From mboxrd@z Thu Jan 1 00:00:00 1970 From: Marcelo Ricardo Leitner Subject: Re: Question about synchronize_net() in AF_PACKET close() Date: Wed, 10 Sep 2014 10:19:34 -0300 Message-ID: <54104FE6.4030404@redhat.com> References: <20140909.150000.1647602729214485795.davem@davemloft.net> Reply-To: mleitner@redhat.com Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: David Miller , netdev@vger.kernel.org, paulmck@linux.vnet.ibm.com, stephen@networkplumber.org To: martin@martingkelly.com Return-path: Received: from mx1.redhat.com ([209.132.183.28]:21859 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750734AbaIJNTt (ORCPT ); Wed, 10 Sep 2014 09:19:49 -0400 In-Reply-To: <20140909.150000.1647602729214485795.davem@davemloft.net> Sender: netdev-owner@vger.kernel.org List-ID: Em 09-09-2014 19:00, David Miller escreveu: > From: Martin Kelly > Date: Tue, 9 Sep 2014 14:44:56 -0700 > >> In net/packet/af_packet.c, I noticed the following few lines within >> packet_release: >> >> synchronize_net(); >> /* >> * Now the socket is dead. No more input will appear. >> */ >> sock_orphan(sk); >> sock->sk = NULL; >> >> From testing and code analysis, I have found that it appears to be safe >> to move sock_orphan and sock->sk = NULL before the synchronize_net() >> call like so: >> >> /* >> * Now the socket is dead. No more input will appear. >> */ >> sock_orphan(sk); >> sock->sk = NULL; >> synchronize_net(); >> >> Could some RCU and/or networking experts chime in about whether this a >> safe operation? For all I know, there is some deep, fundamental reason >> why those lines are in the order they are. On the other hand, perhaps >> there is not. > > The synchronize_net() is also there to protect against the prot hook > which can run asynchronously from the core packet input path on any > cpu. > > You probably want to reference commit: > > commit 808f5114a9206fee855117d416440e1071ab375c > Author: stephen hemminger > Date: Mon Feb 22 07:57:18 2010 +0000 > > packet: convert socket list to RCU (v3) > > which put the synchronize_net() there in the first place. And also http://patchwork.ozlabs.org/patch/181605/ Which was an attempt on improving it. Marcelo