That should work. Just replace
your_post_type with the name of the post type. Also, little known fact: the
save_post hook passes the post’s ID as an argument.
function save_my_metadata($ID = false, $post = false)
if($post->post_type != 'your_post_type')
update_post_meta($ID, 'my_metadata', $_POST['my_metadata']);
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