RFR: 8229451: ZGC: Make some roots invisible to the heap iterator

[Per Liden]

Thanks Erik!

/Per

On 8/22/19 10:03 AM, Erik Österlund wrote:
> Hi Per,
> 
> Looks reasonable.
> 
> /Erik
> 
> On 2019-08-21 15:43, Per Liden wrote:
>> Returned to this patch and did some more testing, and realized that 
>> the UnhandledOops checker will complain about the raw oop. Adjusted 
>> the code to avoid that.
>>
>> Diff: http://cr.openjdk.java.net/~pliden/8229451/webrev.1-diff
>> Full: http://cr.openjdk.java.net/~pliden/8229451/webrev.1
>>
>> Testing: Passed tier1-3
>>
>> cheers,
>> Per
>>
>> On 8/14/19 10:27 AM, Per Liden wrote:
>>> Thanks Erik!
>>>
>>> I agree that another path in ZRootIterator is unfortunate, but the 
>>> alternatives I've managed to come up with tend to be worse.
>>>
>>> /Per
>>>
>>> On 8/14/19 9:57 AM, Erik Osterlund wrote:
>>>> Hi Per,
>>>>
>>>> Unfortunate with another special path in the root iterator. But 
>>>> alternatives also look bad. Looks good.
>>>>
>>>> Thanks,
>>>> /Erik
>>>>
>>>>> On 13 Aug 2019, at 10:42, Per Liden <per.liden at oracle.com> wrote:
>>>>>
>>>>> JDK-8227226 can temporarily create long[] objects on the heap, 
>>>>> which later become oop arrays, when the array initialization has 
>>>>> been completed. This is fine from a sampling/reporting point of 
>>>>> view (the things done in the MemAllocator::Allocation destructor), 
>>>>> since that only happens after the final klass pointer has been 
>>>>> installed. However, if a heap iteration (via ZHeapIterator) happens 
>>>>> before the final klass pointer has been installed, it will then see 
>>>>> the long[]. As far as I can tell, this isn't a big deal, unless 
>>>>> that heap iteration is out to JVMTI-tag all long[] instances. In 
>>>>> that case, we tag a long[] which will later become an oop array, 
>>>>> which seems wrong and potentially problematic. To avoid this, we 
>>>>> want to be able to hide these roots from the heap iterator until 
>>>>> the final klass pointer has been installed.
>>>>>
>>>>> The approach here is that these temporary long[] objects are not 
>>>>> kept alive in a Handle, but instead treated as a special root in 
>>>>> ZThreadLocalData, that can optionally be made invisible to the 
>>>>> ZRootsIterator.
>>>>>
>>>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8229451
>>>>> Webrev: http://cr.openjdk.java.net/~pliden/8229451/webrev.0
>>>>>
>>>>> /Per
>>>>
>