diff --git a/core/src/main/scala/org/apache/spark/shuffle/IndexShuffleBlockResolver.scala b/core/src/main/scala/org/apache/spark/shuffle/IndexShuffleBlockResolver.scala
index 9b27f648134e..f23e51c4983f 100644
--- a/core/src/main/scala/org/apache/spark/shuffle/IndexShuffleBlockResolver.scala
+++ b/core/src/main/scala/org/apache/spark/shuffle/IndexShuffleBlockResolver.scala
@@ -310,13 +310,13 @@ private[spark] class IndexShuffleBlockResolver(
             val mapTaskIds = taskIdMapsForShuffle.computeIfAbsent(
               shuffleId, _ => new OpenHashSet[Long](8)
             )
-            mapTaskIds.add(mapId)
+            mapTaskIds.synchronized { mapTaskIds.add(mapId) }
 
           case ShuffleDataBlockId(shuffleId, mapId, _) =>
             val mapTaskIds = taskIdMapsForShuffle.computeIfAbsent(
               shuffleId, _ => new OpenHashSet[Long](8)
             )
-            mapTaskIds.add(mapId)
+            mapTaskIds.synchronized { mapTaskIds.add(mapId) }
 
           case _ => // Unreachable
         }