RFR: 8275730: Relax memory constraint on MultiThreadedRefCounter

[Zhengyu Gu]

On Wed, 3 Nov 2021 03:36:46 GMT, Yumin Qi <minqi at openjdk.org> wrote:

>> It is just an atomic counter, only needs atomic guarantee.
>
> add (sub) itself also with a parameter of atomic_memory_order, but this change is more concise.
> LGTM.

Thanks, @yminqi

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PR: https://git.openjdk.java.net/jdk/pull/6069