Page History

...

When after this, the consumer calls a callback, the callback must be able to invoke the consumer again. This is allowed because the thread-local variable corresponds to the top of the stack. Therefore, code that is not aware of this KIP (all programs in existence till now) will continue to work as before.
The callback may now chose to access the thread-local variable (using getThreadAccessKey), and store the access key on the local-variable of another thread (using setThreadAccessKey), thereby allowing that thread to access the consumer. Because acquire  immediately and atomically stores a new access key, it is not possible for multiple threads to use a valid access key concurrently.
When a callback passes on the its access-key to another thread, it must wait for that other thread to complete before returning from the callback.

...