single function to dismiss all open view controllers

You can call :

self.view.window!.rootViewController?.dismiss(animated: false, completion: nil)

Should dismiss all view controllers above the root view controller.

Updated answer for Swift 4 and swift 5

UIApplication.shared.keyWindow?.rootViewController?.dismiss(animated: true, completion: nil)

and when you use navigationController

self.navigationController?.popToRootViewController(animated: true)

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