as noted someone else, it is lock contention that doesn't scale, not mutable shared state. lock-free data structures, patterns like RCU ... in many cases these will scale entirely appropriately to the case at hand. A lot of situations that require high-scale mutable shared state have an inherent asymmetry to the data usage (e.g. one consumer, many writers; many consumers; one writer) that nearly always allow a better pattern than "wrap it in a mutex".

No, it's the mutable shared state that is the problem. Lock contention is just downstream of the same problems as any other mutable shared state.

> patterns like RCU

RCU isn't mutable shared state! It's sharing immutable state! That's the whole paradigm.