RFR: Fix ShenandoahPurgeSATBTask [v2]

[Roman Kennke]

On Thu, 18 Mar 2021 16:19:46 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

>> Ugh. Right. Looks good!
>
>> shenandoahConcurrentMark
> 
> Thanks for pointing out. I will fix it in jdk/jdk

Actually, now that I think about it a little more, I believe you should leave the claiming code where it was. It was one closure per task, and that should correctly claim the threads. Now you let all tasks see all the threads, which is wrong. Or I'm missing something.

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PR: https://git.openjdk.java.net/shenandoah/pull/23