static atomic_t access_count = ATOMIC_INIT(-1);

static int driver_open(struct inode *devicefile,
                       struct file *instance)
{
  if (atomic_inc_and_test(&access_count) != 0) { // true, falls ==0
    return 0; // Zugriff erlaubt
  }
  atomic_dec(&access_count);
  return -EBUSY;
}

static int driver_release(struct inode *devicefile,
                          struct file *instance)
{
  atomic_dec(&access_count);
  return 0;
}