使用二进制信号量实现计数信号量

Question

我已经实现了某种用户级线程system.I在使用二进制信号量实现计数信号量方面需要一些帮助实现（如下所述的向上和向下功能）。这是我实现二进制信号量的界面：

typedef enum BinSemStatus{
   locked,
   unlocked
} BinSemStatus;


struct semaphore {
BinSemStatus status;
int bid;
};

int bsem_alloc();//allocate a new binary semaphore,return its descriptor
void bsem_free(int id);
void bsem_down(int id);
void bsem_up(int id);

这里是计数信号量接口的接口：

struct counting_semaphore* counting_alloc(uint value);
counting_free(struct counting_semaphore* sem);

// If the value representing the count of
// the semaphore variable is not negative, decrement it by 1. If the 
// semaphore variable is now
// negative, the thread executing acquire is blocked until the value is 
// greater or equal to 1.
// Otherwise, the thread continues execution.
void up(struct counting_semaphore* sem);
// Increments the value of semaphore
// variable by 1.
void down(struct counting_semaphore* sem);

我尝试做的是 void up(struct counting_semaphore* sem) 锁定 value.But 如下所示，这不是 enough.I 在有问题的情况下添加了评论。

struct counting_semaphore {
int binary_descriptor;
int value;
}; 

void down(struct counting_semaphore *sem){
  bsem_down(sem->binary_descriptor);
  if (sem->value > 0){
    sem->value--;
  }
  else{
     //not sure what to do here, maybe use anather semaphore in some way?
  } 
  bsem_up(sem->binary_descriptor);
 }
void up(struct counting_semaphore *sem){
  bsem_down(sem->binary_descriptor);
  sem->value++;
  bsem_up(sem->binary_descriptor);
}

Answer 1

当sem->value到0时，线程阻塞，是时候重新调度了。你没有显示你的调度代码，所以我不能给出具体的建议。可能调度程序应该代表线程调用 bsem_up(sem->binary_descriptor);。

Answer 2

临界区由 sem->binary_descriptor1 保护，每个操作 down(S) 和 up(S) 都使用它来正确更新 sem.value

的值

更新两个操作后，仅当值为正时才释放 sem->binary_descriptor2 信号量。

S.value 永远不会为负，因为任何执行 down(S) 的进程都在 sem->binary_descriptor2.

处被阻塞

  struct counting_semaphore* counting_semaphore_alloc(int value) {

    struct counting_semaphore* sem = malloc(sizeof(struct counting_semaphore));
    sem->value = value;
    sem->binary_descriptor1= bsem_alloc();//locks the value
    sem->binary_descriptor2= bsem_alloc();//locks the sing of the value
    if (value <= 0) {
        bsem_down(sem->binary_descriptor2);
    }
    return sem;
    }

void down(struct counting_semaphore *sem){
    bsem_down(sem->binary_descriptor2);
    bsem_down(sem->binary_descriptor1);
    sem->value--;
    if (sem->value>0){
        bsem_up(sem->binary_descriptor2);
    }
    bsem_up(sem->binary_descriptor1);
}


void up(struct counting_semaphore* sem) {
    bsem_down(sem->binary_descriptor1);
    sem->value++;
    if (sem->value == 1) {
        bsem_up(sem->binary_descriptor2);
    }
    bsem_up(sem->binary_descriptor1);
}

使用二进制信号量实现计数信号量

Implementing Counting-Semaphore Using Binary-Semaphore

c

synchronization

thread-safety

semaphore

binary-semaphore