From owner-freebsd-net@FreeBSD.ORG Fri Nov 9 09:59:22 2012 Return-Path: Delivered-To: freebsd-net@FreeBSD.org Received: from mx1.freebsd.org (mx1.freebsd.org [69.147.83.52]) by hub.freebsd.org (Postfix) with ESMTP id 0225649C; Fri, 9 Nov 2012 09:59:21 +0000 (UTC) (envelope-from fabien.thomas@netasq.com) Received: from work.netasq.com (gwlille.netasq.com [91.212.116.1]) by mx1.freebsd.org (Postfix) with ESMTP id B15918FC12; Fri, 9 Nov 2012 09:59:21 +0000 (UTC) Received: from [10.2.1.1] (unknown [10.2.1.1]) by work.netasq.com (Postfix) with ESMTPSA id D30ED2705341; Fri, 9 Nov 2012 10:59:19 +0100 (CET) Subject: Re: [patch] reducing arp locking Mime-Version: 1.0 (Apple Message framework v1283) Content-Type: text/plain; charset=iso-8859-1 From: Fabien Thomas In-Reply-To: <509CC776.9010200@FreeBSD.org> Date: Fri, 9 Nov 2012 10:59:18 +0100 Content-Transfer-Encoding: quoted-printable Message-Id: <37E1F76F-D951-4B36-AF00-039DA1CC5CF3@netasq.com> References: <509AEDAC.10002@FreeBSD.org> <509B884F.7040106@networx.ch> <509B88B1.3070905@FreeBSD.org> <49EE4F42-6162-40F4-9DE0-1ACA1289B225@netasq.com> <509CC776.9010200@FreeBSD.org> To: "Alexander V. Chernikov" X-Mailer: Apple Mail (2.1283) Cc: freebsd-net@FreeBSD.org, freebsd-hackers@FreeBSD.org X-BeenThere: freebsd-net@freebsd.org X-Mailman-Version: 2.1.14 Precedence: list List-Id: Networking and TCP/IP with FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 09 Nov 2012 09:59:22 -0000 Le 9 nov. 2012 =E0 10:05, Alexander V. Chernikov a =E9crit : > On 09.11.2012 12:51, Fabien Thomas wrote: >>=20 >> Le 8 nov. 2012 =E0 11:25, Alexander V. Chernikov a =E9crit : >>=20 >>> On 08.11.2012 14:24, Andre Oppermann wrote: >>>> On 08.11.2012 00:24, Alexander V. Chernikov wrote: >>>>> Hello list! >>>>>=20 >>>>> Currently we need to acquire 2 read locks to perform simple 6-byte >>>>> copying from arp record to packet >>>>> ethernet header. >>>>>=20 >>>>> It seems that acquiring lle lock for fast path (main traffic flow) = is >>>>> not necessary even with >>>>> current code. >>>>>=20 >>>>> My tests shows ~10% improvement with this patch applied. >>>>>=20 >>>>> If nobody objects I plan to commit this change at the end of next = week. >>>>=20 >>>> This is risky and prone to race conditions. The copy of the MAC = address >>>> should be done while the table read lock is held to protect against = the >>> It is done exactly as you say: table read lock is held. >>=20 >> How do you protect from entry update if i've a ref to the entry ? >> You can end up doing bcopy of a partial mac address. > I see no problems in copying incorrect mac address in that case: > if host mac address id updated, this is, most likely, another host, = and several packets being lost changes nothing. Sending packet to a bogus mac address is not really nothing :) >=20 > However, there can be some realistic scenario where this can be the = case (L2 load balancing/failover). I'll update in_arpinput() to do lle = removal/insertion in that case. >=20 >> la_preempt modification is also write access to an unlocked = structure. > This one changes nothing: > current code does this under _read_ lock. Under the table lock not the entry lock ? Table lock is here to protect the table if I've understood the code = correctly. If i get an exclusive reference to the entry you will end up reading and = writing to the entry without any lock. >=20 >>=20 >>=20 >>>=20 >>>> entry going away. You can either return with table lock held and = drop >>>> it after the copy, or you could a modified lookup function that = takes a >>>> pointer for the copy destination, do the copy with the read lock, = and then >>>> return. If no entry is found an error is returned and obviously no = copy >>>> is done. >>>>=20 >>>=20 >>>=20 >>> -- >>> WBR, Alexander >>>=20 >>>=20 >>> _______________________________________________ >>> freebsd-hackers@freebsd.org mailing list >>> http://lists.freebsd.org/mailman/listinfo/freebsd-hackers >>> To unsubscribe, send any mail to = "freebsd-hackers-unsubscribe@freebsd.org" >>=20 >>=20 >=20