openmp c 睡觉理发师

Question

我是 OpenMP 的新手，出于学习目的，我正在尝试解决睡着的理发师问题。但是，我无法按照我的意愿运行代码。

这就是我希望代码运行的方式：

Creating customer 0
Creating customer 1
Creating customer 2
Creating customer 3
Creating customer 4
Sleeping
Customer: 3 waiting for seats
Customer: 3 ready, waiting for barber
Customer: 1 waiting for seats
Customer: 1 ready, waiting for barber
Customer: 2 waiting for seats
Customer: 2 ready, waiting for barber
Customer: 4 waiting for seats
Customer: 4 ready, waiting for barber
Customer: 0 waiting for seats
Customer: 0 ready, waiting for barber
Barber: waiting for customer
Barber: waiting for seats
Customer: 2 is done. Leaving..
Number of customers hair cutted: 1
Barber: waiting for customer
Barber: waiting for seats
Customer: 1 is done. Leaving..

但是它实际上是这样运行的：

Creating customer 0
Creating customer 1
Creating customer 2
Creating customer 3
Creating customer 4
Customer: 0 waiting for seats
Customer: 0 ready, waiting for barber
Barber: waiting for customer
Barber: waiting for seats
Customer: 0 is done. Leaving..
Customer: 1 waiting for seats
Customer: 1 ready, waiting for barber
Number of customers hair cutted: 1
Barber: waiting for customer
Barber: waiting for seats
Customer: 1 is done. Leaving..
Customer: 2 waiting for seats
Customer: 2 ready, waiting for barber
Number of customers hair cutted: 2
Barber: waiting for customer
etc..

如您所见，就像一次只有一个客户在等待信号量，而他们不知何故在等待其他客户任务完成。

代码：

void customer(int threadid)
{
    int hairCutted = 0;
    while (!hairCutted) {
        printf("Customer: %ld waiting for seats\n", (long)threadid);
        sem_wait(&accessWRSeats);
        if (numOfFreeSeats >0) {
            numOfFreeSeats--;
            sem_post(&custReady);
            sem_post(&accessWRSeats);
            printf("Customer: %ld ready, waiting for barber\n", (long)threadid);
            sem_wait(&barberReady);
            hairCutted = 1;
        }
        else {
            sem_post(&accessWRSeats);
            hairCutted = 1;
        }
    }

    printf("Customer: %ld is done. Leaving..\n", (long)threadid);
}


void createCustomers(void)
{
    int i;    

    printf("Random number: %d\n", randNum1);

    #pragma omp parallel
    {
        #pragma omp single firstprivate(i)
        for (i = 0; i < randNum1; ++i)
        {
            #pragma omp task
            customer(i);
            printf("Creating customer %d\n", i);
        }
    }

    printf("Sleeping\n");
    sleep(60);
    int j;
    printf("Random number: %d\n", randNum2);
    #pragma omp parallel
    {
        #pragma omp single firstprivate(i)
        for (j = 0; j < randNum2; ++j, ++i)
        {
            #pragma omp task
            customer(i);
            printf("Creating customer %d\n", i);
        }
    }
}

Answer 1

看看这段代码：

sem_wait(&accessWRSeats);
if (numOfFreeSeats >0)
{
   ...
}
else
{
  sem_post(&accessWRSeats);
  hairCutted = 1;
}

如果没有空位，顾客为什么要理发？ （这意味着理发店已经满员，因此理发师正忙于照顾其他客户。） 他应该发信号accessWRSeats，离开，改天再试……