Fix BatchNorm backward synchronization

src/operator/nn/batch_norm.cc

@@ -653,6 +653,9 @@ then set ``gamma`` to 1 and its gradient to 0.
 NNVM_REGISTER_OP(_backward_BatchNorm)
 .set_num_inputs(8)
 .set_num_outputs(3)
+.set_attr<nnvm::FMutateInputs>("FMutateInputs", [](const nnvm::NodeAttrs& attrs) {
+  return std::vector<uint32_t>{6, 7};   // moving_mean, moving_var
+})
 .set_attr<nnvm::TIsBackward>("TIsBackward", true)
 .set_attr<FInferStorageType>("FInferStorageType", BatchNormStorageType)
 #if MXNET_USE_MKLDNN == 1

tests/python/unittest/test_gluon.py

@@ -665,6 +665,32 @@ def transpose(shape):
         assert (layer(x).shape==ceil_out_shape)
 
 
+@with_seed()
+@pytest.mark.parametrize('variable', ['running_var', 'running_mean'])
+def test_batchnorm_backward_synchronization(variable):
+    """
+    Tests if synchronization of BatchNorm running variables is done correctly.
+    If not, the test sometimes fails - depending on the timing.
+    """
+    ctx = mx.test_utils.default_context()
+
+    for _ in range(20):
+        layer = nn.BatchNorm()
+        layer.initialize(ctx=ctx)
+        for _ in range(3):
+            data = mx.nd.random.normal(loc=10, scale=2, shape=(1, 3, 10, 10), ctx=ctx)
+            with mx.autograd.record():
+                out = layer(data)
+            out.backward()
+
+        # check if each read give the same value
+        var1 = getattr(layer, variable).data().asnumpy()
+        for _ in range(10):
+            var2 = getattr(layer, variable).data().asnumpy()
+            if (var1 != var2).any():
+                raise AssertionError("Two consecutive reads of " + variable + " give different results")
+
+
 @with_seed()
 def test_batchnorm():
     layer = nn.BatchNorm(in_channels=10)