RFR: 8264788: Make SequentialSubTasksDone use-once [v2]

[Thomas Schatzl]

On Thu, 8 Apr 2021 09:02:20 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> Thomas Schatzl has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   ayang review
>
> Looking at how `SequentialSubTasksDone` is used, I don't really see the benefit of having `assert(_num_claimed == _num_tasks)` in the destructor. If we drop that assertion, `Atomic::add` is more readable.
> 
> I prefer `Atomic::add` with the assertion removed, but this version is fine as well.

I changed to the `Atomic::add` but kept the assert (modified).

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PR: https://git.openjdk.java.net/jdk/pull/3372