RFR: 8255527: Shenandoah: Let ShenadoahGCStateResetter disable barriers [v2]

[Zhengyu Gu]

On Wed, 28 Oct 2020 11:14:38 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> JDK-8255036 introduced a regression: the new ShenandoahGCStateResetter does not actually disable barriers: it only records old state (without changing it) and then restores that state (i.e a no-op). 
>> 
>> Testing: hotspot_gc_shenandoah (also on conc-weakref branch, where the bug did manifest)
>
> Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Make heap local variable const for consistency

Marked as reviewed by zgu (Reviewer).

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PR: https://git.openjdk.java.net/jdk/pull/899