複数のmodalを同時に閉じる方法

2つのmodalを同時に閉じたいとき、

[self.presentingViewController.presentingViewController dismissViewControllerAnimated:YES completion:NULL];

するとできました。一番深い階層のmodalを指定すると、その前のものは全て閉じることができるみたいです。
(http://stackoverflow.com/questions/8798491/dismissing-multiple-view-controllers-in-ios-5)

UIViewControllerのドキュメントにも書いてあります。

If you present several view controllers in succession, thus building a stack of presented view controllers, calling this method on a view controller lower in the stack dismisses its immediate child view controller and all view controllers above that child on the stack. When this happens, only the top-most view is dismissed in an animated fashion; any intermediate view controllers are simply removed from the stack. The top-most view is dismissed using its modal transition style, which may differ from the styles used by other view controllers lower in the stack.

(https://developer.apple.com/library/ios/documentation/uikit/reference/UIViewController_Class/Reference/Reference.html#//apple_ref/occ/instm/UIViewController/dismissViewControllerAnimated:completion:)