fix sparse update bug

Author: jacquesqiao

paddle/fluid/operators/distributed/variable_response.cc

@@ -151,6 +151,7 @@ bool VariableResponse::CopySelectRowsData(
     ::google::protobuf::io::CodedInputStream* input,
     const platform::DeviceContext& ctx, int length) {
   auto* slr = GetVar()->GetMutable<framework::SelectedRows>();
+  slr->mutable_rows()->clear();
   slr->mutable_rows()->resize(length /
                               framework::SizeOfType(typeid(int64_t)));  // int64
   int64_t* rows_data = slr->mutable_rows()->data();

paddle/fluid/operators/listen_and_serv_op.cc

@@ -165,12 +165,13 @@ void ListenAndServOp::RunSyncLoop(
                           recv_scope);
     VLOG(2) << "run all blocks spent " << GetTimestamp() - ts << "(ms)";
 
-    rpc_service_->SetCond(distributed::kRequestGet);
-    rpc_service_->WaitBarrier(distributed::kRequestGet);
-    rpc_service_->ResetBarrierCounter();
     // reset received sparse vars to avoid reuse it in the next mini-batch
     dynamic_cast<distributed::RequestSendHandler *>(request_send_handler_.get())
         ->ResetSparseVarRecorder();
+
+    rpc_service_->SetCond(distributed::kRequestGet);
+    rpc_service_->WaitBarrier(distributed::kRequestGet);
+    rpc_service_->ResetBarrierCounter();
   }  // while(true)
 }