Operating system 两个可以'；t保持计数器值的一致性

这是TAS代码

boolean TAS(*target) 
{    
    boolean rv = *target;    //Save the initial value
    *target = true;      //Set lock
    return rv;       //Return initial value
}

以下代码表示使用TA在线程安全计数器上执行增量操作的两次不同尝试，以保持计数器值的一致性

代码1

Counter::Increment() {    
    while (TAS(&lock))       ;    
    value++;    
    lock = FREE;    
    memory_barrier(); 
} 

代码2

Counter::Increment() {
   while (lock == BUSY && TAS(&lock))       ;    
   value++;    
   lock = FREE;    
   memory_barrier(); 
} 

---

假设

空闲==false

和

忙碌==true

，则无法处理一致性的每个代码将出现什么问题，以下是一个打破互斥的流程：

Start thread 1:
    Increment
        TAS
            rv = *lock; // FREE
Switch to thread 2:
    Increment
        TAS
            rv = *lock; // FREE
            *lock = BUSY;
            return rv; // FREE
        Load value 0 from memory and push it into the stack
Switch to thread 1:
            *lock = BUSY;
            return rv; // FREE
        Load value 0 from memory and push it into the stack
        Push 1 into the stack
        Pop two integers from the stack and add them
        Store the result (0 + 1) into value
Switch to thread 2:
        Push 1 into the stack
        Pop two integers from the stack and add them
        Store the result (0 + 1) into value