RFR: 8274196: Crashes in VM_HeapDumper::work after JDK-8252842 [v3]

[Lin Zang]

> 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.

Lin Zang has updated the pull request incrementally with one additional commit since the last revision:

  Add missing read barrier

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5681/files
  - new: https://git.openjdk.java.net/jdk/pull/5681/files/91f1c349..c1658e19

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5681&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5681&range=01-02

  Stats: 34 lines in 1 file changed: 16 ins; 16 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5681.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5681/head:pull/5681

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