diff --git a/core/src/scheduler/job/SearchJob.h b/core/src/scheduler/job/SearchJob.h
index 1e586090b9baff5272054c29ed60d220e409cbcb..40cf59e24e4b5b16ca3085b8b5d621769c8f647b 100644
--- a/core/src/scheduler/job/SearchJob.h
+++ b/core/src/scheduler/job/SearchJob.h
@@ -90,6 +90,10 @@ class SearchJob : public Job {
         return index_files_;
     }
 
+    std::mutex& mutex() {
+        return mutex_;
+    }
+
  private:
     uint64_t topk_ = 0;
     uint64_t nq_ = 0;
diff --git a/core/src/scheduler/task/SearchTask.cpp b/core/src/scheduler/task/SearchTask.cpp
index 1bf1caff76e641c0ca25a4fc3ab4a22fe5cbc2ab..7daa6268e8f5a0f6d37a5a2699c16dcce55fec21 100644
--- a/core/src/scheduler/task/SearchTask.cpp
+++ b/core/src/scheduler/task/SearchTask.cpp
@@ -219,8 +219,11 @@ XSearchTask::Execute() {
 
             // step 3: pick up topk result
             auto spec_k = index_engine_->Count() < topk ? index_engine_->Count() : topk;
-            XSearchTask::MergeTopkToResultSet(output_ids, output_distance, spec_k, nq, topk, metric_l2,
+            {
+                std::unique_lock<std::mutex> lock(search_job->mutex());
+                XSearchTask::MergeTopkToResultSet(output_ids, output_distance, spec_k, nq, topk, metric_l2,
                                               search_job->GetResult());
+            }
 
             span = rc.RecordSection(hdr + ", reduce topk");
             //            search_job->AccumReduceCost(span);