io_uring/net: add IORING_ACCEPT_POLL_FIRST flag

Similarly to how polling first is supported for receive, it makes sense
to provide the same for accept. An accept operation does a lot of
expensive setup, like allocating an fd, a socket/inode, etc. If no
connection request is already pending, this is wasted and will just be
cleaned up and freed, only to retry via the usual poll trigger.

Add IORING_ACCEPT_POLL_FIRST, which tells accept to only initiate the
accept request if poll says we have something to accept.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/uapi/linux/io_uring.h

@@ -380,6 +380,7 @@ enum io_uring_op {
  */
 #define IORING_ACCEPT_MULTISHOT	(1U << 0)
 #define IORING_ACCEPT_DONTWAIT	(1U << 1)
+#define IORING_ACCEPT_POLL_FIRST	(1U << 2)
 
 /*
  * IORING_OP_MSG_RING command types, stored in sqe->addr

io_uring/net.c

@@ -1487,6 +1487,9 @@ void io_sendrecv_fail(struct io_kiocb *req)
 		req->cqe.flags |= IORING_CQE_F_MORE;
 }
 
+#define ACCEPT_FLAGS	(IORING_ACCEPT_MULTISHOT | IORING_ACCEPT_DONTWAIT | \
+			 IORING_ACCEPT_POLL_FIRST)
+
 int io_accept_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 {
 	struct io_accept *accept = io_kiocb_to_cmd(req, struct io_accept);
@@ -1499,7 +1502,7 @@ int io_accept_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 	accept->flags = READ_ONCE(sqe->accept_flags);
 	accept->nofile = rlimit(RLIMIT_NOFILE);
 	accept->iou_flags = READ_ONCE(sqe->ioprio);
-	if (accept->iou_flags & ~(IORING_ACCEPT_MULTISHOT | IORING_ACCEPT_DONTWAIT))
+	if (accept->iou_flags & ~ACCEPT_FLAGS)
 		return -EINVAL;
 
 	accept->file_slot = READ_ONCE(sqe->file_index);
@@ -1530,6 +1533,10 @@ int io_accept(struct io_kiocb *req, unsigned int issue_flags)
 	struct file *file;
 	int ret, fd;
 
+	if (!(req->flags & REQ_F_POLLED) &&
+	    accept->iou_flags & IORING_ACCEPT_POLL_FIRST)
+		return -EAGAIN;
+
 retry:
 	if (!fixed) {
 		fd = __get_unused_fd_flags(accept->flags, accept->nofile);