Visa - Round Trip Flights

You are perfectly allowed to do that. Anyway, you should look at your comprehensive itinerary as two separate ones:

Canada - Shanghai - Tokyo

Tokyo - Shanghai - Canada.

They are both eligible for visa free transit. The first one for 144 hour visa free transit because you will stay in Shanghai for more than 24 hours and the second one for 24 hour visa free transit because you will stay in Shanghai for less than 24 hours. In both cases you don't need any kind of visa!

You will printout your flight tickets and have those printouts with you. Also, you will declare at check-in in Canada that you will travel to Tokyo via Shanghai using 144 hour visa free transit.

Upon your landing in Shanghai, you will approach 144 hour visa free transit counter where you will fill out the blue form and after a short inspection, you will get a sticker and be allowed to exit the airport.

On your way back, you will declare in Tokyo that you will travel to Canada via Shanghai using 24 hour visa free transit. Upon arrival in Shanghai, assuming that your layover will be a short one, you will just remain in a transit area waiting for your flight to Canada. That's it.