radix_tree_tag_get() is not as safe as the docs make out [ver #2]

[safe/jmp/linux-2.6] / include / linux / radix-tree.h

diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h
index c5da749..55ca73c 100644 (file)
--- a/include/linux/radix-tree.h
+++ b/include/linux/radix-tree.h
@@ -121,6 +121,13 @@ do {                                                                       \
  * (Note, rcu_assign_pointer and rcu_dereference are not needed to control
  * access to data items when inserting into or looking up from the radix tree)
  *
+ * Note that the value returned by radix_tree_tag_get() may not be relied upon
+ * if only the RCU read lock is held.  Functions to set/clear tags and to
+ * delete nodes running concurrently with it may affect its result such that
+ * two consecutive reads in the same locked section may return different
+ * values.  If reliability is required, modification functions must also be
+ * excluded from concurrency.
+ *
  * radix_tree_tagged is able to be called without locking or RCU.
  */