futex-condition-wait.lisp

(with-pinned-objects (queue me)
  (setf (waitqueue-token queue) me)
  (release-mutex mutex)
  ;; Now we go to sleep using futex-wait. If anyone else
  ;; manages to grab MUTEX and call CONDITION-NOTIFY during
  ;; this comment, it will change the token, and so futex-wait
  ;; returns immediately instead of sleeping. Ergo, no lost
  ;; wakeup. We may get spurious wakeups, but that's ok.
  (setf status
        (case (allow-with-interrupts
                (futex-wait (waitqueue-token-address queue)
                            (get-lisp-obj-address me)
                            ;; our way of saying "no
                            ;; timeout":
                            (or to-sec -1)
                            (or to-usec 0)))
          ((1)
           ;;  1 = ETIMEDOUT
           :timeout)
          (t
           ;; -1 = EWOULDBLOCK, possibly spurious wakeup
           ;;  0 = normal wakeup
           ;;  2 = EINTR, a spurious wakeup
           :ok))))

futex_wait.c

static inline int sys_futex(void *futex, int op, int val, struct timespec *rel)
{
    return syscall(SYS_futex, futex, op, val, rel);
}

int
futex_wait(int *lock_word, int oldval, long sec, unsigned long usec)
{
  struct timespec timeout;
  int t;

  if (sec<0) {
      t = sys_futex(lock_word, futex_wait_op(), oldval, 0);
  }
  else {
      timeout.tv_sec = sec;
      timeout.tv_nsec = usec * 1000;
      t = sys_futex(lock_word, futex_wait_op(), oldval, &timeout);
  }
  if (t==0)
      return 0;
  else if (errno==ETIMEDOUT)
      return 1;
  else if (errno==EINTR)
      return 2;
  else
      /* EWOULDBLOCK and others, need to check the lock */
      return -1;
}