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 <rost...@goodmis.org>
> > Signed-off-by: Paul E. McKenney <paul...@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)
> >
>
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