On Fri, Mar 21, 2014 at 4:04 AM, Mark Evenson <evenson@panix.com> wrote:
On 3/20/14, 23:59, James M. Lawrence wrote:
Hi,
The original bug appears to still be there. For example this will hang:
(defun test () (let ((object (cons nil nil))) (threads:synchronized-on object (threads:object-wait object 0.000000001))))
1.4.0-dev-svn-14653 Java_HotSpot(TM)_Server_VM-Oracle_Corporation-1.7.0_04-b20 i386-Linux-3.2.0-24-generic-pae
Sigh. Yes, this is still not resolved with THREADS:OBJECT-WAIT in abcl-1.4.0-dev, although CL:SLEEP is fixed.
Apparently my thinking on this was incomplete. For the SLEEP, degrading to thread yield make sense for intervals less than a nanosecond. For OBJECT-WAIT is it really the case that you wish to degrade to a thread yield without ever having tried to wait for a notification on the synchronized object?
In pthreads, no matter what timeout is given to pthread_cond_timedwait, it will always release and reacquire the mutex. A certain-to-expire timeout just means we are operating within the Planck time of the system. Because pthread_cond_timedwait can return spuriously, the information it provides is not particularly useful on its own. We must always check to see if "something happened", for instance whether data produced by another thread has arrived. It may have arrived even if pthread_cond_timedwait returned ETIMEDOUT.
From what I understand of java.lang.Object.wait, the model is similar. The main difference seems to be the weirdo interpretation of 0 = infinity. It's discontinuous: passing 0.00000001 yields a totally different animal than passing 0. I should be able to wait one system Planck time unit (i.e. wait 0, without 0 meaning infinity), but I can't.
I need to add a workaround to the ABCL implementation of bordeaux-threads:condition-wait. If the user passes a timeout of zero then the this should translate to TIMED-WAIT of LEAST-POSITIVE-SINGLE-FLOAT. On your end, I think you just need to avoid rounding a non-zero value to zero. If the reader rounds a literal to zero then there's nothing you can do, but 0.000000001 is well before that problem. The separation of the reader phase in Lisp does not mix well with the extreme discontinuity between 0 and non-zero values with TIMED-WAIT. Best, lmj