From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932406AbVI3A1n (ORCPT ); Thu, 29 Sep 2005 20:27:43 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S932410AbVI3A1n (ORCPT ); Thu, 29 Sep 2005 20:27:43 -0400 Received: from 22.107.233.220.exetel.com.au ([220.233.107.22]:46861 "EHLO arnor.apana.org.au") by vger.kernel.org with ESMTP id S932406AbVI3A1n (ORCPT ); Thu, 29 Sep 2005 20:27:43 -0400 Date: Fri, 30 Sep 2005 10:27:19 +1000 To: "Paul E. McKenney" Cc: Suzanne Wood , Robert.Olsson@data.slu.se, davem@davemloft.net, linux-kernel@vger.kernel.org, netdev@oss.sgi.com, walpole@cs.pdx.edu Subject: Re: [RFC][PATCH] identify in_dev_get rcu read-side critical sections Message-ID: <20050930002719.GC21062@gondor.apana.org.au> References: <200509292330.j8TNUSmH019572@rastaban.cs.pdx.edu> <20050930002346.GP8177@us.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20050930002346.GP8177@us.ibm.com> User-Agent: Mutt/1.5.9i From: Herbert Xu Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Sep 29, 2005 at 05:23:46PM -0700, Paul E. McKenney wrote: > > Is there any case where __in_dev_get() might be called without > needing to be wrapped with rcu_dereference()? If so, then I > agree (FWIW, given my meagre knowledge of Linux networking). Yes. All paths that call __in_dev_get() under the rtnl do not need rcu_dereference (or any RCU at all) since the rtnl prevents any ip_ptr modification from occuring. > However, rcu_dereference() only generates a memory barrier on DEC > Alpha, so there is normally no penalty for using it in the NULL-pointer > case. So, when using rcu_dereference() unconditionally simplifies > the code, it may make sense to "just do it". Here is what the code would look like: rcu_read_lock(); in_dev = dev->ip_ptr; if (in_dev) { in_dev = rcu_dereference(in_dev); atomic_inc(&in_dev->refcnt); } rcu_read_unlock(); return in_dev; Cheers, -- Visit Openswan at http://www.openswan.org/ Email: Herbert Xu ~{PmV>HI~} Home Page: http://gondor.apana.org.au/~herbert/ PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt