RFR 8215047: Task terminators do not complete termination in consistent state

[zgu at redhat.com]

Hi,

> > > I will think a bit more about the change while waiting for the
> > > test
> > > results.
> > > 
> 
> Testing showed good results, no issues.

Thanks!

> 
> I would like to ask if I understood the changes correctly: to me it
> seems that some of these changes are the actual fix, others seem to
> be
> enhancements.
> 
> So the changes in owstTaskTerminator.cpp:68-74 seem to be the actual
> fix for the issue - it needs to recheck _offered_termination within
> the
> lock at that point because of the mentioned race.
I would say that this is actually an enhancement. Without it, merely
delays the termination of this particular thread. 

Filed RFE: https://bugs.openjdk.java.net/browse/JDK-8217794

> 
> There are additional improvements to both terminators
> (owstTaskTerminator.cpp:171-174, taskqueue.cpp:220-232) that may
> result
> in potential needless additional rounds of offer_termination().
> I.e. before returning a result that indicates to the caller to
> continue
> doing work for another round, instead if in the meantime everyone has
> offered termination, we are obviously done too.

> 
> If I am correct, I would strongly prefer if these changes (the
> enhancements) would be changed separately.
> 
> Also, the code in taskqueue.cpp:220-232 is worth an extra method with
> a
> description indicating why we can return true in offer_termination in
> that case (because obviously, if all threads are sure to have offered
> termination, between the evaluation of the condition and trying to
> decrement the _offer_termination variable, work has been completed.

Okay. But I want to point out that it is not necessary a race, because
TerminatorTerminator may decide to request an abort based on external
conditions, which may have nothing to do with the progress of the
termination.

Updated Webrev: http://cr.openjdk.java.net/~zgu/JDK-8215047/webrev.02/i
ndex.html

Passed hotspot_gc test, will rerun all other tests.

> As an additional enhancement, it might be worth doing padding of the
> ParallelTaskTerminator::_offered_termination variable.
> 
I filed a separate RFE: https://bugs.openjdk.java.net/browse/JDK-821778
5


-Zhengyu


> Thanks,
>   Thomas
> 
>