Re: [PATCH] rcu: Is it safe to enter an RCU read-side critical section?

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

On Fri, Sep 06, 2013 at 12:59:41PM +0200, Frederic Weisbecker wrote:
> On Thu, Sep 05, 2013 at 12:52:34PM -0700, Paul E. McKenney wrote:
> > There is currently no way for kernel code to determine whether it
> > is safe to enter an RCU read-side critical section, in other words,
> > whether or not RCU is paying attention to the currently running CPU.
> > Given the large and increasing quantity of code shared by the idle loop
> > and non-idle code, the this shortcoming is becoming increasingly painful.
> > 
> > This commit therefore adds rcu_watching_this_cpu(), which returns true
> > if it is safe to enter an RCU read-side critical section on the currently
> > running CPU.  This function is quite fast, using only a __this_cpu_read().
> > However, the caller must disable preemption.
> > 
> > Reported-by: Steven Rostedt <rostedt@goodmis.org>
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > 
> >  include/linux/rcupdate.h |    1 +
> >  kernel/rcutree.c         |   12 ++++++++++++
> >  2 files changed, 13 insertions(+)
> > 
> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > index 15d33d9..1c7112c 100644
> > --- a/include/linux/rcupdate.h
> > +++ b/include/linux/rcupdate.h
> > @@ -225,6 +225,7 @@ extern void rcu_idle_enter(void);
> >  extern void rcu_idle_exit(void);
> >  extern void rcu_irq_enter(void);
> >  extern void rcu_irq_exit(void);
> > +extern bool rcu_watching_this_cpu(void);
> >  
> >  #ifdef CONFIG_RCU_USER_QS
> >  extern void rcu_user_enter(void);
> > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > index a06d172..7b8fcee 100644
> > --- a/kernel/rcutree.c
> > +++ b/kernel/rcutree.c
> > @@ -710,6 +710,18 @@ EXPORT_SYMBOL_GPL(rcu_lockdep_current_cpu_online);
> >  #endif /* #if defined(CONFIG_PROVE_RCU) && defined(CONFIG_HOTPLUG_CPU) */
> >  
> >  /**
> > + * rcu_watching_this_cpu - are RCU read-side critical sections safe?
> > + *
> > + * Return true if RCU is watching the running CPU, which means that this
> > + * CPU can safely enter RCU read-side critical sections.  The caller must
> > + * have at least disabled preemption.
> > + */
> > +bool rcu_watching_this_cpu(void)
> > +{
> > +	return !!__this_cpu_read(rcu_dynticks.dynticks_nesting);
> > +}
> 
> There is also rcu_is_cpu_idle().

Good point, thank you!  I was clearly in autonomic-reflex mode yesterday.  :-/

Here is the rcutree version:

int rcu_is_cpu_idle(void)
{
	int ret;

	preempt_disable();
	ret = (atomic_read(&__get_cpu_var(rcu_dynticks).dynticks) & 0x1) == 0;
	preempt_enable();
	return ret;
}

And here is the rcutiny version:

int rcu_is_cpu_idle(void)
{
	return !rcu_dynticks_nesting;
}

Steve, could you please use rcu_is_cpu_idle()?  I will revert yesterday's
redundancy.

							Thanx, Paul

> Thanks.
> 
> > +
> > +/**
> >   * rcu_is_cpu_rrupt_from_idle - see if idle or immediately interrupted from idle
> >   *
> >   * If the current CPU is idle or running at a first-level (not nested)
> > 
>