The articles code for mutex_lock is this

  mutex_lock(unsigned int mutex)
  {
    while (!cmpxchg(&mutex, UNLOCKED, LOCKED))
      futex(&mutex, FUTEX_WAIT, LOCKED);
  }

Now consider what happens if someone calls it

  volatile unsigned int my_mutex = UNLOCKED;
  mutex_lock(my_mutex);
  ...
  mutex_unlock(my_mutex);

my_mutex is copied into the mutex parameter. The mutex parameter is updated to locked. mutex_lock exits, which destroys the mutex variable. my_mutex still has the variable UNLOCKED.