volatile int seq = 0, lock = 1, n = 0;

void lock() {
  atomic_inc(n);
  while(1) {
    lseq = seq;
    if (atomic_dec(&lock))
      return; 
    futex_wait(&seq, lseq);
  }
}

void unlock() {
  lock = 1;
  atomic_inc(seq);
  atomic_dec(n);  
  if (n > 0) 
    futex_wake(&seq, 1);
}