How to avoid infinite loop in save_post callback

You can remove the callback from the save_post hook, update the post and then re-add the call back to the hook. The Codex gives an example.

add_action('save_post', 'wpse51363_save_post');

function wpse51363_save_post($post_id) {

    //Check it's not an auto save routine
     if ( defined('DOING_AUTOSAVE') && DOING_AUTOSAVE ) 
          return;

    //Perform permission checks! For example:
    if ( !current_user_can('edit_post', $post_id) ) 
          return;

    //Check your nonce!

    //If calling wp_update_post, unhook this function so it doesn't loop infinitely
    remove_action('save_post', 'wpse51363_save_post');

    // call wp_update_post update, which calls save_post again. E.g:
    wp_update_post(array('ID' => $post_id, 'post_status' => 'private'));

    // re-hook this function
    add_action('save_post', 'wpse51363_save_post');
}