From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752882Ab1HAFXv (ORCPT ); Mon, 1 Aug 2011 01:23:51 -0400 Received: from e4.ny.us.ibm.com ([32.97.182.144]:50056 "EHLO e4.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752637Ab1HAFXp (ORCPT ); Mon, 1 Aug 2011 01:23:45 -0400 Date: Sun, 31 Jul 2011 22:23:42 -0700 From: "Paul E. McKenney" To: David Miller Cc: eric.dumazet@gmail.com, shemminger@vyatta.com, mchan@broadcom.com, linux-kernel@vger.kernel.org Subject: Re: RCU: how to suppress warnings from rcu_assign_pointer? Message-ID: <20110801052342.GD2307@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20110729003843.GA2373@linux.vnet.ibm.com> <20110729105058.2cb9fa86@nehalam.ftrdhcpuser.net> <1312028225.2873.104.camel@edumazet-laptop> <20110730.201609.2157787571393748046.davem@davemloft.net> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20110730.201609.2157787571393748046.davem@davemloft.net> 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 Sat, Jul 30, 2011 at 08:16:09PM -0700, David Miller wrote: > From: Eric Dumazet > Date: Sat, 30 Jul 2011 14:17:05 +0200 > > > Le vendredi 29 juillet 2011 à 10:50 -0700, Stephen Hemminger a écrit : > >> Gcc now generates warnings from rcu_assign_pointer when passed the > >> address of something for example: > >> rcu_assign_pointer(dev_queue->qdisc, &noop_qdisc); > >> This warning is harmless and should be surpressed but there maybe > >> other cases where we want that Gcc warning. > >> > >> I tried various combinations of in rcu_assign_pointer macro > >> #pragma GCC diagnostic push > >> #pragma GCC diagnostic ignored "-Wlogical-op" > >> ... > >> #pragma GCC diagnostic pop > >> but macro's and pragma's don't nest with the correct scope for > >> this. > >> > >> Maybe some one with more Gcc foo and time to waste could take > >> a crack at it. > > > > I would just remove the test from rcu_assign_pointer(). > > > > We now have RCU_INIT_POINTER() for places we dont want/afford the > > smp_wmb() > > Agreed: > > Acked-by: David S. Miller Very good!!! Many thanks to the three of you! I have queued this, adding Eric's Signed-off-by. Eric, could you please reply either giving me explicit permission to do this or telling me that you would rather that I apply under my own Signed-off-by? (I would guess that the former is OK with you, but I really do need to know for sure.) Please see below for the current state of this commit in my on-laptop -rcu tree. I have also added a second commit upgrading comment headers to call out exactly when RCU_INIT_POINTER() may be used. Thanx, Paul ------------------------------------------------------------------------ rcu: Make rcu_assign_pointer() unconditionally insert a memory barrier Recent changes to gcc give warning messages on rcu_assign_pointers()'s checks that allow it to determine when it is OK to omit the memory barrier. Stephen Hemminger tried a number of gcc tricks to silence this warning, but #pragmas and CPP macros do not work together in the way that would be required to make this work. However, we now have RCU_INIT_POINTER(), which already omits this memory barrier, and which therefore may be used when assigning NULL to an RCU-protected pointer that is accessible to readers. This commit therefore makes rcu_assign_pointer() unconditionally emit the memory barrier. Reported-by: Stephen Hemminger Signed-off-by: Eric Dumazet Acked-by: David S. Miller Signed-off-by: Paul E. McKenney diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index ea80396..b2e5fe8 100644 --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -443,9 +443,7 @@ extern int rcu_my_thread_group_empty(void); }) #define __rcu_assign_pointer(p, v, space) \ ({ \ - if (!__builtin_constant_p(v) || \ - ((v) != NULL)) \ - smp_wmb(); \ + smp_wmb(); \ (p) = (typeof(*v) __force space *)(v); \ })