RFR: 8369019: Improve timed-park mechanism in ObjectMonitor for virtual thread support [v4]

[David Holmes]

On Mon, 6 Oct 2025 22:34:29 GMT, Patricio Chilano Mateo <pchilanomate at openjdk.org> wrote:

>> Please review the following fix. When blocking in `ObjectMonitor::enter_internal` we currently use timed-park for pinned virtual threads. This is done to alleviate some potential deadlocks cases where the successor is an unmounted virtual thread that cannot run. In particular this could happen during class loading/initialization if all other carriers are blocked waiting for the same class to be loaded/initialized.
>> This mechanism should be extended to cover `ObjectMonitor::reenter_internal` used in `Object.wait` (notification case). Also, the criteria to decide whether to do a timed-park should be based on whether there are unmounted vthreads already in the `_entry_list`, and not just if this is a pinned virtual thread. This covers mixed usages of the same ObjectMonitor between virtual threads and platform threads. This will become more relevant once we bring the changes currently in the fibers branch to preempt virtual threads during klass initialization.
>> 
>> These changes have been running in the loom pipeline for a couple of months already. I also added a new test case to test/jdk/java/lang/Thread/virtual/MonitorWaitNotify.java which deadlocks without these changes.
>> 
>> Thanks,
>> Patricio
>
> Patricio Chilano Mateo has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Move test to RetryMonitorEnterWhenPinned.java and add timed-variants

src/hotspot/share/runtime/objectMonitor.inline.hpp line 147:

> 145: 
> 146: inline void ObjectMonitor::inc_unmounted_vthreads() {
> 147:   assert(_unmounted_vthreads >= 0, "");

Suggestion:

  assert(_unmounted_vthreads >= 0, "invariant");

Here and below - thanks.

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PR Review Comment: https://git.openjdk.org/jdk/pull/27597#discussion_r2409285515