linux-kernel - Re: [PATCH tip/core/urgent 1/2] rcu: Prevent soft-lockup complaints about no-CBs CPUs

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Date:	Sat, 5 Jan 2013 09:54:40 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	edumazet@...gle.com, darren@...art.com, sbw@....edu,
	patches@...aro.org, Paul Gortmaker <paul.gortmaker@...driver.com>
Subject: Re: [PATCH tip/core/urgent 1/2] rcu: Prevent soft-lockup complaints
 about no-CBs CPUs

On Sat, Jan 05, 2013 at 06:21:01PM +0100, Frederic Weisbecker wrote:
> Hi Paul,
> 
> 2013/1/5 Paul E. McKenney <paulmck@...ux.vnet.ibm.com>:
> > From: Paul Gortmaker <paul.gortmaker@...driver.com>
> >
> > The wait_event() at the head of the rcu_nocb_kthread() can result in
> > soft-lockup complaints if the CPU in question does not register RCU
> > callbacks for an extended period.  This commit therefore changes
> > the wait_event() to a wait_event_interruptible().
> >
> > Reported-by: Frederic Weisbecker <fweisbec@...il.com>
> > Signed-off-by: Paul Gortmaker <paul.gortmaker@...driver.com>
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > ---
> >  kernel/rcutree_plugin.h |    3 ++-
> >  1 files changed, 2 insertions(+), 1 deletions(-)
> >
> > diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> > index f6e5ec2..43dba2d 100644
> > --- a/kernel/rcutree_plugin.h
> > +++ b/kernel/rcutree_plugin.h
> > @@ -2366,10 +2366,11 @@ static int rcu_nocb_kthread(void *arg)
> >         for (;;) {
> >                 /* If not polling, wait for next batch of callbacks. */
> >                 if (!rcu_nocb_poll)
> > -                       wait_event(rdp->nocb_wq, rdp->nocb_head);
> > +                       wait_event_interruptible(rdp->nocb_wq, rdp->nocb_head);
> >                 list = ACCESS_ONCE(rdp->nocb_head);
> >                 if (!list) {
> >                         schedule_timeout_interruptible(1);
> > +                       flush_signals(current);
> 
> Why is that needed?

To satisfy my paranoia.  ;-)  And in case someone ever figures out some
way to send a signal to a kthread.

							Thanx, Paul

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