RFR: 8212107: VMThread issues and cleanup [v8]

[Daniel D.Daugherty]

On Sun, 27 Sep 2020 19:11:42 GMT, Robbin Ehn <rehn at openjdk.org> wrote:

>> We simplify the vmThread by removing the queue and refactor the the main loop.
>> This solves the issues listed:
>> - It can create an extra safepoint directly after a safepoint.
>> - It's not safe for a non-JavaThread to add safepoint to queue while GC do oops do.
>> - The exposure of the vm operation is dangerous if it's a handshake.
>> - The code is a hornets nest with the repetition of checks and branches
>> 
>> Passes t1-8, and a benchmark run.
>> 
>> If you want a smaller diff the commits contains the incremental progress and each commit passed t1.
>
> Robbin Ehn has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Removed unused declartion - Claes

I'm okay with leaving SafepointALot as you have it now
and leaving any future cleanup/refinement to a new RFE.

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Marked as reviewed by dcubed (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/228