RFR: 8358645: Access violation in ThreadsSMRSupport::print_info_on during thread dump

[David Holmes]

We started observing these occasional crashes involving the JfrSamplerThread (which is a NonJavaThread) during JFR's termination thread dump. Examination of the `print_info_on` code showed that it was making an invalid assumption that it is safe to walk the thread's thread-list at a safepoint, as that is not true when you are dealing with a NonJavaThread, as it is not held at the safepoint and so the list you are walking can disappear whilst you examine it. I added some test code to make this more likely and was able to trigger similar crashes - see JBS for details.

The simple fix is to only walk JavaThreads.

Testing
 - tier 5 JFR tests
 - selected tests with JFR explicitly enabled
 - tier 1-3 (sanity)

Thanks

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Commit messages:
 - 8358645: Access violation in ThreadsSMRSupport::print_info_on during thread dump

Changes: https://git.openjdk.org/jdk/pull/25963/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25963&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8358645
  Stats: 4 lines in 1 file changed: 1 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/25963.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25963/head:pull/25963

PR: https://git.openjdk.org/jdk/pull/25963