Attending a conference that my abusive ex-supervisor is also attending, how I should react?
The best revenge is living well. You have a good start on that working on your topic with a nice supervisor, and attending the conference as a volunteer.

Ideally, treat him as just another academic you happen to know, about on the level of a professor who taught one of your undergraduate courses. I understand that may be difficult, but it tells him that his treatment of you was just a minor roadbump in your life. He is too unimportant to be even worth your anger. For a racist, seeing you happily getting on with your life will hurt far more than if you let him see he still has emotional power over you.