From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756205Ab2IFO31 (ORCPT ); Thu, 6 Sep 2012 10:29:27 -0400 Received: from casper.infradead.org ([85.118.1.10]:45358 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756166Ab2IFO3Z convert rfc822-to-8bit (ORCPT ); Thu, 6 Sep 2012 10:29:25 -0400 Message-ID: <1346941673.18408.21.camel@twins> Subject: Re: [PATCH tip/core/rcu 18/23] rcu: Add random PROVE_RCU_DELAY to grace-period initialization From: Peter Zijlstra To: "Paul E. McKenney" 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, rostedt@goodmis.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, eric.dumazet@gmail.com, darren@dvhart.com, fweisbec@gmail.com, sbw@mit.edu, patches@linaro.org Date: Thu, 06 Sep 2012 16:27:53 +0200 In-Reply-To: <1346350718-30937-18-git-send-email-paulmck@linux.vnet.ibm.com> References: <20120830181811.GA29154@linux.vnet.ibm.com> <1346350718-30937-1-git-send-email-paulmck@linux.vnet.ibm.com> <1346350718-30937-18-git-send-email-paulmck@linux.vnet.ibm.com> Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7BIT X-Mailer: Evolution 3.2.2- Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 2012-08-30 at 11:18 -0700, Paul E. McKenney wrote: > > 1. CPU 0 completes a grace period, but needs an additional > grace period, so starts initializing one, initializing all > the non-leaf rcu_node strcutures and the first leaf rcu_node > structure. Because CPU 0 is both completing the old grace > period and starting a new one, it marks the completion of > the old grace period and the start of the new grace period > in a single traversal of the rcu_node structures. > > Therefore, CPUs corresponding to the first rcu_node structure > can become aware that the prior grace period has ended, but > CPUs corresponding to the other rcu_node structures cannot > yet become aware of this. > > 2. CPU 1 passes through a quiescent state, and therefore informs > the RCU core. Because its leaf rcu_node structure has already > been initialized, so this CPU's quiescent state is applied to > the new (and only partially initialized) grace period. > > 3. CPU 1 enters an RCU read-side critical section and acquires > a reference to data item A. Note that this critical section > will not block the new grace period. > > 4. CPU 16 exits dyntick-idle mode. Because it was in dyntick-idle > mode, some other CPU informed the RCU core of its extended > quiescent state for the past several grace periods. This means > that CPU 16 is not yet aware that these grace periods have ended. > > 5. CPU 16 on the second leaf rcu_node structure removes data item A > from its enclosing data structure and passes it to call_rcu(), > which queues a callback in the RCU_NEXT_TAIL segment of the > callback queue. > > 6. CPU 16 enters the RCU core, possibly because it has taken a > scheduling-clock interrupt, or alternatively because it has > more than 10,000 callbacks queued. It notes that the second > most recent grace period has ended (recall that it cannot yet > become aware that the most recent grace period has completed), > and therefore advances its callbacks. The callback for data > item A is therefore in the RCU_NEXT_READY_TAIL segment of the > callback queue. > > 7. CPU 0 completes initialization of the remaining leaf rcu_node > structures for the new grace period, including the structure > corresponding to CPU 16. > > 8. CPU 16 again enters the RCU core, again, possibly because it has > taken a scheduling-clock interrupt, or alternatively because > it now has more than 10,000 callbacks queued. It notes that > the most recent grace period has ended, and therefore advances > its callbacks. The callback for data item A is therefore in > the RCU_NEXT_TAIL segment of the callback queue. > > 9. All CPUs other than CPU 1 pass through quiescent states, so that > the new grace period completes. Note that CPU 1 is still in > its RCU read-side critical section, still referencing data item A. > > 10. Suppose that CPU 2 is the last CPU to pass through a quiescent > state for the new grace period, and suppose further that CPU 2 > does not have any callbacks queued. It therefore traverses > all of the rcu_node structures, marking the new grace period > as completed, but does not initialize a new grace period. > > 11. CPU 16 yet again enters the RCU core, yet again possibly because > it has taken a scheduling-clock interrupt, or alternatively > because it now has more than 10,000 callbacks queued. It notes > that the new grace period has ended, and therefore advances > its callbacks. The callback for data item A is therefore in > the RCU_DONE_TAIL segment of the callback queue. This means > that this callback is now considered ready to be invoked. > > 12. CPU 16 invokes the callback, freeing data item A while CPU 1 > is still referencing it. This is the same scenario as the previous patch (17), right? However did you find a 12-stage race like that, is that PROMELA goodness or are you training to beat some chess champion?