Rethinking the futex API

> Why must it make a syscall in the uncontested case?

It doesn't have to be. But this isn't just a simple compare and swap. You are checking a vector, which requires many steps. These steps have to be done atomically along with the following futex_waitv() if needed. How are you going to do it?

I've been programming long enough to be very confident you can' trust your average programmer to get that sort of thing right. Hell, I'm pretty confident I won't get it right on the first try. That in itself isn't a problem: you wrap it in a user space library so they can't get it wrong.

But then, if you are implementing a user space library anyway, and the library could implement the futex_waitv() in user space and it would be faster to boot in the uncontested case, what have you gained by doing it in the kernel?