Re: Re: [RFC][PATCH -mm 5/6] Freezer: Use freezing timeout more efficiently

[Oleg Nesterov <oleg@tv-sign.ru>]

On 07/10, Rafael J. Wysocki wrote:
>
> +	if (todo && freeze_user_space && !time_after(jiffies, end_time)) {
> +		/*
> +		 * Some tasks have not been able to freeze.  They might be stuck
> +		 * in TASK_UNINTERRUPTIBLE waiting for the frozen tasks.  Try to
> +		 * thaw the tasks that have frozen without clearing the freeze
> +		 * requests of the remaining tasks and repeat.
> +		 */
> +		read_lock(&tasklist_lock);
> +		do_each_thread(g, p) {
> +			if (frozen(p)) {
> +				p->flags &= ~PF_FROZEN;
> +				wake_up_process(p);

I just noticed we don't use thaw_process(), this means that the retry doesn't
play well with wait_event_freezable() introduced in the previous patch.

Suppose that kthread_stop(T) hangs and blocks the freezer, and T does

	while (!kthread_should_stop()) {
		wait_event_freezable(...);
		do_something();
	}

and it is freezed. We clear PF_FROZEN but not TIF_FREEZE, wait_event_freezable
checks freezing() and goes to refrigerator again.

Another problem is that we only count UNINTERRUPTIBLE tasks to make a decision
about retry, while wait_event_freezable() sleeps NTERRUPTIBLE.

Oleg.