说明：  使用临界区机制同步线程，假如一个银行系统有两个线程执行取款任务。一个使用存折在柜台取款，一个使用银行卡在ATM取款。若不加控制，很可能账户余额不足于两次取款的总额，但还可以把钱取走。
(Critical mechanism for the use of thread synchronization, if a banking system there are two threads to implement task teller. A teller at the counter using the passbook, a bank card in the ATM withdrawals. If unchecked, is likely to account balances less than twice the total amount of money, but can also take away money.)