How to read / write pipe in order with different workgroups and CUs?

Hello~

 i am trying to use Pipe in OpenCL2.0, but in my code I use many different workgroups and CUs. 

I have defined below attribute

__attribute__((num_compute_units(2))) 

In my code  it produces random values and put into different workgroups , for example 2 workgroups of 4x4 dimension each , 

write into the pipe, and then read from the pipe.

 I found that that when CU = 1, values write and read from pipe are in order, but when CU > 2, write / read are not in order.

Does anyone has idea about how to read / write pipe in order with many different workgroups and CUs?

Thanks in advance!