[$] Thread-identity switcheroo for io_uring

The io_uringsubsystem is all about asynchronous execution; applications count on itto not block - unless explicitly requested to. Within io_uring, maintainingthe "never blocks" guarantee has sometimes been a challenge, given thatmany paths in the kernel were never designed for asynchronous execution.This problem has been worked around, but at a significant cost toperformance. Now, io_uring maintainer Jens Axboe has posted an RFC patch setwith a somewhat radical (and potentially scary) solution to the problem.