RFR(S) 8223951: Shenandoah: Only need to update thread roots during final update refs

[Aleksey Shipilev]

On 5/17/19 4:17 PM, Roman Kennke wrote:
>>> After switching to strong to-space invariant and pre-evacuating all root, thread roots are only
>>> places that may contain to-space oops after updating references during normal concurrent cycles, we
>>> only need to update them during final updaterefs pause, which should reduce pause time.
>>>
>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8223951
>>> Webrev: http://cr.openjdk.java.net/~zgu/JDK-8223951/webrev.00/
>>
>> Looks fine to me.
>>
>> Aside: I wonder how would be get the thread root with from-space ptr if we don't have safepoints
>> between loads and the LRBs that give us to-space ptrs after potential evac.
> 
> I don't think we cann guarantee that we don't get a safepoint between
> load and LRB.

Ah, that sucks. Oh well! But that is the only plausible way we can get the non-updated thread root,
right?

-Aleksey