--- old/src/hotspot/share/trace/tracingExport.cpp	2018-04-11 10:52:07.000000000 -0400
+++ new/src/hotspot/share/trace/tracingExport.cpp	2018-04-11 10:52:07.000000000 -0400
@@ -36,5 +36,7 @@
 void TracingExport::set_sampler_thread_with_lock(Thread* thread) {
   // Grab Threads_lock to avoid conflicts with Thread-SMR scans.
   MutexLocker ml(Threads_lock);
-  _sampler_thread = thread;
+  assert(thread == NULL || _sampler_thread == NULL, "_sampler_thread should not already be set.");
+  // To match with the lock-free load_acquire():
+  OrderAccess::release_store(&_sampler_thread, thread);
 }