Re: [PATCH 01/13] powerpc: Add rcu_read_lock() to gup_fast() implementation

Paul E. McKenney
2010-04-16T13:43:39Z

On Fri, Apr 16, 2010 at 04:54:51PM +1000, Benjamin Herrenschmidt wrote:
> On Thu, 2010-04-15 at 07:28 -0700, Paul E. McKenney wrote:
> >
> > Of course, with call_rcu_sched(), the corresponding RCU read-side
> > critical
> > sections are non-preemptible. Therefore, in CONFIG_PREEMPT_RT, these
> > read-side critical sections must use raw spinlocks.
> >
> > Can the code in question accommodate these restrictions?
>
> What we protect against is always code that hard-disable IRQs (though
> there seem to be a bug in the hugepages code there...). Would that
> work ?

From the perspective of call_rcu_sched() and synchronize_sched(),
the following things mark RCU-sched read-side critical sections:

1. rcu_read_lock_sched() and rcu_read_unlock_sched().

2. preempt_disable() and preempt_enable(), along with anything
else that disables preemption.

3. local_bh_disable() and local_bh_enable(), along with anything
else that disables bottom halves.

4. local_irq_disable() and local_irq_enable(), along wiht anything
else that disables hardirqs.

5. Handlers for NMIs.

So I believe that in this case call_rcu_sched() is your friend. ;-)

Thanx, Paul