Yeah, SERIALIZABLE will guarantee you get enough locks to avoid race conditions, it's just very slow. The only way to do better in theory is to have your client code aware of all other client code, even then idk how exactly you'd make that take the right locks.

Also idk what the Rust suggestion is. Nothing in Rust would guard against this kind of race condition if you replicated the example over there. You can do mutex or even RwMutex on a single struct, but that doesn't force you to hold struct A's read mutex while you write to struct B.