From e59e125714804124de8a9a38c9f056835ecba6c4 Mon Sep 17 00:00:00 2001
From: reminisce <wujun.nju@gmail.com>
Date: Fri, 21 Jul 2017 13:38:05 -0700
Subject: [PATCH] More comments

---
 src/ndarray/ndarray.cc | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/src/ndarray/ndarray.cc b/src/ndarray/ndarray.cc
index 48875408e38a..4a402def143d 100644
--- a/src/ndarray/ndarray.cc
+++ b/src/ndarray/ndarray.cc
@@ -1131,6 +1131,12 @@ void NDArray::SyncCopyFromNDArray(const NDArray& src, int i, int j) {
   }
   // The copy operation was pushed to engine to execute.
   // Need to wait here for it being completed.
+  // The reason for pushing the copy operation to engine
+  // is because when copying data from a sparse tensor
+  // to the current one, that sparse ndarray's storage_shape/aux_shape
+  // may not be ready or changed and we need to ensure
+  // thread safty for reading the correct shape info to allocate
+  // memory for the current ndarray.
   WaitToRead();
 }