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

[Kim Barrett]

> On Mar 27, 2019, at 4:37 AM, Roman Kennke <rkennke at redhat.com> wrote:
>>> Also, the resetting logic warrants a test, it is so rare, it will otherwise not get executed, but if it fails, it would be catastrophic.
>> I was hoping the resetting code was simple enough to not require a
>> test.  I will see if I can come up with a sensible gtest.
> 
> I guess the 'design' of the claiming protocol is just making almost everything not-obvious. I think for this reason alone I shall go ahead and work on limiting/eliminating its usage outside of thread.cpp anyway.

Another function similar to possibly_parallel_threads_do that operated
on all threads seems like it would be a good thing to have.  Possibly
stealing the existing name and renaming the old operation.  I'd like
the defer that to a followup RFE rather than as part of this bug fix.


> If it's too hard to come up with a gtest, then go ahead with your proposed change.

It turned out not to be *too* difficult to add a gtest.  New webrevs:

full: http://cr.openjdk.java.net/~kbarrett/8221102/open.01/
incr: http://cr.openjdk.java.net/~kbarrett/8221102/open.01.inc/