Writing diary is a way to protect yourself

Lots of times, he can stab you on your back without you knowing since he is two levels higher. If you were laid off or fired, you can sue your company, and the diary would be the evidence. Remember to include who, what, how, when, where. 

One of my friends was laid off by her company, she is the only high level female Chinese director. She did not have any writing evidence, she could not sue her company even though she believed there has been discrimination.

my boss is white American man, he did not think I would take notes, leaving along going to HR. He was still lying about me the day before he was fired. I am in a relatively high level management position, HR could handle things differently if you level is not significant. Then the notes are even more important.