900-4FE0-88AC-40F10DAE2030@yahoo.com>
MIME-Version: 1.0
Content-Type: Text/Plain;
  charset="iso-8859-1"
Content-Transfer-Encoding: 7bit
Message-Id: <201308291201.04131.jhb@freebsd.org>
X-Greylist: Sender succeeded SMTP AUTH, not delayed by milter-greylist-4.2.7
 (bigwig.baldwin.cx); Thu, 29 Aug 2013 12:02:19 -0400 (EDT)
Cc: FreeBSD Net <freebsd-net@freebsd.org>, Adrian Chadd <adrian@freebsd.org>,
 freebsd-current@freebsd.org, Robert Watson <rwatson@freebsd.org>,
 "Alexander V. Chernikov" <melifaro@ipfw.ru>
X-BeenThere: freebsd-net@freebsd.org
X-Mailman-Version: 2.1.14
Precedence: list
List-Id: Networking and TCP/IP with FreeBSD <freebsd-net.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/options/freebsd-net>,
 <mailto:freebsd-net-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-net>
List-Post: <mailto:freebsd-net@freebsd.org>
List-Help: <mailto:freebsd-net-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-net>,
 <mailto:freebsd-net-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Thu, 29 Aug 2013 16:02:21 -0000

On Thursday, August 29, 2013 11:37:08 am Scott Long wrote:
> 
> On Aug 29, 2013, at 7:42 AM, John Baldwin <jhb@freebsd.org> wrote:
> 
> > On Saturday, August 24, 2013 10:16:33 am Robert Watson wrote:
> >> There are a number of other places in the kernel where migration to an rmlock 
> >> makes sense -- however, some care must be taken for four reasons: (1) while 
> >> read locks don't experience line contention, write locking becomes observably 
> >> e.g., rmlocks might not be suitable for tcbinfo; (2) rmlocks, unlike rwlocks, 
> >> more expensive so is not suitable for all rwlock line contention spots -- 
> >> implement reader priority propagation, so you must reason about; and (3) 
> >> historically, rmlocks have not fully implemented WITNESS so you may get less 
> >> good debugging output.  if_lagg is a nice place to use rmlocks, as 
> >> reconfigurations are very rare, and it's really all about long-term data 
> >> stability.
> > 
> > 3) should no longer be an issue.  rmlocks now have full WITNESS and assertion
> > support (including an rm_assert).
> > 
> > However, one thing to consider is that rmlocks pin readers to CPUs while the
> > read lock is held (which rwlocks do not do).
> 
> And this is not a problem for the application that we're giving it in the
> lagg driver.

That is likely true.  I was merely tweaking Robert's general guidelines re: rmlock.

-- 
John Baldwin