Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758986AbYF0Fwc (ORCPT ); Fri, 27 Jun 2008 01:52:32 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1758665AbYF0Fv4 (ORCPT ); Fri, 27 Jun 2008 01:51:56 -0400 Received: from E23SMTP02.au.ibm.com ([202.81.18.163]:41180 "EHLO e23smtp02.au.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754624AbYF0Fvi (ORCPT ); Fri, 27 Jun 2008 01:51:38 -0400 Date: Fri, 27 Jun 2008 11:19:59 +0530 From: Dhaval Giani To: Dipankar Sarma Cc: Gautham R Shenoy , "Paul E. McKenney" , Ingo Molnar , laijs@cn.fujitsu.com, Peter Zijlstra , lkml , Rusty Russel Subject: Re: [PATCH] fix rcu vs hotplug race Message-ID: <20080627054959.GB3309@linux.vnet.ibm.com> Reply-To: Dhaval Giani References: <20080623103700.GA4043@linux.vnet.ibm.com> <20080623105844.GC28192@elte.hu> <20080623114941.GB3160@in.ibm.com> <20080624110144.GA8695@elte.hu> <20080626152728.GA24972@linux.vnet.ibm.com> <20080627044738.GC3419@in.ibm.com> <20080627051855.GD26167@in.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20080627051855.GD26167@in.ibm.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1780 Lines: 53 On Fri, Jun 27, 2008 at 10:48:55AM +0530, Dipankar Sarma wrote: > On Fri, Jun 27, 2008 at 10:17:38AM +0530, Gautham R Shenoy wrote: > > IMHO the warning is a spurious one. > > Here's the timeline. > > CPU_A CPU_B > > -------------------------------------------------------------- > > cpu_down(): . > > . . > > . . > > stop_machine(): /* disables preemption, . > > * and irqs */ . > > . . > > . . > > take_cpu_down(); . > > . . > > . . > > . . > > cpu_disable(); /*this removes cpu . > > *from cpu_online_map . > > */ . > > . . > > . . > > restart_machine(); /* enables irqs */ . > > ------WINDOW DURING WHICH rcp->cpumask is stale --------------- > > . call_rcu(); > > . /* disables irqs here */ > > . .force_quiescent_state(); > > .CPU_DEAD: .for_each_cpu(rcp->cpumask) > > . . smp_send_reschedule(); > > . . > > . . WARN_ON() for offlined CPU! > > . > > Exactly. The call_rcu()s are coming from a different subsystem > and can happen anytime during the CPU hotplug path. So, RCU subsystem > doesn't have anything to do to keep rcu->cpumask consistent. > It is *safe* even if we miss poking a cpu or two while > forcing quiescent state in all CPUs. The worst that can happen > is a delay in grace period. No correctness problem here. > One question. What is preventing a CPU from clearing its mask after we have checked whether it is online but before we have called into smp_send_reschedule? -- regards, Dhaval -- 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/