Integrated: 8369019: Improve timed-park mechanism in ObjectMonitor for virtual thread support

[Patricio Chilano Mateo]

On Wed, 1 Oct 2025 22:13:58 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

This pull request has now been integrated.

Changeset: 9feb8f21
Author:    Patricio Chilano Mateo <pchilanomate at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/9feb8f21b5d000f8901938f1dde89638c79ca805
Stats:     206 lines in 4 files changed: 180 ins; 3 del; 23 mod

8369019: Improve timed-park mechanism in ObjectMonitor for virtual thread support

Reviewed-by: dholmes, alanb

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PR: https://git.openjdk.org/jdk/pull/27597