6944166: G1: explicit GCs are not always handled correctly

G1 was not handling explicit GCs correctly in many ways. It does now. See the CR for the list of improvements contained in this changeset.

Reviewed-by: iveresov, ysr, johnc

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

@@ -266,6 +266,12 @@ void ConcurrentMarkThread::run() {
       _cm->clearNextBitmap();
       _sts.leave();
     }
+
+    // Update the number of full collections that have been
+    // completed. This will also notify the FullGCCount_lock in case a
+    // Java thread is waiting for a full GC to happen (e.g., it
+    // called System.gc() with +ExplicitGCInvokesConcurrent).
+    g1->increment_full_collections_completed(true /* outer */);
   }
   assert(_should_terminate, "just checking");