RFR: 8261504: Shenandoah: reconsider ShenandoahJavaThreadsIterator::claim memory ordering

[Zhengyu Gu]

On Wed, 10 Feb 2021 12:00:38 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> JDK-8256298 added the thread iterator for thread roots, and I don't think we need the Hotspot's default memory_order_conservative, which emits two-way memory fences around the CASes at least on AArch64 and PPC64. The simple "relaxed" should do.
> 
> Additional testing:
>  - [x] Linux x86_64 hotspot_gc_shenandoah
>  - [x] Linux AArch64 hotspot_gc_shenandoah
>  - [x] Linux AArch64 tier1 with Shenandoah

Looks good.

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Marked as reviewed by zgu (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/2506