Re: [PATCH] hotplug: Optimize {get,put}_online_cpus()

[Oleg Nesterov]

And somehow I didn't notice that cpuhp_set_state() doesn't look right,

On 09/19, Peter Zijlstra wrote:
>  void cpu_hotplug_begin(void)
>  {
> -	cpu_hotplug.active_writer = current;
> +	lockdep_assert_held(&cpu_add_remove_lock);
>  
> -	for (;;) {
> -		mutex_lock(&cpu_hotplug.lock);
> -		if (likely(!cpu_hotplug.refcount))
> -			break;
> -		__set_current_state(TASK_UNINTERRUPTIBLE);
> -		mutex_unlock(&cpu_hotplug.lock);
> -		schedule();
> -	}
> +	__cpuhp_writer = current;
> +
> +	/* After this everybody will observe _writer and take the slow path. */
> +	synchronize_sched();
> +
> +	/* Wait for no readers -- reader preference */
> +	cpuhp_wait_refcount();
> +
> +	/* Stop new readers. */
> +	cpuhp_set_state(1);

But this stops all readers, not only new. Even if cpuhp_wait_refcount()
was correct, a new reader can come right before cpuhp_set_state(1) and
then it can call another recursive get_online_cpus() right after.

Oleg.

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