+
+ /*
+ * We have a dilemma here. The node's slot[0] must not be
+ * NULLed in case there are concurrent lookups expecting to
+ * find the item. However if this was a bottom-level node,
+ * then it may be subject to the slot pointer being visible
+ * to callers dereferencing it. If item corresponding to
+ * slot[0] is subsequently deleted, these callers would expect
+ * their slot to become empty sooner or later.
+ *
+ * For example, lockless pagecache will look up a slot, deref
+ * the page pointer, and if the page is 0 refcount it means it
+ * was concurrently deleted from pagecache so try the deref
+ * again. Fortunately there is already a requirement for logic
+ * to retry the entire slot lookup -- the indirect pointer
+ * problem (replacing direct root node with an indirect pointer
+ * also results in a stale slot). So tag the slot as indirect
+ * to force callers to retry.
+ */
+ if (root->height == 0)
+ *((unsigned long *)&to_free->slots[0]) |=
+ RADIX_TREE_INDIRECT_PTR;
+