linux-kernel - Re: [PATCH -tip/core/rcu] Fix rcu_migrate_callback() to allow for multiple waiters

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Date:	Wed, 19 Aug 2009 20:45:39 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	linux-kernel@...r.kernel.org, mathieu.desnoyers@...ymtl.ca,
	mingo@...e.hu, josht@...ux.vnet.ibm.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, dvhltc@...ibm.com, niv@...ibm.com,
	tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
	hugh.dickins@...cali.co.uk, benh@...nel.crashing.org
Subject: Re: [PATCH -tip/core/rcu] Fix rcu_migrate_callback() to allow for
	multiple waiters

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 <mathieu.desnoyers@...ymtl.ca>
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > ---
> > 
> >  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
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