The problem isn't just the mainline source code, you also have drivers which of course vary for each device. Then even completely beyond control of the kernel,

The problem isn't just the mainline source code, you also have drivers which of course vary for each device. Then even completely beyond control of the kernel, devices have their own RAM caches, which means that even if the device reports that something was written, it might not have been. So there's no way to absolutely be sure.