Automate a dependency graph with parallel programming [closed]

I found a C/Linux exercise in a book, and I propose a code as a solution, please feel free to correct it.

Here is the exercise :

Taking into consideration the following dependency graph, which express dependency relations between 4 tasks of a program :

We want to create 4 processes (S1 to S4) which will be executed in parallel in order to automate the dependency graph above. Each process Si will be based on the model 1 below :

Propose a solution and clearly justify it.

Here is the code :

sem_t free;
sem_t busy;
static int x1 = 1, x2 = 1, x3 = 1;

void *s1(void *z)
{
    float b = 0;
    sem_wait(&busy);
    b = (54 * 91) - 43;
    printf("  (54 * 91) - 43 = %f  #  Task 1 completed.\n\n", b);
    x1 = 0;
    sem_post(&free);
}

void *s2(void *z)
{
    float c;
    sem_wait(&busy);
    c = (30 + 71) - 63;
    printf("  (30 + 71) - 63 = %f  #  Task 2 completed.\n\n", c);
    x2 = 0;
    sem_post(&free);
}

void *s3(void *z)
{
    float d;
    sem_wait(&busy);
    d = (65 - 40) * 58;
    printf("  (65 - 40) * 58 = %f  #  Task 3 completed.\n\n", d);
    x3 = 0;
    sem_post(&free);
}

void *s4(void *z)
{
    float e;
    sem_wait(&busy);
    e = 81 - (35 * 46);
    printf("  81 - (35 * 46) = %f  #  Task 4 completed.\n\n", e);
    sem_post(&free);
}

int main(void)
{
    pthread_t tr;

    sem_init(&free, 0, 5);
    sem_init(&busy, 0, 0);

    pthread_create(&tr, NULL, s1, NULL);

    if (x1 == 0)
    {
        pthread_create(&tr, NULL, s2, NULL);
        pthread_create(&tr, NULL, s3, NULL);
    }

    if ((x1 == 0) && (x3 == 0))
    {
        pthread_create(&tr, NULL, s4, NULL);
    }

    pthread_exit(NULL);
}