From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751264AbWAIT7S (ORCPT ); Mon, 9 Jan 2006 14:59:18 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751270AbWAIT7S (ORCPT ); Mon, 9 Jan 2006 14:59:18 -0500 Received: from e32.co.us.ibm.com ([32.97.110.150]:29862 "EHLO e32.co.us.ibm.com") by vger.kernel.org with ESMTP id S1751264AbWAIT7Q (ORCPT ); Mon, 9 Jan 2006 14:59:16 -0500 Date: Mon, 9 Jan 2006 11:59:33 -0800 From: "Paul E. McKenney" To: Oleg Nesterov Cc: linux-kernel@vger.kernel.org, Dipankar Sarma , Manfred Spraul , Linus Torvalds , Andrew Morton , vatsa@in.ibm.com Subject: Re: [PATCH 2/5] rcu: don't check ->donelist in __rcu_pending() Message-ID: <20060109195933.GE14738@us.ibm.com> Reply-To: paulmck@us.ibm.com References: <43C165BC.F7C6DCF5@tv-sign.ru> <20060109185944.GB15083@us.ibm.com> <43C2C818.65238C30@tv-sign.ru> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <43C2C818.65238C30@tv-sign.ru> User-Agent: Mutt/1.4.1i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 09, 2006 at 11:31:20PM +0300, Oleg Nesterov wrote: > "Paul E. McKenney" wrote: > > > > On Sun, Jan 08, 2006 at 10:19:24PM +0300, Oleg Nesterov wrote: > > > ->donelist becomes != NULL only in rcu_process_callbacks(). > > > > > > rcu_process_callbacks() always calls rcu_do_batch() when > > > ->donelist != NULL. > > > > > > rcu_do_batch() schedules rcu_process_callbacks() again if > > > ->donelist was not flushed entirely. > > > > > > So ->donelist != NULL means that rcu_tasklet is either > > > TASKLET_STATE_SCHED or TASKLET_STATE_RUN, we don't need to > > > check it in __rcu_pending(). > > > > As Vatsa noted, this is needed if the CPU-hotplug case moves > > from ->donelist to ->donelist. It could be omitted if CPU-hotplug > > instead moves from ->donelist to ->nextlist, as is the case in Oleg's > > patch. The extra grace-period delay should not be a problem for the > > presumably rare hotplug case, but: > > Just to be sure. So do you agree that CPU-hotplug is buggy now (without > that patch) ? Hmmm... So your thought is that __rcu_offline_cpu() moves nxtlist and curlist, but not donelist, but then returns to rcu_offline_cpu(), which might well do the tasklet_kill_immediate() before the tasklet completed processing all of donelist. Seems plausible to me. If true, your patch adding the following statement to the ed of __rcu_offline_cpu seems like a reasonable fix: rcu_move_batch(this_rdp, rdp->donelist, rdp->donetail); Vatsa, is there something that Oleg and I are missing? > > o the extra test in __rcu_pending() should be quite inexpensive, > > since the cacheline is already loaded given the earlier tests. > > Yes, it was a cleanup, not an optimization. > > > o although tasklet_schedule() looks to be perfectly reliable > > right now, and although any bugs in tasklet_schedule() must > > be fixed, having RCU leakage be the major symptom of > > tasklet_schedule() failure sounds quite unfriendly to me. > > > > So I am not (yet) convinced that this patch is the way to go. > > Ok, I agree. Sounds good! Thanx, Paul