--- a/src/hotspot/share/jfr/periodic/sampling/jfrThreadSampler.cpp	Wed Apr 17 12:43:41 2019 +0530
+++ b/src/hotspot/share/jfr/periodic/sampling/jfrThreadSampler.cpp	Wed Apr 17 09:25:27 2019 +0200
@@ -359,7 +359,7 @@
   }
 
   bool ret = false;
-  thread->set_trace_flag();
+  thread->set_trace_flag();  // Provides StoreLoad, needed to keep read of thread state from floating up.
   if (JAVA_SAMPLE == type) {
     if (thread_state_in_java(thread)) {
       ret = sample_thread_in_java(thread, frames, max_frames);