Re: [PATCH, RFC] v4 scalable classic RCU implementation

[Manfred Spraul <manfred@colorfullife.com>]

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Hi Paul,

Paul E. McKenney wrote:
> o	The rcu_pending() and rcu_needs_cpu() primitives are now
> 	much more aggressive about permitting CPUs to enter dynticks
> 	idle mode.  Only CPUs that have RCU callbacks are kept out
> 	of dynticks idle mode.
>   

I've noticed that right now rcu_enter_nohz() can be nested within 
rcu_irq_enter():
irq_exit() first calls tick_nohz_stop_sched_tick(), then rcu_irq_exit().
And tick_nohz_stop_sched_tick() can switch into nohz mode.

Is that intentional? Does rcupreempt support that? It broke my rcustate 
code on x86-64.

I would prefer if something like the attached patch is applied. What do 
you think?
Do you need the patch as well?

--
    Manfred

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diff --git a/kernel/softirq.c b/kernel/softirq.c
index ba20a90..cca5a83 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -284,10 +284,10 @@ void irq_exit(void)
 		invoke_softirq();
 
 #ifdef CONFIG_NO_HZ
+	rcu_irq_exit(0);
 	/* Make sure that timer wheel updates are propagated */
 	if (!in_interrupt() && idle_cpu(smp_processor_id()) && !need_resched())
 		tick_nohz_stop_sched_tick(0);
-	rcu_irq_exit(0);
 #endif
 	preempt_enable_no_resched();
 }