From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753178Ab0CFBp6 (ORCPT ); Fri, 5 Mar 2010 20:45:58 -0500 Received: from relay1-d.mail.gandi.net ([217.70.183.193]:49414 "EHLO relay1-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751721Ab0CFBp5 (ORCPT ); Fri, 5 Mar 2010 20:45:57 -0500 Date: Fri, 5 Mar 2010 17:45:13 -0800 From: Josh Triplett To: Steven Rostedt Cc: "Paul E. McKenney" , linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca, dvhltc@us.ibm.com, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, Frederic Weisbecker , Ingo Molnar Subject: Re: [PATCH tip/core/rcu 1/3] ftrace: replace read_barrier_depends() with rcu_dereference_raw() Message-ID: <20100306014513.GA13858@feather> References: <20100305230256.GA9410@linux.vnet.ibm.com> <1267830207-9474-1-git-send-email-paulmck@linux.vnet.ibm.com> <1267838945.10871.1786.camel@gandalf.stny.rr.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1267838945.10871.1786.camel@gandalf.stny.rr.com> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Mar 05, 2010 at 08:29:05PM -0500, Steven Rostedt wrote: > On Fri, 2010-03-05 at 15:03 -0800, Paul E. McKenney wrote: > > Replace the calls to read_barrier_depends() in ftrace_list_func() with > > rcu_dereference_raw() to improve readability. The reason that we use > > rcu_dereference_raw() here is that removed entries are never freed, > > instead they are simply leaked. This is one of a very few cases where > > use of rcu_dereference_raw() is the long-term right answer. And I don't > > yet know of any others. ;-) > > > > Cc: Steven Rostedt > > Acked-by: Steven Rostedt > > Thanks Paul! > > > Cc: Frederic Weisbecker > > Cc: Ingo Molnar > > Signed-off-by: Paul E. McKenney > > --- > > > > > @@ -154,8 +159,7 @@ static int __register_ftrace_function(struct ftrace_ops *ops) > > * the ops->next pointer is valid before another CPU sees > > * the ops pointer included into the ftrace_list. > > */ > > - smp_wmb(); > > - ftrace_list = ops; > > + rcu_assign_pointer(ftrace_list, ops); > > [ Off topic ] > > I looked at rcu_assign_pointer() and it is: > > #define rcu_assign_pointer(p, v) \ > ({ \ > if (!__builtin_constant_p(v) || \ > ((v) != NULL)) \ > smp_wmb(); \ > (p) = (v); \ > }) > > My question is, why that crazy if? The only time that will fail is if we > are assigning the constant NULL to p. What makes NULL so important here? > Can't there be a case when assigning NULL to p will require that wmb()? The barrier ensures that the reader can't see the new p and the old *p. Since you can't look at *NULL, that concern doesn't apply. - Josh Triplett