From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751470Ab0CSWtj (ORCPT ); Fri, 19 Mar 2010 18:49:39 -0400 Received: from e5.ny.us.ibm.com ([32.97.182.145]:49479 "EHLO e5.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751212Ab0CSWti (ORCPT ); Fri, 19 Mar 2010 18:49:38 -0400 Date: Fri, 19 Mar 2010 15:49:32 -0700 From: "Paul E. McKenney" To: Alexey Dobriyan Cc: Mathieu Desnoyers , akpm@linux-foundation.org, Ingo Molnar , linux-kernel@vger.kernel.org, laijs@cn.fujitsu.com, dipankar@in.ibm.com, 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, eric.dumazet@gmail.com Subject: Re: [RFC patch 2/3] tree/tiny rcu: Add debug RCU head objects (v3) Message-ID: <20100319224932.GE2520@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20100319204739.574593298@efficios.com> <20100319205233.474028085@efficios.com> <20100319221000.GA5276@x200> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20100319221000.GA5276@x200> 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, Mar 20, 2010 at 12:10:00AM +0200, Alexey Dobriyan wrote: > On Fri, Mar 19, 2010 at 04:47:41PM -0400, Mathieu Desnoyers wrote: > > Helps finding racy users of call_rcu(), which results in hangs because list > > entries are overwritten and/or skipped. > > > > This new patch version is based on the debugobjects with the newly introduced > > "active state" tracker. > > > > Non-initialized entries are all considered as "statically initialized". An > > activation fixup (triggered by call_rcu()) takes care of performing the debug > > object initialization without issuing any warning. Since we cannot increase the > > size of struct rcu_head, I don't see much room to put an identifier for > > statically initialized rcu_head structures. So for now, we have to live without > > "activation without explicit init" detection. But the main purpose of this debug > > option is to detect double-activations (double call_rcu() use of a rcu_head > > before the callback is executed), which is correctly addressed here. > > > > This also detects potential internal RCU callback corruption, which would cause > > the callbacks to be executed twice. > > Is this useful? > > Basic usage is so there no double call_rcu(): > > if (atomic_dec_and_test()) > call_rcu() I believe that it is. There have been a few cases of call_rcu() being invoked twice without a grace period between the two invocations. Mathieu's patch would catch this sort of misbehavior. That said, I do agree that if everyone followed the rules, there would be no need for Mathieu's patch -- and there would be no need for much else, besides. ;-) Thanx, Paul