[Ecls-list] Slightly disruptive change (in threads)

[Juan Jose Garcia-Ripoll]

I have finally convinced myself that there is no way to reuse the operating
system mutexes from pthreads or Windows if we still want to have
interruptible threads.

As a possible fix I have uploaded to git/CVS an implementation based on
libatomics' CAS (compare-and-swap) combined with some simple-minded wait
scheme (very similar to SBCL's). The code looks very simple. It consist on
a function, get_lock_inner(), which is executed with disable threads,
followed by some code that decides whether to wait and for how long.

The difference with respect to pthreads is that get_lock_inner() stores in
the lock two values, the owner and the counter, which are enough to know
whether a lock is owned or not. With that, WITH-LOCK becomes implementable
with lisp functions and no special magic (See also below).

I would appreciate if you could test it and discuss here both the stability
and the philosophy of the implementation.

Best,

Juanjo

(defmacro with-lock ((lock-form &rest options) &body body)
  (ext:with-unique-names (lock owner count)
    `(let* ((,lock ,lock-form)
            (,owner (mp:lock-owner ,lock))
    (,count (mp:lock-count ,lock)))
       (without-interrupts
           (unwind-protect
                (with-restored-interrupts
                    (mp::get-lock ,lock)
                  (locally , at body))
             (when (and (eq mp:*current-process* (mp:lock-owner ,lock))
(or (not (eq ,owner mp:*current-process*))
    (> (mp:lock-count ,lock) ,count)))
               (mp::giveup-lock ,lock)))))))

static cl_fixnum
get_lock_inner(cl_object lock, cl_object own_process)
{
        if (AO_compare_and_swap_full((AO_t*)&(lock->lock.owner),
     (AO_t)Cnil, (AO_t)own_process)) {
 return lock->lock.counter = 1;
} else if (lock->lock.owner == own_process) {
                if (!lock->lock.recursive) {
return -1;
}
                return ++lock->lock.counter;
        } else {
return 0;
}
}


-- 
Instituto de Física Fundamental, CSIC
c/ Serrano, 113b, Madrid 28006 (Spain)
http://juanjose.garciaripoll.googlepages.com
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