RFR: 8344049: Shenandoah: Eliminate init-update-refs safepoint [v4]

[William Kemper]

On Tue, 14 Jan 2025 20:12:31 GMT, Y. Srinivas Ramakrishna <ysr at openjdk.org> wrote:

>> William Kemper has updated the pull request incrementally with two additional commits since the last revision:
>> 
>>  - Encapsulate and document a method for making concurrent gc_state changes
>>  - Control thread doesn't need a gc lab, also make gclabs for safepoint workers parsable
>
> src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp line 1268:
> 
>> 1266: 
>> 1267:   // The handshake won't touch non-java threads, so do those separately.
>> 1268:   Threads::non_java_threads_do(&prepare_for_update_refs);
> 
> Which non-Java threads need to prepare for update refs? (i.e. which make use of this state predicate and/or participate in update refs phase.)
> 
> Would be good to document that somewhere (or may be repeat it here).

I will change the comment here. Specifically, we are most interested in changing the gc state for the worker threads, but also the control thread and vm thread and any other thread that might touch an oop.

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PR Review Comment: https://git.openjdk.org/jdk/pull/22688#discussion_r1915606414