From 2ab12ca2480979ba4d651adff7fef6daf3a86ce7 Mon Sep 17 00:00:00 2001
From: Yu Yang <yuyang18@baidu.com>
Date: Tue, 17 Apr 2018 10:26:04 +0800
Subject: [PATCH] Add comments and clean code

---
 paddle/fluid/pybind/pybind.cc   | 6 ++++--
 paddle/fluid/pybind/tensor_py.h | 9 +++++++++
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/paddle/fluid/pybind/pybind.cc b/paddle/fluid/pybind/pybind.cc
index 254c4a5942..5121987f92 100644
--- a/paddle/fluid/pybind/pybind.cc
+++ b/paddle/fluid/pybind/pybind.cc
@@ -505,13 +505,15 @@ All parameter, weight, gradient are variables in Paddle.
                                   scope, local_scopes, allow_op_delay);
            })
       .def("bcast_params", &ParallelExecutor::BCastParamsToGPUs)
+      // NOTE: even we return a vec<Scope*>* to Python use reference policy.
+      // We still cannot get local_scope from this vector, since the element
+      // of vec<Scope*> will be freed by Python GC. We can only return Scope*
+      // one by one and mark them as reference.
       .def("local_scopes",
            [](ParallelExecutor &self) -> std::vector<Scope *> * {
              return &self.GetLocalScopes();
            },
            py::return_value_policy::reference)
-      .def("local_scopes_len",
-           [](ParallelExecutor &self) { return self.GetLocalScopes().size(); })
       .def("local_scope", [](ParallelExecutor &self,
                              size_t i) { return self.GetLocalScopes()[i]; },
            py::return_value_policy::reference)
diff --git a/paddle/fluid/pybind/tensor_py.h b/paddle/fluid/pybind/tensor_py.h
index c9cad15a74..159d1d5f4e 100644
--- a/paddle/fluid/pybind/tensor_py.h
+++ b/paddle/fluid/pybind/tensor_py.h
@@ -190,6 +190,10 @@ void PyCUDATensorSetFromArray(
       static_cast<const platform::CUDADeviceContext *>(pool.Get(place));
   paddle::platform::GpuMemcpyAsync(dst, array.data(), sizeof(T) * array.size(),
                                    cudaMemcpyHostToDevice, dev_ctx->stream());
+  // NOTE: For safety, here wait the copy complete.
+  // It because the CPU array.data() could be destroyed after this method.
+  // If we make this method async, it could be copied data from a memory buffer
+  // that has been freed.
   dev_ctx->Wait();
 }
 
@@ -217,6 +221,11 @@ void PyCUDATensorSetFromArray(
   paddle::platform::GpuMemcpyAsync(dst, array.data(),
                                    sizeof(uint16_t) * array.size(),
                                    cudaMemcpyHostToDevice, dev_ctx->stream());
+  // NOTE: For safety, here wait the copy complete.
+  // It because the CPU array.data() could be destroyed after this method.
+  // If we make this method async, it could be copied data from a memory buffer
+  // that has been freed.
+  dev_ctx->Wait();
 }
 
 template <typename T>
-- 
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