diff --git a/llvm/lib/Transforms/Instrumentation/DataFlowSanitizer.cpp b/llvm/lib/Transforms/Instrumentation/DataFlowSanitizer.cpp
--- a/llvm/lib/Transforms/Instrumentation/DataFlowSanitizer.cpp
+++ b/llvm/lib/Transforms/Instrumentation/DataFlowSanitizer.cpp
@@ -162,6 +162,12 @@
              "load or return with a nonzero label"),
     cl::Hidden);
 
+// Experimental feature that inserts callbacks for certain data events.
+// Currently callbacks are only inserted for stores.
+//
+// If this flag is set to true, the user must provide definitions for the
+// following callback functions:
+//   void __dfsan_store_callback(dfsan_label Label);
 static cl::opt<bool> ClEventCallbacks(
     "dfsan-event-callbacks",
     cl::desc("Insert calls to __dfsan_*_callback functions on data events."),