io_uring: pick up link work on submit reference drop

If work completes inline, then we should pick up a dependent link item
in __io_queue_sqe() as well. If we don't do so, we're forced to go async
with that item, which is suboptimal.

This also fixes an issue with io_put_req_find_next(), which always looks
up the next work item. That should only be done if we're dropping the
last reference to the request, to prevent multiple lookups of the same
work item.

Outside of being a fix, this also enables a good cleanup series for 5.7,
where we never have to pass 'nxt' around or into the work handlers.
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -1483,10 +1483,10 @@ static void io_free_req(struct io_kiocb *req)
 __attribute__((nonnull))
 static void io_put_req_find_next(struct io_kiocb *req, struct io_kiocb **nxtptr)
 {
+	if (refcount_dec_and_test(&req->refs)) {
 		io_req_find_next(req, nxtptr);
-
-	if (refcount_dec_and_test(&req->refs))
 		__io_free_req(req);
+	}
 }
 
 static void io_put_req(struct io_kiocb *req)
@@ -4749,7 +4749,7 @@ static void __io_queue_sqe(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 
 err:
 	/* drop submission reference */
-	io_put_req(req);
+	io_put_req_find_next(req, &nxt);
 
 	if (linked_timeout) {
 		if (!ret)