I realize now that you were proposing unlocking and relocking the mutex several times. Generally, if someone unlocks a mutex in the middle, they're already thinking about races with other callers (even if the code isn't cancelled in between).

You can definitely argue that developers should think about await points the same way they think about letting go of the mutex entirely, in case cancellation happens. Are mutexes conducive to that kind of thinking? Practically, I've found this to be very easy to get wrong.