Re: [patch 20/20] rcu: Make CPU_DYING_IDLE an explicit call

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Fri, Feb 26, 2016 at 06:43:44PM -0000, Thomas Gleixner wrote:
> Make the RCU CPU_DYING_IDLE callback an explicit function call, so it gets
> invoked at the proper place.
> 
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

A question below...

> ---
>  include/linux/cpu.h      |    4 +---
>  include/linux/notifier.h |    2 ++
>  include/linux/rcupdate.h |    4 +---
>  kernel/cpu.c             |    1 +
>  kernel/rcu/tree.c        |   26 +++++++++++++++-----------
>  kernel/sched/idle.c      |    2 --
>  6 files changed, 20 insertions(+), 19 deletions(-)
> 
> Index: b/include/linux/cpu.h
> ===================================================================
> --- a/include/linux/cpu.h
> +++ b/include/linux/cpu.h
> @@ -101,9 +101,7 @@ enum {
>  					* Called on the new cpu, just before
>  					* enabling interrupts. Must not sleep,
>  					* must not fail */
> -#define CPU_DYING_IDLE		0x000B /* CPU (unsigned)v dying, reached
> -					* idle loop. */
> -#define CPU_BROKEN		0x000C /* CPU (unsigned)v did not die properly,
> +#define CPU_BROKEN		0x000B /* CPU (unsigned)v did not die properly,
>  					* perhaps due to preemption. */
> 
>  /* Used for CPU hotplug events occurring while tasks are frozen due to a suspend
> Index: b/include/linux/notifier.h
> ===================================================================
> --- a/include/linux/notifier.h
> +++ b/include/linux/notifier.h
> @@ -47,6 +47,8 @@
>   * runtime initialization.
>   */
> 
> +struct notifier_block;
> +
>  typedef	int (*notifier_fn_t)(struct notifier_block *nb,
>  			unsigned long action, void *data);
> 
> Index: b/include/linux/rcupdate.h
> ===================================================================
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -332,9 +332,7 @@ void rcu_init(void);
>  void rcu_sched_qs(void);
>  void rcu_bh_qs(void);
>  void rcu_check_callbacks(int user);
> -struct notifier_block;
> -int rcu_cpu_notify(struct notifier_block *self,
> -		   unsigned long action, void *hcpu);
> +void rcu_report_dead(unsigned int cpu);
> 
>  #ifndef CONFIG_TINY_RCU
>  void rcu_end_inkernel_boot(void);
> Index: b/kernel/cpu.c
> ===================================================================
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -762,6 +762,7 @@ void cpuhp_report_idle_dead(void)
>  	BUG_ON(st->state != CPUHP_AP_OFFLINE);
>  	st->state = CPUHP_AP_IDLE_DEAD;
>  	complete(&st->done);

What prevents the other CPU from killing this CPU at this point, so
that this CPU does not tell RCU that it is dead?

I agree that the odds should be low, but there are all manner of things
that might delay a CPU for just a little bit too long...

Or am I missing something subtle here?

							Thanx, Paul

> +	rcu_report_dead(smp_processor_id());
>  }
> 
>  #else
> Index: b/kernel/rcu/tree.c
> ===================================================================
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -4247,6 +4247,21 @@ static void rcu_prepare_cpu(int cpu)
>  		rcu_init_percpu_data(cpu, rsp);
>  }
> 
> +#ifdef CONFIG_HOTPLUG_CPU
> +void rcu_report_dead(unsigned int cpu)
> +{
> +	struct rcu_state *rsp;
> +
> +	/* QS for any half-done expedited RCU-sched GP. */
> +	preempt_disable();
> +	rcu_report_exp_rdp(&rcu_sched_state,
> +			   this_cpu_ptr(rcu_sched_state.rda), true);
> +	preempt_enable();
> +	for_each_rcu_flavor(rsp)
> +		rcu_cleanup_dying_idle_cpu(cpu, rsp);
> +}
> +#endif
> +
>  /*
>   * Handle CPU online/offline notification events.
>   */
> @@ -4278,17 +4293,6 @@ int rcu_cpu_notify(struct notifier_block
>  		for_each_rcu_flavor(rsp)
>  			rcu_cleanup_dying_cpu(rsp);
>  		break;
> -	case CPU_DYING_IDLE:
> -		/* QS for any half-done expedited RCU-sched GP. */
> -		preempt_disable();
> -		rcu_report_exp_rdp(&rcu_sched_state,
> -				   this_cpu_ptr(rcu_sched_state.rda), true);
> -		preempt_enable();
> -
> -		for_each_rcu_flavor(rsp) {
> -			rcu_cleanup_dying_idle_cpu(cpu, rsp);
> -		}
> -		break;
>  	case CPU_DEAD:
>  	case CPU_DEAD_FROZEN:
>  	case CPU_UP_CANCELED:
> Index: b/kernel/sched/idle.c
> ===================================================================
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -220,8 +220,6 @@ static void cpu_idle_loop(void)
>  			rmb();
> 
>  			if (cpu_is_offline(smp_processor_id())) {
> -				rcu_cpu_notify(NULL, CPU_DYING_IDLE,
> -					       (void *)(long)smp_processor_id());
>  				cpuhp_report_idle_dead();
>  				arch_cpu_idle_dead();
>  			}
> 
>