RFR: 8274196: Crashes in VM_HeapDumper::work after JDK-8252842

[Coleen Phillimore]

On Fri, 24 Sep 2021 13:13:39 GMT, Lin Zang <lzang at openjdk.org> wrote:

> The root cause for crash in ZGC is that the JNIHandles are processed before object iteration. And ZGC would update the JNIHandles at object iteration with read barrier. So the crash is cause by accessing the invalid address which can be dummy info after zgc, and hence crash.
> 
> The lock rank issue can be fixed because the related mutexes are acquired in safepoint. so the safepoint_check_required could be safepoint_check_always.
> 
> The Epsilon issue is caused by wrong _num_dumper_thread calculated when the gang==NULL.

The lock stuff looks ok, but please have at least one of the original reviewers review the change.

test/hotspot/jtreg/ProblemList-zgc.txt line 49:

> 47: 
> 48: serviceability/dcmd/gc/HeapDumpAllTest.java                   8274196   linux-all,windows-all
> 49: serviceability/dcmd/gc/HeapDumpTest.java                      8274196   linux-all,windows-all

Before you push, you'll need to do a merge from mainline and should un-ProblemList sun/tools/jmap/BasicJMapTest.java.

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Marked as reviewed by coleenp (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/5681