RFR: 8221102: Allow GC threads to participate in threads claiming protocol

[Kim Barrett]

> On Mar 25, 2019, at 4:19 PM, Roman Kennke <rkennke at redhat.com> wrote:
> 
> 
> 
> Am 25.03.19 um 20:42 schrieb Kim Barrett:
>> 
>> We could give less exposure to the claiming mechanism by providing a
>> second function similar to the existing possibly_parallel_threads_do
>> that does hit all the threads.  The only problem I have with that is
>> naming.  Maybe the old function should be renamed to
>> possibly_parallel_java_threads_and_vm_thread_do, for consistency with
>> the serial function.
> 
> Zhengyu found a problem with my approach to include all workers in the
> protocol: workers that would be created at safepoints would start out
> with 0 claim token, and the final verification would fail because it
> wants all threads to be at global thread_parity.

Yes, that could be a problem.  For the current cases where we do
iterate over all threads, we get away with it because we don't do the
final thread count verification; that verification is only associated
with the subset iteration of possibly_parallel_oops_do.  Interesting.
The couple of places currently doing all-thread iteration look like
they should be fine with missing some that are under construction.
But that kind of brings us back around to the NJT initialization race
question from the other thread.  I'll continue this thought over there.

>  It could be fixed
> fairly easily by letting all threads start with current global parity.

I suspect that introduces a race to get the current global parity.

> But as you say, this might just be one case of assumptions and there
> might be more lurking around. So let's go with your approach for now.
> 
> Roman

OK.  So does that count as a "review"?  And I still need a second.