[$] Thread-identity switcheroo for io_uring

The io_uring subsystem is all about asynchronous execution; applications count on it to not block — unless explicitly requested to. Within io_uring, maintaining the "never blocks" guarantee has sometimes been a challenge, given that many paths in the kernel were never designed for asynchronous execution. This problem has been worked around, but at a significant cost to performance. Now, io_uring maintainer Jens Axboe has posted an RFC patch set with a somewhat radical (and potentially scary) solution to the problem.