diff --git a/cpp/src/scheduler/task/SearchTask.cpp b/cpp/src/scheduler/task/SearchTask.cpp
index b32c547022..46d429ab56 100644
--- a/cpp/src/scheduler/task/SearchTask.cpp
+++ b/cpp/src/scheduler/task/SearchTask.cpp
@@ -271,7 +271,7 @@ Status XSearchTask::MergeResult(SearchContext::Id2DistanceMap &distance_src,
         return Status::OK();
     }
 
-    merge_mutex_.lock();
+    std::unique_lock<std::mutex> lock(merge_mutex_);
     if (distance_target.empty()) {
         distance_target.swap(distance_src);
         return Status::OK();
@@ -331,7 +331,6 @@ Status XSearchTask::MergeResult(SearchContext::Id2DistanceMap &distance_src,
     }
 
     distance_target.swap(distance_merged);
-    merge_mutex_.unlock();
 
     return Status::OK();
 }