io_uring: add support for IORING_OP_POLL
This is basically a direct port of bfe4037e, which implements a one-shot poll command through aio. Description below is based on that commit as well. However, instead of adding a POLL command and relying on io_cancel(2) to remove it, we mimic the epoll(2) interface of having a command to add a poll notification, IORING_OP_POLL_ADD, and one to remove it again, IORING_OP_POLL_REMOVE. To poll for a file descriptor the application should submit an sqe of type IORING_OP_POLL. It will poll the fd for the events specified in the poll_events field. Unlike poll or epoll without EPOLLONESHOT this interface always works in one shot mode, that is once the sqe is completed, it will have to be resubmitted. Reviewed-by: NHannes Reinecke <hare@suse.com> Based-on-code-from: Christoph Hellwig <hch@lst.de> Signed-off-by: NJens Axboe <axboe@kernel.dk>
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