From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753164Ab0BWQQL (ORCPT ); Tue, 23 Feb 2010 11:16:11 -0500 Received: from e6.ny.us.ibm.com ([32.97.182.146]:42540 "EHLO e6.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752297Ab0BWQQJ (ORCPT ); Tue, 23 Feb 2010 11:16:09 -0500 Date: Tue, 23 Feb 2010 08:16:02 -0800 From: "Paul E. McKenney" To: Arnd Bergmann Cc: Mathieu Desnoyers , 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: <20100223161602.GD6700@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> <201002231654.09724.arnd@arndb.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <201002231654.09724.arnd@arndb.de> 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 04:54:09PM +0100, Arnd Bergmann wrote: > On Tuesday 23 February 2010, Mathieu Desnoyers wrote: > > 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. > > Like the annotations adding __rcu to each rcu protected pointer, we'd > also have to add annotations to static initialization of those. E.g. > something like > > #define DEFINE_RCU_VAR(type, name, pointer) \ > type __rcu *name = (__force type)pointer > > should do for simple initializations, and I guess I can come up > with similar solutions if we need something more fancy. We would also need something for initialization of structure fields. Does __force work in that case as well? > > 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. > > I assume that it's never wrong to do a pointer assignment using > rcu_assign_pointer, or to do a dereference using rcu_dereference, > even if you hold a mutex or stop_machine. I would also guess that > the performance impact of doing so is not measurable. If both are > true for all corner cases, we could just use the rcu primitives > for any access on those variables for consistency reasons. Seems like a good default position, but please see below. > If there are cases where it does not work, we need to come up with > names for new primitives that just do the assignment or dereference > with __force but no actual synchronization. Some data structures are shared by RCU and non-RCU code, with struct list_head being the most prominent example. Making the "next" pointer as __rcu might be OK, but there are a -lot- of non-RCU uses of struct list_head. Would we really want to introduce rcu_dereference() to all non-RCU list-traversal primitives, or do we need to do something else? Thanx, Paul