RFR 8231293: Shenandoah: Traversal fails JVMTI tests

[Roman Kennke]

Why is that a problem?

As far as I know, we don't concurrently clean native roots with 
traversal GC. Or do we?

Roman

> The problem is caused by LRB trying to evacuate dead oops.
> At this point, traversal is completed, therefore, no more evacuation 
> should happen. We should complete the marking bitmap and arm native 
> barrier to prevent from-space oops from leaking to system.
> 
> There are duplicated works among fixup roots and parallel cleaning, 
> let's address them in DKJDK-8231324.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8231293
> Webrev: http://cr.openjdk.java.net/~zgu/JDK-8231293/webrev.00/index.html
> 
> Test:
>    hotspot_gc_shenandoah (fastdebug and release)
>    vmTestbase/nsk/jvmti (fastdebug and release) normal and traversal
>    modes.
> 
> Thanks,
> 
> -Zhengyu
>