RFR: 8254029: ObjectMonitor cleanup/minor bug-fix changes extracted from JDK-8253064

[Daniel D.Daugherty]

On Mon, 19 Oct 2020 11:22:21 GMT, Erik Österlund <eosterlund at openjdk.org> wrote:

>> 8254029: ObjectMonitor cleanup/minor bug-fix changes extracted from JDK-8253064
>
> Looks great in general. I only have one question regarding when deflation would not be called by a Java thread (which
> is now checked for).

@fisk - Thanks for the review. Please let me know if I have resolved
your question.

> src/hotspot/share/runtime/synchronizer.cpp line 2366:
> 
>> 2364:   GVars.stw_random = os::random();
>> 2365:
>> 2366:   if (self->is_Java_thread()) {
> 
> When would this not be run by a Java thread?

`ObjectSynchronizer::do_final_audit_and_print_stats()` calls:

> L2958:   ObjectSynchronizer::deflate_idle_monitors();

and do_final_audit_and_print_stats() is called from two places:

- void VM_Exit::doit() - The do_final_audit_and_print_stats() call happens
  when the VMThread is executing the VM_Exit VM-op. That happens as
  a result of a vm_exit() call a.k.a. System.exit().

- VMThread::run() - The do_final_audit_and_print_stats() call happens
  after the VMThread has been terminated and finished processing
  VM-ops. So this is the jni_DestroyJavaVM() path that happens when
  the program falls off the end of main() and that thread does an
  orderly shutdown of the VMThread.

A key piece of this changeset is moving the final audit to be executed
by the VMThread on one of its two exit code paths. The final audit
includes a calls to deflate_idle_monitors() to make sure things are as
clean as possible before doing the audit_and_print_stats() call.

None of this happens if the right logging is not enabled.

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PR: https://git.openjdk.java.net/jdk/pull/641