diff --git a/ipc/util.h b/ipc/util.h
index 77336c2bdb9c..c692010e6f0a 100644
--- a/ipc/util.h
+++ b/ipc/util.h
@@ -112,6 +112,9 @@ int ipcperms(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp, short flg);
  * Objects are reference counted, they start with reference count 1.
  * getref increases the refcount, the putref call that reduces the recount
  * to 0 schedules the rcu destruction. Caller must guarantee locking.
+ *
+ * refcount is initialized by ipc_addid(), before that point call_rcu()
+ * must be used.
  */
 int ipc_rcu_getref(struct kern_ipc_perm *ptr);
 void ipc_rcu_putref(struct kern_ipc_perm *ptr,