From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Miller Subject: Re: [PATCH] net: remove superfluous call to synchronize_net() Date: Fri, 17 Apr 2009 04:56:31 -0700 (PDT) Message-ID: <20090417.045631.159492677.davem@davemloft.net> References: <49E5FF5E.50409@cosmosbay.com> <20090415215454.GU6766@linux.vnet.ibm.com> <49E6C4C7.3050105@cosmosbay.com> Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit Cc: paulmck@linux.vnet.ibm.com, netdev@vger.kernel.org To: dada1@cosmosbay.com Return-path: Received: from 74-93-104-97-Washington.hfc.comcastbusiness.net ([74.93.104.97]:52439 "EHLO sunset.davemloft.net" rhost-flags-OK-FAIL-OK-OK) by vger.kernel.org with ESMTP id S1755412AbZDQL4k (ORCPT ); Fri, 17 Apr 2009 07:56:40 -0400 In-Reply-To: <49E6C4C7.3050105@cosmosbay.com> Sender: netdev-owner@vger.kernel.org List-ID: From: Eric Dumazet Date: Thu, 16 Apr 2009 07:40:23 +0200 > [PATCH] net: remove superfluous call to synchronize_net() > > inet_register_protosw() function is responsible for adding a new > inet protocol into a global table (inetsw[]) that is used with RCU rules. > > As soon as the store of the pointer is done, other cpus might see > this new protocol in inetsw[], so we have to make sure new protocol > is ready for use. All pending memory updates should thus be committed > to memory before setting the pointer. > This is correctly done using rcu_assign_pointer() > > synchronize_net() is typically used at unregister time, after > unsetting the pointer, to make sure no other cpu is still using > the object we want to dismantle. Using it at register time > is only adding an artificial delay that could hide a real bug, > and this bug could popup if/when synchronize_rcu() can proceed > faster than now. > > This saves about 13 ms on boot time on a HZ=1000 8 cpus machine ;) > (4 calls to inet_register_protosw(), and about 3200 us per call) > > Signed-off-by: Eric Dumazet I think this change is fine, so I'm adding it to net-next-2.6 If you guys want to continue discussing the merits of putting comments in every spot where we lack a RCU sync call, that's your call. :-)