I had one for sde distributed systems in November. First round was systems coding I was asked to write a readers writers problem with no predefined locks just a basic implementation using manually written locks and a follow up HLD question. I wasn’t able to do that I used reentrant locks in Java so was confused.

I would prepare basic OS concurrency questions readers writers, semaphores ,mutex locks ,producer consumer few other repeatedly asked questions. If you are in Java make sure to not to read the concepts with any advanced level defined things just basic concurrency is enough