Re: [PATCH 4/4 RFC] rcu: New rcu_user_enter_irq() and rcu_user_exit_irq() APIs

[Josh Triplett <josh@joshtriplett.org>]

On Mon, Nov 28, 2011 at 05:00:36PM -0800, Paul E. McKenney wrote:
> On Mon, Nov 28, 2011 at 01:53:23PM -0800, Josh Triplett wrote:
> > On Mon, Nov 28, 2011 at 10:24:47PM +0100, Frederic Weisbecker wrote:
> > > A CPU running in adaptive tickless mode wants to enter into
> > > RCU extended quiescent state while running in userspace. This
> > > way we can shut down the tick that is usually needed on each
> > > CPU for the needs of RCU.
> > 
> > Very awesome.  I've wanted to see this change for a long time.  Thanks!
> 
> I am a fan, also.  ;-)
> 
> [ . . . ]
> 
> > > @@ -503,6 +515,18 @@ void rcu_user_exit(void)
> > >  	__rcu_idle_exit();
> > >  }
> > >  
> > > +void rcu_user_exit_irq(void)
> > > +{
> > > +	unsigned long flags;
> > > +	struct rcu_dynticks *rdtp;
> > > +
> > > +	local_irq_save(flags);
> > > +	rdtp = &__get_cpu_var(rcu_dynticks);
> > > +	WARN_ON_ONCE(rdtp->dynticks_nesting != 1);
> > > +	rdtp->dynticks_nesting = (LLONG_MAX / 2) + 1;
> > > +	local_irq_restore(flags);
> > > +}
> > > +
> > 
> > Any chance that either of these two needs a memory barrier of some kind,
> > to prevent leakage of operations from between them?  Or can you count on
> > no RCU-protected operations occurring during (or leaking into) the
> > extended quiescent state?
> 
> There is no need for a memory barrier on rdtp->dynticks_nesting because
> it is used (aside from state dumping) only by the local CPU.  In contrast,
> changes to ->dynticks are visible to other CPUs, hence the memory barriers
> around changes to ->dynticks.
> 
> Information flows within the CPU from ->dynticks_nesting to ->dynticks,
> which is externally visible.

Thanks for the clarification; just wanted to make sure I understood the
various data involved here.

Now that I understand that:

Reviewed-by: Josh Triplett <josh@joshtriplett.org>