CS 537 - Quiz #5

1. Remember how I said it's easy to implement semaphores with locks and CVs,
but difficult the other way around? Let's look at that:

    typedef struct __cond_t {
    	 sem_t s;
    } cond_t;

    void cond_init(cond_t *c) {
    	 sem_init(&c->s, 0);
    }

    void cond_wait(cond_t *c, mutex_t *m) {
    	 mutex_unlock(&m); // release lock and go to sleep
	 sem_wait(&c->s);
	 mutex_lock(&m);   // grab lock before returning
    }

    void cond_signal(cond_t *c) {
    	 sem_post(&c->s);  // wake up one waiter (if there is one)
    }

Why doesn't this work properly? (Hint: think about how you would expect a CV to
behave if it signals and there's nothing in the queue. What happens next?)