On Sun, Feb 28 2016, NeilBrown [off-list ref] wrote:

+static int wake_slot_function(wait_queue_t *wait, unsigned mode, int sync,
+			      void *arg)
+{
+	struct wait_bit_key *key = arg;
+	struct wait_slot_queue *wait_slot =
+		container_of(wait, struct wait_slot_queue, wait);
+	void **slot;
+
+	if (wait_slot->root != key->flags ||
+	    wait_slot->index != key->timeout)
+		/* Not waking this waiter */
+		return 0;
+	if (wait_slot->state != SLOT_WAITING)
+		/* Should be impossible.... */
+		return 1;
+	if (key->bit_nr == -3)
+		/* Was just deleted, no point in doing a lookup */
+		wait_slot = NULL;
+	else
+		wait_slot->ret = __radix_tree_lookup(
+			wait_slot->root, wait_slot->index, NULL, &slot);
+	if (!wait_slot->ret || !radix_tree_exceptional_entry(wait_slot->ret)) {
+		wait_slot->state = SLOT_GONE;
+		return 1;
+	}
+	if (slot_locked(slot))
+		/* still locked */
+		return 0;
+	wait_slot->ret = lock_slot(slot);
+	wait_slot->state = SLOT_LOCKED;
+	return 1;
+}

Sorry, just realized that this should:
  return autoremove_wake_function(wait, mode, sync, arg);

instead of "return 1;"

NeilBrown