Date: Fri, 6 Sep 2013 11:33:20 -0400
From: Steven Rostedt <rostedt@goodmis.org>
To: paulmck@linux.vnet.ibm.com
Cc: Frederic Weisbecker <fweisbec@gmail.com>, linux-kernel@vger.kernel.org,
        mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com,
        akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
        josh@joshtriplett.org, niv@us.ibm.com, tglx@linutronix.de,
        peterz@infradead.org, dhowells@redhat.com, edumazet@google.com,
        darren@dvhart.com, sbw@mit.edu
Subject: Re: [PATCH] rcu: Is it safe to enter an RCU read-side critical
 section?
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	<20130906105934.GF20519@somewhere>
	<20130906151851.GQ3966@linux.vnet.ibm.com>
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On Fri, 6 Sep 2013 08:18:52 -0700
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:

> 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.
> 

I can't use plain preempt_disable() in function tracing.

Also, since it's a misnomer to say the cpu is idle in NO_HZ_FULL when
we are coming from userspace, can we rename that?

Perhaps we can also have a __rcu_is_cpu_tracking() (or whatever), with
the "__" appended that does not do the preempt disable.

-- Steve
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