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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Sat, Jan 05, 2013 at 06:21:01PM +0100, Frederic Weisbecker wrote:
> Hi Paul,
> 
> 2013/1/5 Paul E. McKenney <paulmck@linux.vnet.ibm.com>:
> > From: Paul Gortmaker <paul.gortmaker@windriver.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@gmail.com>
> > Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
> > Signed-off-by: Paul E. McKenney <paulmck@linux.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