From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758056AbZBXOus (ORCPT ); Tue, 24 Feb 2009 09:50:48 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1756634AbZBXOuk (ORCPT ); Tue, 24 Feb 2009 09:50:40 -0500 Received: from e3.ny.us.ibm.com ([32.97.182.143]:47636 "EHLO e3.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756622AbZBXOuj (ORCPT ); Tue, 24 Feb 2009 09:50:39 -0500 Date: Tue, 24 Feb 2009 06:50:35 -0800 From: "Paul E. McKenney" To: Nick Piggin Cc: linux-kernel@vger.kernel.org, vegard.nossum@gmail.com, mingo@elte.hu, stable@kernel.org, akpm@linux-foundation.org, npiggin@suse.de, penberg@cs.helsinki.fi Subject: Re: [PATCH] v2 Teach RCU that idle task is not quiscent state at boot Message-ID: <20090224145035.GA12157@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20090223161611.GA9563@linux.vnet.ibm.com> <20090223204332.GA19861@linux.vnet.ibm.com> <200902241502.57833.nickpiggin@yahoo.com.au> <20090224050854.GD7173@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20090224050854.GD7173@linux.vnet.ibm.com> User-Agent: Mutt/1.5.15+20070412 (2007-04-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Feb 23, 2009 at 09:08:54PM -0800, Paul E. McKenney wrote: > On Tue, Feb 24, 2009 at 03:02:56PM +1100, Nick Piggin wrote: > > On Tuesday 24 February 2009 07:43:32 Paul E. McKenney wrote: > > > > > @@ -80,6 +81,10 @@ void wakeme_after_rcu(struct rcu_head *head) > > > void synchronize_rcu(void) > > > { > > > struct rcu_synchronize rcu; > > > + > > > + if (num_online_cpus() == 1) > > > + return; /* If UP, synchronize_rcu() is a grace period! */ > > > + > > > > Nice patch... May I just suggest a comment to say that this is > > also a correctness requirement due to the grace period holdoff > > for early boot? > > Excellent point! If Ingo reports that this actually fixes the hang that > he saw, I will add such a comment. > > (And glad you like the patch!) Gah!!! I forgot about preemptable RCU, which the previous patch breaks completely on single-CPU systems. A fix is in the works. Thanx, Paul