RFR: 8254103: Shenandoah: Move updating thread roots to concurrent phase [v2]

[Roman Kennke]

On Wed, 7 Oct 2020 16:57:23 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

>> Due to non-atomic LRB, there can be from-space oops in thread roots after evacuation.
>> To enforce to-space invariant, Shenandoah updates thread roots during final reference updating pause, but it can be
>> done outside pause via thread handshake.
>> Test:
>> 
>> - [x] hotspot_gc_shenandoah
>
> Zhengyu Gu has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Update

Looks good to me! Thanks!

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Marked as reviewed by rkennke (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/541