3DS lightweight mutex

mutex.cpp

Handle arbiter;

struct Mutex {
  // 1 = unlocked
  // 0 = locked
  // -n = contended
  s32 val = 1;
  
  void acquire() {
    while (__atomic_fetch_sub(&val, 1, __ATOMIC_ACQUIRE) != 1) {
      // Block if val < 0
      svcArbitrateAddress(arbiter, &val, WaitIfLessThan, 0, -1);
      __atomic_fetch_add(&val, 1, __ATOMIC_RELEASE);
    }
  }
  
  void release() {
    if (__atomic_fetch_add(&val, 1, __ATOMIC_RELEASE) < 0) {
      // Wake up 1 thread
      svcArbitrateAddress(arbiter, &val, Signal, 1, -1);
    }
  }
};