HDFS-14618. Incorrect synchronization of ArrayList field (ArrayList is thread-unsafe). Contributed by Paul Ward.

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/PendingReconstructionBlocks.java

@@ -132,7 +132,9 @@ class PendingReconstructionBlocks {
   public void clear() {
     synchronized (pendingReconstructions) {
       pendingReconstructions.clear();
-      timedOutItems.clear();
+      synchronized (timedOutItems) {
+        timedOutItems.clear();
+      }
       timedOutCount = 0L;
     }
   }