RFR: 8339725: Concurrent GC crashed due to GetMethodDeclaringClass

[Liang Mao]

Hi,

It's a fix for 8339725. I think getting the oop from Klass::java_mirror() should use a ON_PHANTOM_OOP_REF decorator here which could make sure the oop would be kept alive in concurrent marking and return nullptr while in concurrent reference processing and unloading.

test/hotspot/jtreg/runtime and gc are clean.

Thanks,
Liang

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Commit messages:
 - 8339725: Concurrent GC crashed due to GetMethodDeclaringClass

Changes: https://git.openjdk.org/jdk/pull/20907/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20907&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339725
  Stats: 5 lines in 3 files changed: 3 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/20907.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20907/head:pull/20907

PR: https://git.openjdk.org/jdk/pull/20907