static atomic_t bytes_available = ATOMIC_INIT(0);

#define READ_POSSIBLE (atomic_read(&bytes_available) != 0)

ssize_t driver_read(struct file *instance, char __user *buffer,
                    size_t max_bytes_to_read, loff_t *offset)
{
  size_t to_copy, not_copied;
  char kernelmem[128]; // lokale Variable: Schutz über Spinlocks kann entfallen

  if (!READ_POSSIBLE && (instance->f_flags & O_NONBLOCK)) {
    // keine Daten vorhanden und Nonblocking-Mode
    return -EAGAIN;
  }
  if (wait_event_interruptible( wq_read, READ_POSSIBLE)) {
    // während des Schlafens durch ein Signal unterbrochen
    return -ERESTARTSYS;
  }
  to_copy = min((size_t)atomic_read(&bytes_available), max_bytes_to_read);
  not_copied = copy_to_user(buffer, kernelmem, to_copy);
  atomic_sub(to_copy - not_copied, &bytes_available);
  return to_copy - not_copied;
}