From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755907Ab0IWRXn (ORCPT ); Thu, 23 Sep 2010 13:23:43 -0400 Received: from e5.ny.us.ibm.com ([32.97.182.145]:56489 "EHLO e5.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754493Ab0IWRXm (ORCPT ); Thu, 23 Sep 2010 13:23:42 -0400 Date: Thu, 23 Sep 2010 10:23:37 -0700 From: "Paul E. McKenney" To: Arnd Bergmann Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca, josh@joshtriplett.org, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, eric.dumazet@gmail.com Subject: Re: [PATCH tip/core/rcu 5/8] rcu: only one evaluation of arg in rcu_dereference_check() unless sparse Message-ID: <20100923172337.GF2406@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20100923005033.GA22006@linux.vnet.ibm.com> <1285203053-22068-5-git-send-email-paulmck@linux.vnet.ibm.com> <201009231513.40126.arnd@arndb.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <201009231513.40126.arnd@arndb.de> 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 Thu, Sep 23, 2010 at 03:13:39PM +0200, Arnd Bergmann wrote: > On Thursday 23 September 2010, Paul E. McKenney wrote: > > The current version of the __rcu_access_pointer(), __rcu_dereference_check(), > > and __rcu_dereference_protected() macros evaluate their "p" argument > > three times, not counting typeof()s. This is bad news if that argument > > contains a side effect. This commit therefore evaluates this argument > > only once in normal kernel builds. However, the straightforward approach > > defeats sparse's RCU-pointer checking, so when __CHECKER__ is defined, > > the additional pair of evaluations of the "p" argument are performed in > > order to permit sparse to detect misuse of RCU-protected pointers. > > > > Signed-off-by: Paul E. McKenney > > Cc: Arnd Bergmann > > Sorry for not following up with the alternative approach yet. We can always > do that later, for your patch should go into 2.6.36 to fix any problems > arising from unintended macro side-effects. > > Acked-by: Arnd Bergmann No problem, when you get me a better approach, I will happily queue it! Thanx, Paul