io_uring: enable EPOLLEXCLUSIVE for accept poll

When polling sockets for accept, use EPOLLEXCLUSIVE. This is helpful
when multiple accept SQEs are submitted.

For O_NONBLOCK sockets multiple queued SQEs would previously have all
completed at once, but most with -EAGAIN as the result. Now only one
wakes up and completes.

For sockets without O_NONBLOCK there is no user facing change, but
internally the extra requests would previously be queued onto a worker
thread as they would wake up with no connection waiting, and be
punted. Now they do not wake up unnecessarily.

Co-developed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Dylan Yudaken <dylany@fb.com>
Link: https://lore.kernel.org/r/20220325093755.4123343-1-dylany@fb.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -967,6 +967,7 @@ struct io_op_def {
 	/* set if opcode supports polled "wait" */
 	unsigned		pollin : 1;
 	unsigned		pollout : 1;
+	unsigned		poll_exclusive : 1;
 	/* op supports buffer selection */
 	unsigned		buffer_select : 1;
 	/* do prep async if is going to be punted */
@@ -1061,6 +1062,7 @@ static const struct io_op_def io_op_defs[] = {
 		.needs_file		= 1,
 		.unbound_nonreg_file	= 1,
 		.pollin			= 1,
+		.poll_exclusive		= 1,
 	},
 	[IORING_OP_ASYNC_CANCEL] = {
 		.audit_skip		= 1,
@@ -6280,7 +6282,8 @@ static int io_arm_poll_handler(struct io_kiocb *req, unsigned issue_flags)
 	} else {
 		mask |= POLLOUT | POLLWRNORM;
 	}
-
+	if (def->poll_exclusive)
+		mask |= EPOLLEXCLUSIVE;
 	if (!(issue_flags & IO_URING_F_UNLOCKED) &&
 	    !list_empty(&ctx->apoll_cache)) {
 		apoll = list_first_entry(&ctx->apoll_cache, struct async_poll,