How To Use Condition Variable

The Linux Programming Interface book has a piece of code (producer/consumer) to show how condition variable works:

static pthread_mutex_t mtx = PTHREAD_MUTEX_INITIALIZER;
static pthread_cond_t cond = PTHREAD_COND_INITIALIZER;

static int avail = 0;

while (TRUE) {
    s = pthread_mutex_lock(&mtx);

    while (avail == 0) {   /* Wait for something to consume */
       s = pthread_cond_wait(&cond, &mtx);
    }

    while (avail > 0) {   /* Consume all available units */ 
        avail--;
    }

    s = pthread_mutex_unlock(&mtx);
}

Why we use pthread_mutex_lock in while? why we don't use it in an if?