Q: css_task_iter_advance() && dying_tasks

[Oleg Nesterov]

I never understood the code in kernel/cgroup/ even remotely, most probably
I missed something, but let me ask a couple of stupid questions anyway.

cgroup_exit() does

	css_set_move_task(tsk, cset, NULL, false);
	list_add_tail(&tsk->cg_list, &cset->dying_tasks);

but unless I am totally confused css_task_iter_advance() always ignores
the "dying" sub-threads, so perhaps it should do, say,

	css_set_move_task(tsk, cset, NULL, false);
	if (delay_group_leader(tsk))
		list_add_tail(&tsk->cg_list, &cset->dying_tasks);

and then cgroup_release() can check list_empty(cg_list) before it takes
css_set_lock.

No ?

Or, perhaps we can do even better? Can't cgroup_exit() do something like

	// group_dead should be passed from do_exit()

	css_set_move_task(tsk, cset, NULL, false);

	if (thread_group_leader(tsk) && !group_dead)
		list_add_tail(&tsk->cg_list, &cset->dying_tasks);

	else if (!thread_group_leader(tsk) && group_dead) {
		leader = tsk->group_leader;
		if (!list_empty(leader->cg_list) {
			css_set_skip_task_iters(task_css_set(leader), leader);
			list_del_init(&leader->cg_list);
		}
	}

and then

	- kill the atomic_read(&task->signal->live)) check in
	  css_task_iter_advance()

	- kill the code under css_set_lock in cgroup_release()

Oleg.