进程调度之wait

核心思想：回收子进程描述符

//wait_task_zombie和wait_task_continued回收子进程描述符（注销身份证），子进程彻底消失

do_wait
	=>add_wait_queue(¤t->signal->wait_chldexit,&wait);
	=>repeat:
		current->state = TASK_INTERRUPTIBLE;
		tsk = current;
		do
			list_for_each(_p,&tsk->children)
				p = list_entry(_p, struct task_struct, sibling);
				ret = eligible_child(pid, options, p);
				switch (p->state)
					if (p->exit_state == EXIT_DEAD)
						continue;
					if (p->exit_state == EXIT_ZOMBIE)
						if (ret == 2)
							goto check_continued;
						if (!likely(options & WEXITED))
							continue;
						retval = wait_task_zombie(p, (options & WNOWAIT), infop, stat_addr, ru);
					check_continued:
						retval = wait_task_continued(p, (options & WNOWAIT), infop, stat_addr, ru);