rfc, leader_pid_type()

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

Eric, Pavel.

Without tasklist lock held, task_tgid/task_pgrp/task_session can return the
bogus NULL. Note that the last 2 can return NULL even if task == current.

What do you think if we add yet another helper?

	struct pid *leader_pid_type(struct task_struct *task, enum pid_type type)
	{
		struct pid *ret;
	retry:
		ret = task->group_leader->pids[type].pid;
		if (likely(ret != NULL) || !pid_alive(task))
			return ret;
		/*
		 * We hit the old leader in the middle of de_thread(),
		 * or setsid/setpgrp is in progress.
		 */
		cpu_relax();
		goto retry;
	}

Yes, we already have a lot helpers... The one potential user is
check_kill_permission(), but it can live without it.

What do you think, do you see other possible users? Say, do_task_stat() may
report sid = 0, but this is minor of course...

Oleg.