[armedbear-devel] CLOS thread safety

[David Kirkman]

On Sat, Apr 24, 2010 at 7:50 AM, Erik Huelsmann <ehuels at gmail.com> wrote:
> Can you share some examples which show the issues here? Adding
> thread-safety to clos.lisp - in specific, targeted places - should be
> well doable.
>

It was surprisingly difficult to create a fragment which would reproduce the
errors.  But here it is!


(defclass counter ()
  ((count :initform 0)))

(setf *ncount* 10000)
(setf *nthread* 1000)

(defun count-thread (amount)
  #'(lambda ()
      (let ((c (make-instance 'counter)))
        (dotimes (i *ncount*)
          (incf (slot-value c 'count) amount))
        (format t "count ~A = ~A~%" amount (slot-value c 'count)))))

(dotimes (i *nthread*)
  (threads:make-thread (count-thread i)))

On my machine, a half dozen or so threads will reliably die with either:

Missing slot MOP::NAME

or

Missing slot MOP::PRECEDENCE-LIST


The MOP::NAME error has about 5x more common than the
MOP::PRECEDENCE-LIST error.  The key to reproducing the error is to use a
lot of threads, with *nthread* = 2, the code can run all morning without any
trouble.  But it will eventually give an error:  I originally noticed
this with long
running code and two threads.  Even with *nthread* = 10, it works without error
most of the time.

It also works (I mean gives errors) if you drop the final format in
count-thread.
The format is not thread safe, but it does not seem to cause errors by
itself -- it
just mangles output -- and it helps to see which threads are finishing.

-david k.