[sh/jdk8u] RFR [backport] 8232747: Shenandoah: Concurrent GC should deactivate SATB before processing weak roots

[Roman Kennke]

>>> This is *not* straight forward backport, as sh/jdk8u has yet adopted 
>>> parallel_cleaning() from upstream, which is too risky to backport, IMHO.
>>>
>>> The key point of this patch is that, we should deactivate SATB before 
>>> processing weak roots.
>>>
>>> Original bug: https://bugs.openjdk.java.net/browse/JDK-8232747
>>> Original patch: https://hg.openjdk.java.net/jdk/jdk/rev/ef2b75750838
>>>
>>> sh/jdk8u webrev: 
>>> http://cr.openjdk.java.net/~zgu/shenandoah/JDK-8232747-8u/webrev.00/
>>>
>>> Test:
>>>    hotspot_gc_shenandoah Linux x86_64 and x86_32
>>>
>>
>> Don't we also need:
>>
>> +      set_has_forwarded_objects(false);
>>
> I am assuming you are referring to the one in op_final_mark()? we don't 
> have that branch anymore. I believe that branch is for dealing with 
> piggyback ref update cycle.

Ah ok. Good to push it, then!

Thanks,
Roman