RFR: 8227175: ZGC: ZHeapIterator visits potentially dead objects

[Erik Österlund]

Hi Stefan,

Looks good.

Thanks,
/Erik

On 2019-07-03 15:31, Stefan Karlsson wrote:
> Hi all,
>
> (Sending this RFR to hotspot-dev since it changes CLD claiming.)
>
> Please review this patch to fix the ZHeapIterator to not visit 
> potentially dead objects.
>
> https://cr.openjdk.java.net/~stefank/8227175/
> https://bugs.openjdk.java.net/browse/JDK-8227175
>
> It changes how all heap iterations are done in ZGC. Previously, the 
> marking code visited only the strong CLDs and traced through metadata 
> to find all other CLDs that should be considered alive. The 
> verification code, serviceability heap iterations, and marking without 
> class unloading, skipped the metadata tracing part and visited all 
> CLDs instead. Now, with this patch, all these heap iterations starts 
> with the strong CLDs and trace through the object graph.
>
> One complication with that scheme is that non-GC heap iterations might 
> be executing after concurrent marking has started, but before dead 
> CLDs have been unlinked. To allow the GC marking code and one, at a 
> time, non-GC heap iteration to run at the same time, I've introduced a 
> new claim bit in the CLD claiming byte. I've called it "other", so now 
> we have "strong", "finalizable", and "other". The contract is that the 
> "other" bits should only be used in a safepoint operation, and must be 
> cleared before the operation ends. This way we get mutual exclusion 
> between different users of the "other" bits.
>
> The patch also adds more precise verification of ZGC references.
>
> This patch was written a few weeks ago to make the verification of ZGC 
> references more precise. I've been using it since then to get better 
> verification of other patches and when hunting for bugs. This means 
> I've been running it through tier 1-7 multiple times. The intent was 
> to get this pushed to JDK 14, but now that we've seen that we have an 
> actual bug because of the imprecise nature of the ZHeapIterator, I'd 
> like to get this patch pushed to JDK 13. We considered trying to split 
> this up into two parts, the first part that fixes the heap iterations 
> and the second part that adds the extra ZGC verification, but we thing 
> that would take longer time and be riskier.
>
> Thanks,
> StefanK