[Sbcl-devel] :WAIT-MULTIPLE test case in threads.impure.lisp is bogus

[Tobias C. R. <tc...@fr...>]

(with-test (:name (:condition-variable :wait-multiple))
  (loop repeat 40 do
	(let ((waitqueue (sb-thread:make-waitqueue :name "Q"))
              (mutex (sb-thread:make-mutex :name "M"))
              (failedp nil))
          (format t ".")
          (finish-output t)
          (let ((threads (loop repeat 200
                               collect
                               (sb-thread:make-thread
                                (lambda ()
                                  (handler-case
                                      (sb-sys:with-deadline (:seconds 0.01)
					(sb-thread:with-mutex (mutex)
                                          (sb-thread:condition-wait waitqueue
                                                                    mutex)
                                          (setq failedp t)))
                                    (sb-sys:deadline-timeout (c)
                                      (declare (ignore c)))))))))
            (mapc #'sb-thread:join-thread threads)
            (assert (not failedp))))))

The test case is bogus because it does not account for spurious wakeups
in CONDITION-WAIT.

With the current code (which uses NIL as futex token in CONDITION-WAIT),
spurious wakeups may only come due to the futex() syscall returning
EINTR -- I can't judge whether that's really impossible in the above
code, but it's highly unlikely at least.

Using NIL as futex token is a bug though (as I depicted in the "lost
wakeup" threads of the last month) -- and fixing it, will make spurious
wakeups less unlikely--making this test case fail almost reliably on a
highly parallel machine.

  -T.