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

[Kim Barrett]

> On Mar 26, 2019, at 6:12 AM, Thomas Schatzl <thomas.schatzl at oracle.com> wrote:
> 
> Hi,
> 
> On Thu, 2019-03-21 at 18:25 -0400, Kim Barrett wrote:
>> Please review this change to the thread claiming protocol for
>> parallel iteration over threads. Previously, the protocol used a 1bit
>> claim counter plus zero as a "never claimed" marker. The new protocol
>> uses a uintx counter, while retaining zero as a "never claimed"
>> marker.
>> 
>> 
> [...]
>> We address this by expanding the claim counter to a uintx-sized
>> counter. There is still a theoretical possibility of collision, which
>> 
> 
> [...]
>> 
>> CR:
>> https://bugs.openjdk.java.net/browse/JDK-8221102
>> 
>> Webrev:
>> http://cr.openjdk.java.net/~kbarrett/8221102/open.00/
>> 
>> Testing:
>> mach5 tier1-5.
>> 
> 
> looks good. One minor style nit: I would prefer that in thread.cpp:
> Threads::change_hread_claim_token():4521 the thread_claim_token would
> be set to a particular value (i.e. 1) directly instead of incremented
> from its current original value. This makes that assignment context-
> free. But I am fine with keeping it as is.
> 
> Thanks,
>  Thomas

I’ll make that change.  Thanks for the review.