RFR: 8322853: Should use ConditionalMutexLocker in NativeHeapTrimmerThread::print_state

[Denghui Dong]

Hi all,

Please help review this small fix.

MutexLocker requires that mutex != nullptr, so we should use ConditionalMutexLocker instead.

Also, this patch made '_stop' volatile, since it will be read twice in run() but may be changed.

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Commit messages:
 - remove unused method is_stopped
 - only make _stop volatile
 - make some fields volatile
 - 8322853: Should use ConditionalMutexLocker in NativeHeapTrimmerThread::print_state

Changes: https://git.openjdk.org/jdk/pull/17226/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17226&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8322853
  Stats: 10 lines in 1 file changed: 1 ins; 7 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/17226.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17226/head:pull/17226

PR: https://git.openjdk.org/jdk/pull/17226