From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751274AbVI3BP2 (ORCPT ); Thu, 29 Sep 2005 21:15:28 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751397AbVI3BP2 (ORCPT ); Thu, 29 Sep 2005 21:15:28 -0400 Received: from e1.ny.us.ibm.com ([32.97.182.141]:38617 "EHLO e1.ny.us.ibm.com") by vger.kernel.org with ESMTP id S1751274AbVI3BP1 (ORCPT ); Thu, 29 Sep 2005 21:15:27 -0400 Date: Thu, 29 Sep 2005 18:16:03 -0700 From: "Paul E. McKenney" To: Herbert Xu 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: <20050930011603.GT8177@us.ibm.com> Reply-To: paulmck@us.ibm.com References: <200509292330.j8TNUSmH019572@rastaban.cs.pdx.edu> <20050930002346.GP8177@us.ibm.com> <20050930002719.GC21062@gondor.apana.org.au> <20050930003642.GQ8177@us.ibm.com> <20050930010404.GA21429@gondor.apana.org.au> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20050930010404.GA21429@gondor.apana.org.au> User-Agent: Mutt/1.4.1i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Sep 30, 2005 at 11:04:04AM +1000, Herbert Xu wrote: > On Thu, Sep 29, 2005 at 05:36:42PM -0700, Paul E. McKenney wrote: > > > > > 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; > > > > How about: > > > > rcu_read_lock(); > > in_dev = dev->ip_ptr; > > if (rcu_dereference(in_dev)) { > > atomic_inc(&in_dev->refcnt); > > } > > rcu_read_unlock(); > > return in_dev; > > With this the barrier will taken even when in_dev is NULL. > > I agree this isn't such a big deal since it only impacts Alpha and then > only when in_dev is NULL. But as we already do the branch anyway to > increment the reference count, we might as well make things a little > better for Alpha. OK, how about this instead? rcu_read_lock(); in_dev = dev->ip_ptr; if (in_dev) { atomic_inc(&rcu_dereference(in_dev)->refcnt); } rcu_read_unlock(); return in_dev; Thanx, Paul