This is why we can't have safe cancellation points

I think you could do it the way the kernel does the exception table -- just make a sorted list of pairs of starts and ends of cancellable regions. You only need to check it when your cancellation signal is delivered, and the data cache impact of *that* is basically irrelevant.

But you could do it by flipping the default if you're willing to accept a branch: just test the cancellable flag and jump out of line if needed. This is no worse than the existing musl thing in which each cancellable syscall needs to test the cancallable flag anyway to see if it needs to cancel even without a signal being sent.