Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S262731AbVCXHqI (ORCPT ); Thu, 24 Mar 2005 02:46:08 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S262733AbVCXHqI (ORCPT ); Thu, 24 Mar 2005 02:46:08 -0500 Received: from e35.co.us.ibm.com ([32.97.110.133]:62669 "EHLO e35.co.us.ibm.com") by vger.kernel.org with ESMTP id S262731AbVCXHqE (ORCPT ); Thu, 24 Mar 2005 02:46:04 -0500 Date: Wed, 23 Mar 2005 23:46:13 -0800 From: "Paul E. McKenney" To: Ingo Molnar Cc: linux-kernel@vger.kernel.org, Esben Nielsen Subject: Re: [patch] Real-Time Preemption, -RT-2.6.12-rc1-V0.7.41-07 Message-ID: <20050324074613.GJ1298@us.ibm.com> Reply-To: paulmck@us.ibm.com References: <20050322054345.GB1296@us.ibm.com> <20050322072413.GA6149@elte.hu> <20050322092331.GA21465@elte.hu> <20050322093201.GA21945@elte.hu> <20050322100153.GA23143@elte.hu> <20050322112856.GA25129@elte.hu> <20050323061601.GE1294@us.ibm.com> <20050323063317.GB31626@elte.hu> <20050324052854.GA1298@us.ibm.com> <20050324053456.GA14494@elte.hu> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20050324053456.GA14494@elte.hu> User-Agent: Mutt/1.4.1i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1422 Lines: 29 On Thu, Mar 24, 2005 at 06:34:56AM +0100, Ingo Molnar wrote: > > * Paul E. McKenney wrote: > > > Now, it is true that CPU#2 might record a quiescent state during this > > time, but this will have no effect because -all- CPUs must pass > > through a quiescent state before any callbacks will be invoked. Since > > CPU#1 is refusing to record a quiescent state, grace periods will be > > blocked for the full extent of task 1's RCU read-side critical > > section. > > ok, great. So besides the barriers issue (and the long grace period time > issue), the current design is quite ok. And i think your original flip > pointers suggestion can be used to force synchronization. The thing I am currently struggling with on the flip-pointers approach is handling races between rcu_read_lock() and the flipping. In the earlier implementations that used this trick, you were guaranteed that if you were executing concurrently with one flip, you would do a voluntary context switch before the next flip happened, so that the race was harmless. This guarantee does not work in the PREEMPT_RT case, so more thought will be required. :-/ Thanx, Paul - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/