Both the .htaccess redirect and the Redirects tool in the control panel both redirect the specific URL and all subdirectories. So if you setup a redirect for example.com, it will also redirect example.com/wp, example.com/wp/wp-admin, etc. So if you set a site to redirect back onto a subdirectory of the same domain, it will keep redirecting you until your browser gives up.
If you want to only redirect the root URL, the easiest way is probably to use a PHP redirect. So in the root folder of your domain, you could create a file with the name index.php and these contents:
<?php
header('Location: /wp/');