RFR: 8253778: ShenandoahSafepoint::is_at_shenandoah_safepoint should not access VMThread state from other threads [v2]

[Roman Kennke]

On Wed, 30 Sep 2020 07:47:32 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> There are lots of Shenandoah-related test failures in current runs, triggered by new assert in JDK-8212107. The assert
>> looks legitimate, and Shenandoah code should instead be more relaxed here, by checking it runs in VMThread before
>> accessing its state.  Testing:
>>   - [x] hotspot_gc_shenandoah
>
> Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Revert the not very valid change

Marked as reviewed by rkennke (Reviewer).

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PR: https://git.openjdk.java.net/jdk/pull/424