loop: avoid corruption of ringbuffer

The ringbuffer can't be written to from multiple threads.

When both the main loop and data thread do _invoke, they both write to
the ringbuffer and cause it to be corrupted because the ringbuffer is
not multi-writer safe.

Doing invoke from the thread itself is usually done to flush things out
so we really only need to flush the ringbuffer and call the callback.

See #1451