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

Per Liden per.liden at oracle.com
Wed Aug 14 08:27:12 UTC 2019

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.


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

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