io_uring: add support for backlogged CQ ring

Currently we drop completion events, if the CQ ring is full. That's fine for requests with bounded completion times, but it may make it harder or impossible to use io_uring with networked IO where request completion times are generally unbounded. Or with POLL, for example, which is also unbounded. After this patch, we never overflow the ring, we simply store requests in a backlog for later flushing. This flushing is done automatically by the kernel. To prevent the backlog from growing indefinitely, if the backlog is non-empty, we apply back pressure on IO submissions. Any attempt to submit new IO with a non-empty backlog will get an -EBUSY return from the kernel. This is a signal to the application that it has backlogged CQ events, and that it must reap those before being allowed to submit more IO. Note that if we do return -EBUSY, we will have filled whatever backlogged events into the CQ ring first, if there's room. This means the application can safely reap events WITHOUT entering the kernel and waiting for them, they are already available in the CQ ring.

JAJens Axboe committed 7 years ago