nio4r provides internal locking to ensure that it's safe to use from multiple concurrent threads. Only one thread can select on a ::NIO::Selector at a given time, and while a thread is selecting other threads are blocked from registering or deregistering IO objects. Once a pending select has completed, requests to register/unregister IO objects will be processed.

Wakeup support

The NIO::Selector#wakeup method allows one thread to unblock another thread that's in the middle of an NIO::Selector#select operation. This lets other threads that need to communicate immediately with the selector unblock it so it can process other events that it's not presently selecting on.