diff --git a/bmtrain/pipe/comm.py b/bmtrain/pipe/comm.py
index 32099a0..d65f0f1 100644
--- a/bmtrain/pipe/comm.py
+++ b/bmtrain/pipe/comm.py
@@ -107,7 +107,7 @@ def send_backward_recv_forward(self, backward_grad, need_data=False):
                 data = self.get_data()
             else:
                 forward_state = [None]
-                data = None
+                data = [None]
         return forward_state, data
 
 
diff --git a/bmtrain/pipe/schedule.py b/bmtrain/pipe/schedule.py
index abd898d..29299fb 100644
--- a/bmtrain/pipe/schedule.py
+++ b/bmtrain/pipe/schedule.py
@@ -26,6 +26,8 @@ def backward_func(inp, backward_step, output, grad_output, optim_manager=None):
     if not isinstance(grad_output, Iterable):
         grad_output = [grad_output]
     backward_step(output[0], grad_output[0])
+    current_stream = torch.cuda.current_stream()
+    current_stream.wait_stream(bmt.config['load_stream'])
     input_grad = [None]
     if inp is not None:
         input_grad = []
@@ -164,7 +166,5 @@ def pipeline_forward_backward(model, data_iterator, forward_step, backward_step,
             commander.send_prev(input_grad)
     blocklist = model.get_blocklist()
     # blocklist.reduce_tied_module()
-    
-    bmt.synchronize()
 
     
diff --git a/example/pipe_train.py b/example/pipe_train.py
index c3c757f..64204a0 100644
--- a/example/pipe_train.py
+++ b/example/pipe_train.py
@@ -6,6 +6,7 @@
 from bmtrain.global_var import config
 from bmtrain import inspect
 from bmtrain.pipe import pipeline_forward_backward
+from typing import Iterable
 
 def main():
     bmt.init_distributed(
@@ -51,7 +52,7 @@ def data_loader():
                 torch.full_like(targets, -100, dtype=torch.long)
             )
             pos = torch.arange(enc_input.size(1)).long().cuda().repeat(enc_input.size(0), 1)
-            yield enc_input, pos, pos<enc_length[:, None], targets
+            yield enc_input, pos, mask, targets
     
     if config['tp_size'] > 1:
         loss_func = bmt.loss.FusedCrossEntropy(ignore_index=-100, parallel=True)
@@ -67,7 +68,6 @@ def data_loader():
     bmt.synchronize()
     avg_time_recorder = bmt.utils.AverageRecorder()
     avg_loss_recorder = bmt.utils.AverageRecorder()
-    global_loss_items = []
 
     def forward_step(model, input, data):
         enc_input, pos, mask, targets = data
@@ -89,20 +89,20 @@ def backward_step(output, grad_output):
             output = optim_manager.scale_loss(output)
             output = output / bmt.config['micros'] 
         torch.autograd.backward(output, grad_tensors=grad_output)
-        current_stream = torch.cuda.current_stream()
-        current_stream.wait_stream(bmt.config['load_stream'])
 
         
         
     for iteration in range(10):
         # load data
+        global_loss_items = []
         st = time.time()
         rank = bmt.config["topology"].pipe_rank
         # global_loss, grad_norm = pipeline_forward_backward(model, data_loader(), micro , num_micros, optim_manager)
+        optim_manager.zero_grad()
         pipeline_forward_backward(model, data_loader(), forward_step, backward_step, micro , num_micros)
         grad_norm = optim_manager.clip_grad_norm(optim_manager.optimizers[0].param_groups, 1.0, norm_type=2)
         optim_manager.step()
-        optim_manager.zero_grad()
+        bmt.synchronize()
         # record time and loss
         iteration_time = time.time() - st