Re: [PATCH 07/11] pidns: Wait in zap_pid_ns_processes until pid_ns->nr_hashed == 1

[Oleg Nesterov <oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On 11/16, Eric W. Biederman wrote:
>
> @@ -216,22 +216,15 @@ void zap_pid_ns_processes(struct pid_namespace *pid_ns)
>
>  	/*
>  	 * sys_wait4() above can't reap the TASK_DEAD children.
> -	 * Make sure they all go away, see __unhash_process().
> +	 * Make sure they all go away, see free_pid().
>  	 */
>  	for (;;) {
> -		bool need_wait = false;
> -
> -		read_lock(&tasklist_lock);
> -		if (!list_empty(&current->children)) {
> -			__set_current_state(TASK_UNINTERRUPTIBLE);
> -			need_wait = true;
> -		}
> -		read_unlock(&tasklist_lock);
> -
> -		if (!need_wait)
> +		set_current_state(TASK_UNINTERRUPTIBLE);
> +		if (pid_ns->nr_hashed == 1)
>  			break;
>  		schedule();
>  	}

I agree, the patch itself looks fine.

But, with all other changes I do not understand this part at all.

A task from the parent namespace can do setns + fork at any time
(until nr_hashed >= 0). So ->nr_hashed can be incremented again
after zap_pid_ns_processes() returns.

Or, we can sleep in TASK_UNINTERRUPTIBLE "forever" if this happens
after kill-them-all.

Could you explain why do we need to wait at all? I can be easily
wrong, but at first glance the original reason for this wait has
gone away?

Oleg.