Explain why the following implementation the semaphore P and V operations in terms of futexes doesn't work.

void P(futex_t *f) {
   /* f->val contains the value of the semaphore */
   int v;
   while (1) {
      while ((v = f->val) > 0) {
         /* semaphore is positive */
         if (CAS(&f->val, v, v-1) == v) {
            /* we were able to decrement it without
               interference */
            return;
         }
         /* try again while semaphore is still positive
      }
      /* semaphore is zero: wait for a V operation
      futex_wait(f, 0);
   }
    }

void V(futex_t *f) {
   if (atomic_inc(&f->val) == 0) {
      /* semaphore was zero: there might be
         waiting threads */
      futex_wake(f);
   }
   return;
}