Re: [PATCH] rcu_barrier VS cpu_hotplug: make rcu_barrier can becalled on CPU_DEAD

[Paul E. McKenney]

On Sat, Mar 07, 2009 at 06:54:36PM +0800, Lai Jiangshan wrote:
> 
> cpu hotplug notifier_call for CPU_DEAD are very complex, some notifier_call
> may call rcu_barrier(), if this notifier_call is called earlier than
> rcu_cpu_notify(), it is buggy.
> 
> _cpu_down() -->
> 	some_cpu_hotplug_notify() -->
> 		case CPU_DEAD: rcu_barrier()
> 	rcu_cpu_notify() -->
> 		rcu_offline_cpu()
> 
> When rcu_barrier() is called, rcu_barrier() does not know how to finish
> his work: rcu callbacks in dead cpu have not been migrated to online cpu,
> so rcu_barrier() cannot wait for these rcu callbacks, but rcu_barrier()
> should wait for all queued rcu callbacks.

Hmmm...

What should happen in this case is that rcu_offline_cpu() moves the
rcu_barrier() callback to a surviving CPU, and then the rcu_barrier()
should complete just fine at the end of a subsequent grace period.
In fact, this is why it is so important to keep RCU callbacks ordered
through CPU-hotplug operations.

Or do you have a test that demonstrates a failure in this case?
If so, please share!

						Thanx, Paul

> This patch ensure that rcu_cpu_notify() is called earlier than 
> any other notifier_call.
> 
> Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>
> ---
> diff --git a/include/linux/notifier.h b/include/linux/notifier.h
> index b86fa2f..f7ae5a3 100644
> --- a/include/linux/notifier.h
> +++ b/include/linux/notifier.h
> @@ -238,6 +238,12 @@ static inline int notifier_to_errno(int ret)
>  #define CPU_DYING_FROZEN	(CPU_DYING | CPU_TASKS_FROZEN)
>  #define CPU_STARTING_FROZEN	(CPU_STARTING | CPU_TASKS_FROZEN)
> 
> +/* Register at highest priority so that task migration (migrate_all_tasks)
> + * and rcu callbacks migration happens before everything else.
> + */
> +#define CPUHOTPLUG_TASK_MIGRATE_PRIORITY	10
> +#define CPUHOTPLUG_RCU_MIGRATE_PRIORITY		11
> +
>  /* Hibernation and suspend events */
>  #define PM_HIBERNATION_PREPARE	0x0001 /* Going to hibernate */
>  #define PM_POST_HIBERNATION	0x0002 /* Hibernation finished */
> diff --git a/kernel/rcuclassic.c b/kernel/rcuclassic.c
> index 654c640..040b894 100644
> --- a/kernel/rcuclassic.c
> +++ b/kernel/rcuclassic.c
> @@ -764,6 +764,7 @@ static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
> 
>  static struct notifier_block __cpuinitdata rcu_nb = {
>  	.notifier_call	= rcu_cpu_notify,
> +	.priority	= CPUHOTPLUG_RCU_MIGRATE_PRIORITY,
>  };
> 
>  /*
> diff --git a/kernel/rcupreempt.c b/kernel/rcupreempt.c
> index 5d59e85..8ffcbf0 100644
> --- a/kernel/rcupreempt.c
> +++ b/kernel/rcupreempt.c
> @@ -1407,6 +1407,7 @@ static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
> 
>  static struct notifier_block __cpuinitdata rcu_nb = {
>  	.notifier_call = rcu_cpu_notify,
> +	.priority = CPUHOTPLUG_RCU_MIGRATE_PRIORITY,
>  };
> 
>  void __init __rcu_init(void)
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index 97ce315..fcef06c 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -1503,6 +1503,7 @@ do { \
> 
>  static struct notifier_block __cpuinitdata rcu_nb = {
>  	.notifier_call	= rcu_cpu_notify,
> +	.priority	= CPUHOTPLUG_RCU_MIGRATE_PRIORITY,
>  };
> 
>  void __init __rcu_init(void)
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 8e2558c..56f225e 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -6740,12 +6740,9 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu)
>  	return NOTIFY_OK;
>  }
> 
> -/* Register at highest priority so that task migration (migrate_all_tasks)
> - * happens before everything else.
> - */
>  static struct notifier_block __cpuinitdata migration_notifier = {
>  	.notifier_call = migration_call,
> -	.priority = 10
> +	.priority = CPUHOTPLUG_TASK_MIGRATE_PRIORITY,
>  };
> 
>  static int __init migration_init(void)
> 
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