Re: [PATCH -tip/core/rcu 3/6] Simplify RCU CPU-hotplug notification

[Lai Jiangshan <laijs@cn.fujitsu.com>]

Paul E. McKenney wrote:
> From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> 
> Use the new cpu_notifier() API to simplify RCU's CPU-hotplug
> notifiers, collapsing down to a single such notifier.  This
> makes it trivial to provide the notifier-ordering guarantee
> that rcu_barrier() depends on.  Also remove redundant
> open_softirq() calls from Hierarchical RCU notifier.
> 
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> ---
>  kernel/rcupdate.c   |   16 +++++++++++++++-
>  kernel/rcupreempt.c |   25 ++-----------------------
>  kernel/rcutree.c    |   17 +++++------------
>  3 files changed, 22 insertions(+), 36 deletions(-)
> 
> diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
> index eae29c2..8df1156 100644
> --- a/kernel/rcupdate.c
> +++ b/kernel/rcupdate.c
> @@ -217,9 +217,13 @@ static void rcu_migrate_callback(struct rcu_head *notused)
>  		wake_up(&rcu_migrate_wq);
>  }
>  
> +extern int rcu_cpu_notify(struct notifier_block *self,
> +			  unsigned long action, void *hcpu);
> +
>  static int __cpuinit rcu_barrier_cpu_hotplug(struct notifier_block *self,
>  		unsigned long action, void *hcpu)
>  {
> +	rcu_cpu_notify(self, action, hcpu);
>  	if (action == CPU_DYING) {
>  		/*
>  		 * preempt_disable() in on_each_cpu() prevents stop_machine(),
> @@ -244,8 +248,18 @@ static int __cpuinit rcu_barrier_cpu_hotplug(struct notifier_block *self,
>  
>  void __init rcu_init(void)
>  {
> +	int i;
> +
>  	__rcu_init();
> -	hotcpu_notifier(rcu_barrier_cpu_hotplug, 0);
> +	cpu_notifier(rcu_barrier_cpu_hotplug, 0);
> +
> +	/*
> +	 * We don't need protection against CPU-hotplug here because
> +	 * this is called early in boot, before either interrupts
> +	 * or the scheduler are operational.
> +	 */
> +	for_each_online_cpu(i)
> +		rcu_barrier_cpu_hotplug(NULL, CPU_UP_PREPARE, (void *)(long)i);

I think,
There is only one cpu in cpu online map at early boot time,
it is the current cpu.

Thanks, Lai