Re: [PATCH 01/51] CPU hotplug: Provide lockless versions of callback registration functions

[Oleg Nesterov <oleg@redhat.com>]

On 02/11, Srivatsa S. Bhat wrote:
>
> +static DECLARE_RWSEM(cpu_hotplug_rwsem);
> +
> +void cpu_notifier_register_begin(void)
> +{
> +	down_read(&cpu_hotplug_rwsem);
> +}
> +
> +void cpu_notifier_register_end(void)
> +{
> +	up_read(&cpu_hotplug_rwsem);
> +}
> +
>  /* Serializes the updates to cpu_online_mask, cpu_present_mask */
>  static DEFINE_MUTEX(cpu_add_remove_lock);
>  
> @@ -32,12 +45,14 @@ static DEFINE_MUTEX(cpu_add_remove_lock);
>   */
>  void cpu_maps_update_begin(void)
>  {
> +	down_write(&cpu_hotplug_rwsem);
>  	mutex_lock(&cpu_add_remove_lock);
>  }
>  
>  void cpu_maps_update_done(void)
>  {
>  	mutex_unlock(&cpu_add_remove_lock);
> +	up_write(&cpu_hotplug_rwsem);
>  }

I am a bit confused... If we do this, why we can't simply turn
cpu_add_remove_lock into rw_semaphore?

Oleg.