diff -r a6deb69743d4 -r 4cbfa5077d68 src/hotspot/share/jfr/recorder/service/jfrPostBox.cpp
--- a/src/hotspot/share/jfr/recorder/service/jfrPostBox.cpp	Mon Nov 25 12:32:07 2019 +0100
+++ b/src/hotspot/share/jfr/recorder/service/jfrPostBox.cpp	Mon Nov 25 12:32:40 2019 +0100
@@ -139,7 +139,7 @@
 
 int JfrPostBox::collect() {
   // get pending and reset to 0
-  const int messages = Atomic::xchg(0, &_messages);
+  const int messages = Atomic::xchg(&_messages, 0);
   if (check_waiters(messages)) {
     _has_waiters = true;
     assert(JfrMsg_lock->owned_by_self(), "incrementing _msg_read_serial is protected by JfrMsg_lock");