From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932404AbVI3AXJ (ORCPT ); Thu, 29 Sep 2005 20:23:09 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S932406AbVI3AXJ (ORCPT ); Thu, 29 Sep 2005 20:23:09 -0400 Received: from e33.co.us.ibm.com ([32.97.110.151]:27324 "EHLO e33.co.us.ibm.com") by vger.kernel.org with ESMTP id S932404AbVI3AXH (ORCPT ); Thu, 29 Sep 2005 20:23:07 -0400 Date: Thu, 29 Sep 2005 17:23:46 -0700 From: "Paul E. McKenney" To: Suzanne Wood Cc: herbert@gondor.apana.org.au, 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: <20050930002346.GP8177@us.ibm.com> Reply-To: paulmck@us.ibm.com References: <200509292330.j8TNUSmH019572@rastaban.cs.pdx.edu> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <200509292330.j8TNUSmH019572@rastaban.cs.pdx.edu> User-Agent: Mutt/1.4.1i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Sep 29, 2005 at 04:30:28PM -0700, Suzanne Wood wrote: > > Date: Fri, 30 Sep 2005 07:28:36 +1000 > > From: Herbert Xu > > > On Thu, Sep 29, 2005 at 09:02:29AM -0700, Suzanne Wood wrote: > > > > > > The exchange below suggests that it is equally important > > > to have the rcu_dereference() in __in_dev_get(), so the > > > idea of the only difference between in_dev_get and > > > __in_dev_get being the refcnt may be accepted. > > > With __in_dev_get() it's the caller's responsibility to ensure > > that RCU works correctly. Therefore if any rcu_dereference is > > needed it should be done by the caller. > > This sounds reasonable to me. Does everyone agree? 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). If all __in_dev_get() invocations need to be wrapped in rcu_dereference(), then it seems to me that there would be motivation to bury rcu_dereference() in __in_dev_get(). > > Some callers of __in_dev_get() don't need rcu_dereference at all > > because they're protected by the rtnl. > > > BTW, could you please move the rcu_dereference in in_dev_get() > > into the if clause? The barrier is not needed when ip_ptr is > > NULL. > > The trouble with that may be that there are three events, the > dereference, the assignment, and the conditional test. The > rcu_dereference() is meant to assure deferred destruction > throughout. One only needs an rcu_dereference() once on the data-flow path from fetching the RCU-protected pointer to dereferencing that pointer. If the pointer is NULL, there is no way you can dereference it, so, technically, Herbert is quite correct. 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". Thanx, Paul