From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753130Ab0BWOf2 (ORCPT ); Tue, 23 Feb 2010 09:35:28 -0500 Received: from e9.ny.us.ibm.com ([32.97.182.139]:59602 "EHLO e9.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752666Ab0BWOf0 (ORCPT ); Tue, 23 Feb 2010 09:35:26 -0500 Date: Tue, 23 Feb 2010 06:35:21 -0800 From: "Paul E. McKenney" To: Mathieu Desnoyers Cc: Arnd Bergmann , linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, josh@joshtriplett.org, dvhltc@us.ibm.com, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com Subject: Re: [PATCH tip/core/rcu 0/21] v6 add lockdep-based diagnostics to rcu_dereference() Message-ID: <20100223143520.GA6700@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20100223010435.GA666@linux.vnet.ibm.com> <201002231359.07910.arnd@arndb.de> <20100223131543.GA19174@Krystal> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20100223131543.GA19174@Krystal> User-Agent: Mutt/1.5.15+20070412 (2007-04-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Feb 23, 2010 at 08:15:44AM -0500, Mathieu Desnoyers wrote: > * Arnd Bergmann (arnd@arndb.de) wrote: > > On Tuesday 23 February 2010, Paul E. McKenney wrote: > > > This patch series adds lockdep-based checking to the rcu_dereference() > > > primitive in order to flag misuses of RCU. > > > > While I haven't looked much at this series, I've been thinking about > > adding static diagnostics for rcu_dereference misuse, in the form of > > an __rcu address space qualifier for pointers. Such a patch would > > obviously conflict with this series, so I'd wait for yours to go > > in first, but maybe you like the idea enough to do it yourself ;-). > > > > The observation is that all accesses to an RCU protected pointer > > are either through rcu_dereference, rcu_assign_pointer or one of their > > variants. so it should be possible to add a new address space like we > > have for __iomem, __user and soon __percpu and have sparse check that > > we use RCU consistently on pointers that need it. > > Just to make myself the devil's advocate: how should we consider > initialization of RCU pointers at boot time that happens before any > possible concurrent reader is allowed to run ? I think this case is an > example of valid RCU-pointer access that is not done through the RCU > primitives. It seems valid to perform these RCU-pointer accesses when > serialized by a different exclusion mechanism, in this case being the > guarantee that no concurrent reader are running at early boot. The same > applies to stop_machine(), and, as I come to think of it, we could > probably think of a scheme that dynamically switch from an RCU read-lock > to, e.g., a mutex for all users, wait for RCU quiescent state, and then > serialize all users on the mutex during the update. So while some of > these cases are a bit far-fetched, I think they are valid, and I wonder > how the address space validation would take them into account. And this is an excellent exposition of a few of the initialization issues I referred to in my earlier email! Thanx, Paul