+/*
+ * Parent image reference counting is used to determine when an
+ * image's parent fields can be safely torn down--after there are no
+ * more in-flight requests to the parent image. When the last
+ * reference is dropped, cleaning them up is safe.
+ */
+static void rbd_dev_parent_put(struct rbd_device *rbd_dev)
+{
+ int counter;
+
+ if (!rbd_dev->parent_spec)
+ return;
+
+ counter = atomic_dec_return_safe(&rbd_dev->parent_ref);
+ if (counter > 0)
+ return;
+
+ /* Last reference; clean up parent data structures */
+
+ if (!counter)
+ rbd_dev_unparent(rbd_dev);
+ else
+ rbd_warn(rbd_dev, "parent reference underflow\n");
+}
+
+/*
+ * If an image has a non-zero parent overlap, get a reference to its
+ * parent.
+ *
+ * Returns true if the rbd device has a parent with a non-zero
+ * overlap and a reference for it was successfully taken, or
+ * false otherwise.
+ */
+static bool rbd_dev_parent_get(struct rbd_device *rbd_dev)
+{
+ int counter;
+
+ if (!rbd_dev->parent_spec)
+ return false;
+
+ counter = atomic_inc_return_safe(&rbd_dev->parent_ref);
+ if (counter > 0 && rbd_dev->parent_overlap)
+ return true;
+
+ /* Image was flattened, but parent is not yet torn down */
+
+ if (counter < 0)
+ rbd_warn(rbd_dev, "parent reference overflow\n");
+
+ return false;
+}
+