[armedbear-devel] Bordeaux-threads test hangs

[Mark Evenson]

On 3/25/13 1129 , Anton Vodonosov wrote:
[…]
>
> Mark, the test hangs not always.

Thanks for the confirmation, as I was just coming around to this 
realization as the only way to explain the inconsistencies.

>
> I think this is bug in the test, because the test is not guaranteed to work.
>
> It create 100 threads, each thread waits for (= i *shared*). In every thread i has different
> value, from 0 to 99. So the threads are chained and each thread
> waits white the previous one will increase *shared*.
>
> But the threads use bt:condition-notify to interact, which deliver
> notification to only one of the waiting thread, and there is not guarantee
> it will be right thread.
>
> The tests passes on SBCL. Maybe SBCL always choses to notify
> the first thread in the waiting queue.
>
> But bt:condition-notify contract does not require this.
>
> In short, I think what we see is not a bug in ABCL or bordeaux-threads,
> but a bug in the test.

Digging into the test

(test condition-variable
   (setf *shared* 0)
   (let ((num-procs 100))
     (dotimes (i num-procs)
       (make-thread
        (compile nil
                 `(lambda ()
                    (with-lock-held (*lock*)
                      (loop
                         until (= ,i *shared*)
                         do (condition-wait *condition-variable* *lock*))
                      (incf *shared*))
                    (condition-notify *condition-variable*)))))
     (with-lock-held (*lock*)
       (loop
          until (= num-procs *shared*)
          do (condition-wait *condition-variable* *lock*)))
     (is (equal num-procs *shared*))))

I really don't understand what is being tested here.  Since there is no 
delay in starting the threads, for a non-loaded CPU each thread never 
really invokes the CONDITION-WAIT.  Instead, each thread "sees" that it 
is the correct worker in the chain, calls essentially a no-op 
CONDITION-NOTIFY and then exits.  Wouldn't one want to delay the 
execution of the threads by some random amount before starting things going?


-- 
"A screaming comes across the sky.  It has happened before, but there
is nothing to compare to it now."