Look at the benchmarks. The fairness locks perform worse than if you didn’t have any fairness because you’re wasting a huge amount of CPU cycles trying to create that fairness.

Recursive locks are generally acknowledged as a terrible design pattern.

You don’t get to simultaneously claim that glibc has an optimal lock implementation and that high performance applications should implement their own. That implies the glibc implementation isn’t actually optimal because you’re paying a penalty for features that aren’t actually necessary.