[$] Toward non-blocking asynchronous I/O

The Linux asynchronous I/O (AIO) layer tends to have many critics and fewdefenders, but most people at least expect it to actually be asynchronous. Intruth, an AIO operation can block in the kernel for a number of reasons,making AIO difficult to use in situations where the calling thread trulycannot afford to block. A longstanding patch set aiming to improve thissituation would appear to be nearing completion, but it is more of a stepin the right direction than a true solution to the problem.