nx_mutex.nx

nx_mutex.nx -- futex-based mutex (FUTEX_WAIT / FUTEX_WAKE). Companion to nx_atom (atomic primitives). When SMP arrives, mutual exclusion needs more than a spinlock -- spinning under contention burns CPU and starves the holder. Futex (fast userspace mutex) lets contended waiters sleep in the kernel until the lock is released. State word: 0 = unlocked 1 = locked, no waiters 2 = locked, possibly contended (waiters parked in kernel) Lock fast path: CAS(0, 1). Lock slow path: spin a few times, then transition to 2 + futex_wait. Unlock fast path: store 0; if previous == 2, futex_wake one. Today the sim is single-threaded, so we just track lock/unlock state without actual blocking; the API matches the SMP path so callers don't need to change later. Linux RV64 futex syscall: 98. Pairs with nx_atom for the CAS primitive (which is currently non-atomic since SMP isn't on); will be a no-source-change upgrade once nx_atom emits real LR/SC sequences.