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add_filter( 'wp_nav_menu_items', 'wti_loginout_menu_link', 10, 2 );

function wti_loginout_menu_link( $items, $args ) {
   if ($args->theme_location == 'primary') {
      if (is_user_logged_in()) {
         $items .= '<li class="right"><a href="'. wp_logout_url() .'">Log Out</a></li>';
      } else {
         $items .= '<li class="right"><a href="'. wp_login_url(get_permalink()) .'">Log In</a></li>';
      }
   }
   return $items;
}