Subject: Re: [PATCH RFC] v1 expedited "big hammer" RCU grace periods

From: "Paul E. McKenney" <[email-masked]>
Date: Thu, 23 Apr 2009 08:15:09 -0700

On Thu, Apr 23, 2009 at 02:11:58PM +0800, Lai Jiangshan wrote:
> Paul E. McKenney wrote:

[ . . . ]

> Hi, Paul
> 
> I just typed codes in email, very like these two pathes:
> 
> [PATCH 1/2] sched: Introduce APIs for waiting multi events
> http://lkml.org/lkml/2009/4/14/733
> 
> [PATCH 2/2] rcupdate: use struct ref_completion
> http://lkml.org/lkml/2009/4/14/734
> 
> Lai.
> --------------

Interesting approach!  This would get a second use for your multi-events
waiting code above.  ;-)

Looks like the idea is to have the task doing the
synchronize_rcu_bh_expedited() hold a reference across the process,
and have each rcu_bh_fast_qs() also acquire a reference, which would
be released in the softirq handler synchronize_rcu_bh_expedited_help().

One question -- does this approach correctly handle all the CPU hotplug
scenarios?  (I think that it might, but am not completely certain.)

							Thanx, Paul

> #ifndef CONFIG_SMP
> 
> static void __init synchronize_rcu_expedited_init(void)
> {
> }
> 
> void synchronize_rcu_bh_expedited(void)
> {
> 	cond_resched();
> }
> 
> #else /* #ifndef CONFIG_SMP */
> 
> static DEFINE_MUTEX(synchronize_rcu_bh_mutex);
> static DEFINE_PER_CPU(int, call_only_once); /* is it need ? */
> static struct ref_completion rcu_bh_expedited_completion
> 
> static void synchronize_rcu_bh_expedited_help(struct softirq_action *unused)
> {
> 	if (__get_cpu_var(call_only_once)) {
> 		smp_mb();
> 		ref_completion_put(&rcu_bh_expedited_completion);
> 		__get_cpu_var(call_only_once) = 0;
> 	}
> }
> 
> static void rcu_bh_fast_qs(void *unused)
> {
> 	__get_cpu_var(call_only_once) = 1;
> 	ref_completion_get(&rcu_bh_expedited_completion);
> 	raise_softirq(RCU_EXPEDITED_SOFTIRQ);
> }
> 
> static void __init synchronize_rcu_expedited_init(void)
> {
> 	open_softirq(RCU_EXPEDITED_SOFTIRQ, synchronize_rcu_bh_expedited_help);
> }
> 
> void synchronize_rcu_bh_expedited(void)
> {
> 	mutex_lock(&synchronize_rcu_bh_mutex);
> 
> 	ref_completion_get_init(&rcu_bh_expedited_completion);
> 
> 	smp_call_function(rcu_bh_fast_qs, NULL, 1);
> 
> 	ref_completion_put_init(&rcu_bh_expedited_completion);
> 	ref_completion_wait(&rcu_bh_expedited_completion);
> 
> 	mutex_unlock(&synchronize_rcu_bh_mutex);
> }
> 
> #endif /* #else #ifndef CONFIG_SMP */
>