From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751406AbZHTDpm (ORCPT ); Wed, 19 Aug 2009 23:45:42 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751197AbZHTDpl (ORCPT ); Wed, 19 Aug 2009 23:45:41 -0400 Received: from e6.ny.us.ibm.com ([32.97.182.146]:49964 "EHLO e6.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751185AbZHTDpl (ORCPT ); Wed, 19 Aug 2009 23:45:41 -0400 Date: Wed, 19 Aug 2009 20:45:39 -0700 From: "Paul E. McKenney" To: Lai Jiangshan Cc: linux-kernel@vger.kernel.org, mathieu.desnoyers@polymtl.ca, mingo@elte.hu, josht@linux.vnet.ibm.com, dipankar@in.ibm.com, akpm@linux-foundation.org, dvhltc@us.ibm.com, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, hugh.dickins@tiscali.co.uk, benh@kernel.crashing.org Subject: Re: [PATCH -tip/core/rcu] Fix rcu_migrate_callback() to allow for multiple waiters Message-ID: <20090820034539.GA6847@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20090820013153.GA19944@linux.vnet.ibm.com> <4A8CC365.8040007@cn.fujitsu.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4A8CC365.8040007@cn.fujitsu.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 Thu, Aug 20, 2009 at 11:30:45AM +0800, Lai Jiangshan wrote: > Paul E. McKenney wrote: > > The current interaction between rcu_barrier() and CPU hotplug can result > > in multiple tasks waiting on rcu_migrate_wq: there can be one CPU-hotplug > > notifier, and, because rcu_barrier() invokes wait_migrated_callbacks() > > outside of rcu_barrier_mutex, an arbitrarily large number of tasks > > executing in _rcu_barrier(). This situation could result in hangs, > > because rcu_migrate_callback() would wake up but one task. > > > > This patch addresses this problem by awakening all sleepers. > > > > Located-by: Mathieu Desnoyers > > Signed-off-by: Paul E. McKenney > > --- > > > > rcupdate.c | 9 ++++++++- > > 1 file changed, 8 insertions(+), 1 deletion(-) > > > > diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c > > index bd5d5c8..6c59e1b 100644 > > --- a/kernel/rcupdate.c > > +++ b/kernel/rcupdate.c > > @@ -211,10 +211,17 @@ void rcu_barrier_sched(void) > > } > > EXPORT_SYMBOL_GPL(rcu_barrier_sched); > > > > +/* > > + * Wake up anyone waiting on migration of RCU callbacks from a CPU > > + * going offline. There can be only one CPU-hotplug notifier waiting, > > + * but there can be any number of rcu_barrier() invocations waiting, > > + * due to the fact that rcu_barrier() does its wait outside of the > > + * rcu_barrier_mutex. > > + */ > > static void rcu_migrate_callback(struct rcu_head *notused) > > { > > if (atomic_dec_and_test(&rcu_migrate_type_count)) > > - wake_up(&rcu_migrate_wq); > > + wake_up_all(&rcu_migrate_wq); > > } > > > > extern int rcu_cpu_notify(struct notifier_block *self, > > > > Maybe wake_up_all() is good for readability. But wake_up() > is enough for the logic. > > In wait_migrated_callbacks(), we use wait_event(), The current > thread is queued as a non-exclusive waiter. And wake_up() > wakes up all non-exclusive waiter. (We only have > non-exclusive waiters in the rcu_migrate_wq) And we even discussed this very point on March 18th!!! OK, never mind, this patch has no effect. Thanx, Paul