YARN-8391. Investigate AllocationFileLoaderService.reloadListener locking issue. Contributed by Szilard Nemeth.

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/scheduler/fair/AllocationFileLoaderService.java

@@ -114,7 +114,9 @@ public class AllocationFileLoaderService extends AbstractService {
       reloadThread = new Thread(() -> {
         while (running) {
           try {
-            reloadListener.onCheck();
+            synchronized (this) {
+              reloadListener.onCheck();
+            }
             long time = clock.getTime();
             long lastModified =
                 fs.getFileStatus(allocFile).getModificationTime();