diff --git a/src/sync/sync_tcp.cpp b/src/sync/sync_tcp.cpp
index 2cb4e598e..b21451d1c 100644
--- a/src/sync/sync_tcp.cpp
+++ b/src/sync/sync_tcp.cpp
@@ -258,8 +258,10 @@ class SyncManager {
       size_t nfinished = size_down_in;
       // can pass message down to childs
       for (int i = 0; i < nlink; ++i) {
-        if (i != parent_index && selecter.CheckWrite(links[i].sock)) {
-          links[i].WriteFromArray(sendrecvbuf, size_down_in);
+        if (i != parent_index) {
+          if (selecter.CheckWrite(links[i].sock)) {
+            links[i].WriteFromArray(sendrecvbuf, size_down_in);
+          }
           nfinished = std::min(links[i].size_write, nfinished);
         }
       }
@@ -319,8 +321,10 @@ class SyncManager {
       size_t nfinished = total_size;
       // send data to all out-link
       for (int i = 0; i < nlink; ++i) {
-        if (i != in_link && selecter.CheckWrite(links[i].sock)) {
-          links[i].WriteFromArray(sendrecvbuf_, size_in);
+        if (i != in_link) {
+          if (selecter.CheckWrite(links[i].sock)) {
+            links[i].WriteFromArray(sendrecvbuf_, size_in);
+          }
           nfinished = std::min(nfinished, links[i].size_write);
         }
       }
diff --git a/test/test_allreduce.cpp b/test/test_allreduce.cpp
index 496039e28..4a47d7f55 100644
--- a/test/test_allreduce.cpp
+++ b/test/test_allreduce.cpp
@@ -113,8 +113,11 @@ int main(int argc, char *argv[]) {
   std::string name = sync::GetProcessorName();
   printf("[%d] start at %s\n", rank, name.c_str());
   TestMax(n);
+  printf("[%d] TestMax pass\n", rank);
   TestSum(n);
+  printf("[%d] TestSum pass\n", rank);
   TestReducer(n);
+  printf("[%d] TestReducer pass\n", rank);
   sync::Finalize();
   printf("[%d] all check pass\n", rank);
   return 0;