操作系统之进程同步

critical section

mutex：互斥

progress：前进，没有死锁，如果没有进程在临界区，选择哪个进入临界区的次数是有限的

bounded waiting：有限等待，没有饿死，

等待：

 busy waiting

      锁变量

      严格轮转法

      peterson's solution

hardware solution

       关中断（多处理器，非抢占无效）

       test ans lock（TSL）

 睡眠与等待

       semaphore（同步，初值为0，两个程序，一个释放，另一个等待释放，P ，V ，重点：生产者消费者几种代码）

       blocking queue

       monitor

       message passing

     

    



1.不行

if (!lock) {

                                        if (!lock) {                

                                            
lock = true；

// critical section                           
lock = false;
}

Context switch after the
testing and before the
locking

2.不行，直接跳过了了某一个的临界区

Lock before testing? How?
lockA = true;                     lockB = true;

if (!lockB) {                         if (!lockA) {
// critical section                 // critical section
}                                                }                                 
lockA = false;                     lockB = false;

Context switch after the locking and before the testing

3 不行

Lock before testing? How?
lockA = true;                    lockB = true;
while (lockB); //X             if (!lockA) { //Y
// critical section               /* critical section */}
lockA = false;                   lockB = false;

It works!
at X: if B is not locked, enter CS; otherwise wait;
at Y: if A is not locked, enter CS; otherwise leave
Not symmetric!
 

4 可以

Peterson’s algorithm
while (true) {
lock[i] = true;
turn = j;
while (lock[j] && turn == j);
// critical section
lock[i] = false;
// remain section
}
It works, but rathercomplicated!