RFR: 8253481: Shenandoah: ShenandoahConcurrentRootScanner::oops_do() should always use _claim_strong for CLDClosure

[Zhengyu Gu]

On Wed, 23 Sep 2020 05:45:59 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> _claim_none should only be used for single thread scanning, regardless concurrent or not.
>> 
>>  ShenandoahConcurrentRootScanner declares ShenandoahClassLoaderDataRoots multi-thread capable, so should always use
>>  _claim_strong for scanning CLDG.
>> 
>> This is not a correctness bug. _claim_none results CLDG to be scanned by every worker.
>> 
>> Test:
>>   - [x] hotspot_gc_shenandoah
>
> Please see JDK-8246097 and JDK-8246101 for previous experiments in this area.

Make a dup of JDK-8246101

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PR: https://git.openjdk.java.net/jdk/pull/304