From mboxrd@z Thu Jan 1 00:00:00 1970 From: Corey Minyard Subject: Re: [PATCH 2/2] udp: RCU handling for Unicast packets. Date: Wed, 29 Oct 2008 10:34:32 -0500 Message-ID: <49088288.6050805@acm.org> References: <20081008.114527.189056050.davem@davemloft.net> <49077918.4050706@cosmosbay.com> <490795FB.2000201@cosmosbay.com> <20081028.220536.183082966.davem@davemloft.net> <49081D67.3050502@cosmosbay.com> <49082718.2030201@cosmosbay.com> <4908627C.6030001@acm.org> <490874F2.2060306@cosmosbay.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: David Miller , shemminger@vyatta.com, benny+usenet@amorsen.dk, netdev@vger.kernel.org, paulmck@linux.vnet.ibm.com, Christoph Lameter , a.p.zijlstra@chello.nl, johnpol@2ka.mipt.ru, Christian Bell To: Eric Dumazet Return-path: Received: from vms046pub.verizon.net ([206.46.252.46]:47500 "EHLO vms046pub.verizon.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752410AbYJ2Pev (ORCPT ); Wed, 29 Oct 2008 11:34:51 -0400 Received: from wf-rch.minyard.local ([96.226.138.225]) by vms046.mailsrvcs.net (Sun Java System Messaging Server 6.2-6.01 (built Apr 3 2006)) with ESMTPA id <0K9I00CWAB9MNIW1@vms046.mailsrvcs.net> for netdev@vger.kernel.org; Wed, 29 Oct 2008 10:34:35 -0500 (CDT) In-reply-to: <490874F2.2060306@cosmosbay.com> Sender: netdev-owner@vger.kernel.org List-ID: Eric Dumazet wrote: > Corey Minyard found a race added in commit > 271b72c7fa82c2c7a795bc16896149933110672d > (udp: RCU handling for Unicast packets.) > > "If the socket is moved from one list to another list in-between the > time the hash is calculated and the next field is accessed, and the > socket has moved to the end of the new list, the traversal will not > complete properly on the list it should have, since the socket will > be on the end of the new list and there's not a way to tell it's on a > new list and restart the list traversal. I think that this can be > solved by pre-fetching the "next" field (with proper barriers) before > checking the hash." > > This patch corrects this problem, introducing a new > sk_for_each_rcu_safenext() > macro. You also need the appropriate smp_wmb() in udp_lib_get_port() after sk_hash is set, I think, so the next field is guaranteed to be changed after the hash value is changed. -corey