Re: [GIT PULL rcu/next] rcu commits for 2.6.40

[Frederic Weisbecker <fweisbec@gmail.com>]

On Sun, May 15, 2011 at 10:56:15PM -0700, Paul E. McKenney wrote:
> On Mon, May 16, 2011 at 12:01:17AM +0200, Frederic Weisbecker wrote:
> > On Sat, May 14, 2011 at 11:34:53AM -0700, Paul E. McKenney wrote:
> > >  rcutree.c |   91 +++++++++++++++++++++++++++++++++++++++++++++++++++-----------
> > >  1 file changed, 75 insertions(+), 16 deletions(-)
> > > 
> > > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > > index a4a2ef0..0b541bd 100644
> > > --- a/kernel/rcutree.c
> > > +++ b/kernel/rcutree.c
> > > @@ -308,23 +308,25 @@ static int rcu_implicit_offline_qs(struct rcu_data *rdp)
> > >  
> > >  #ifdef CONFIG_NO_HZ
> > >  
> > > -/**
> > > - * rcu_enter_nohz - inform RCU that current CPU is entering nohz
> > > +/*
> > > + * __rcu_enter_nohz - inform RCU that current CPU is entering nohz
> > >   *
> > >   * Enter nohz mode, in other words, -leave- the mode in which RCU
> > >   * read-side critical sections can occur.  (Though RCU read-side
> > >   * critical sections can occur in irq handlers in nohz mode, a possibility
> > >   * handled by rcu_irq_enter() and rcu_irq_exit()).
> > > + *
> > > + * This variant should only be called from rcu_irq_exit() because it
> > > + * assumes perfect nesting.  The caller must have irqs disabled.
> > >   */
> > > -void rcu_enter_nohz(void)
> > > +void __rcu_enter_nohz(void)
> > >  {
> > > -	unsigned long flags;
> > >  	struct rcu_dynticks *rdtp;
> > >  
> > > -	local_irq_save(flags);
> > >  	rdtp = &__get_cpu_var(rcu_dynticks);
> > > +	WARN_ON_ONCE(rdtp->dynticks_nesting <= 0);
> > 
> > But this warning will always trigger if it's called from rcu_irq_enter()
> > after rcu_enter_nohz(). Right?
> 
> Hello, Frederic,
> 
> Thank you for looking this over!
> 
> But you lost me on this one.  Isn't __rcu_enter_nohz() called from
> rcu_irq_exit() rather than from rcu_irq_enter()?

Oops, right I read it too quickly.