diff --git a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/PendingReconstructionBlocks.java b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/PendingReconstructionBlocks.java
index 956e94f4df1..6628c43a52c 100644
--- a/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/PendingReconstructionBlocks.java
+++ b/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/PendingReconstructionBlocks.java
@@ -134,7 +134,9 @@ class PendingReconstructionBlocks {
    * The total number of blocks that are undergoing reconstruction.
    */
   int size() {
-    return pendingReconstructions.size();
+    synchronized (pendingReconstructions) {
+      return pendingReconstructions.size();
+    }
   }
 
   /**