--- old/src/hotspot/share/gc/parallel/psCompactionManager.cpp	2019-12-03 11:25:09.975990403 +0100
+++ new/src/hotspot/share/gc/parallel/psCompactionManager.cpp	2019-12-03 11:25:09.603978851 +0100
@@ -172,24 +172,25 @@
 }
 
 size_t ParCompactionManager::pop_shadow_region_mt_safe(PSParallelCompact::RegionData* region_ptr) {
+  MonitorLocker ml(_shadow_region_monitor, Mutex::_no_safepoint_check_flag);
   while (true) {
-    MutexLocker ml(_shadow_region_monitor, Mutex::_no_safepoint_check_flag);
-    if (_shadow_region_array->is_empty()) {
-      // Check if the corresponding heap region is available now.
-      // If so, we don't need to get a shadow region anymore,
-      // and we return 0 to indicate this case
-      if (region_ptr->claimed()) {
-        return 0;
-      }
-    } else {
+    if (!_shadow_region_array->is_empty()) {
       return _shadow_region_array->pop();
     }
+    // Check if the corresponding heap region is available now.
+    // If so, we don't need to get a shadow region anymore, and
+    // we return InvalidShadow to indicate such a case.
+    if (region_ptr->claimed()) {
+      return InvalidShadow;
+    }
+    ml.wait(1);
   }
 }
 
 void ParCompactionManager::push_shadow_region_mt_safe(size_t shadow_region) {
-  MutexLocker ml(_shadow_region_monitor, Mutex::_no_safepoint_check_flag);
+  MonitorLocker ml(_shadow_region_monitor, Mutex::_no_safepoint_check_flag);
   _shadow_region_array->push(shadow_region);
+  ml.notify();
 }
 
 void ParCompactionManager::push_shadow_region(size_t shadow_region) {