7020042: G1: Partially remove fix for 6994628

Summary: Disable reference discovery and processing during concurrent marking by disabling fix for 6994628.
Reviewed-by: tonyp, ysr

hotspot/src/share/vm/gc_implementation/g1/concurrentMark.cpp

@@ -3203,8 +3203,12 @@ public:
                CMTask* task)
     : _g1h(g1h), _cm(cm), _task(task)
   {
-    _ref_processor = g1h->ref_processor();
-    assert(_ref_processor != NULL, "should not be NULL");
+    assert(_ref_processor == NULL, "should be initialized to NULL");
+
+    if (G1UseConcMarkReferenceProcessing) {
+      _ref_processor = g1h->ref_processor();
+      assert(_ref_processor != NULL, "should not be NULL");
+    }
   }
 };
 

hotspot/src/share/vm/gc_implementation/g1/g1_globals.hpp

@@ -89,6 +89,11 @@
           "The number of discovered reference objects to process before "   \
           "draining concurrent marking work queues.")                       \
                                                                             \
+  experimental(bool, G1UseConcMarkReferenceProcessing, false,               \
+          "If true, enable reference discovery during concurrent "          \
+          "marking and reference processing at the end of remark "          \
+          "(unsafe).")                                                      \
+                                                                            \
   develop(bool, G1SATBBarrierPrintNullPreVals, false,                       \
           "If true, count frac of ptr writes with null pre-vals.")          \
                                                                             \