Bytes come in to the ring buffer in the interupt.
The bytes are pulled out an processed in the main loop.
The main loop checks the byte count of the ring buffer so it is never stuck.

By the way
Doing too much work in an interupt means the rest on the system is frozen during that time. Waiting for anything without a timeout, unless it is garunteed to happen is a bad idea. The system hangs. The debuging is a pain.