存储之厨：注意：xchg 自带 lock 属性，但cmpxchg 必须显式地加lock 前缀，否则多处理器中无法保证原子性。
The CMPXCHG instruction is commonly used for testing and modifying semaphores. It checks to see if a semaphore is free. If the semaphore is free, it is marked allocated; otherwise it gets the ID of the current owner. This is all done in one uninterruptible operation. In a single-processor system, the CMPXCHG instruction eliminates the need to switch to protection level 0 (to disable interrupts) before executing multiple instructions to te
