Swift has easy access to pthread mutex and you can do that.

serialQueue.async { // Critical section } is GCD's way of doing the same with the added benefit that the calling code does not have to wait.

Problems in the parent code could be solved by using a mutex instead of GCD in this case. The code _has_ to wait for the access token to be loaded and not blocking does not make sense here unless you also introduce some kind of Promises.