RFR: 8369019: Improve timed-park mechanism in ObjectMonitor for virtual thread support [v2]

[Coleen Phillimore]

On Fri, 3 Oct 2025 15:29:25 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:
> 
>   Fix typos

src/hotspot/share/runtime/objectMonitor.cpp line 1108:

> 1106:   // Note that we can have false positives where timed-park is not necessary.
> 1107:   bool do_timed_parked = has_unmounted_vthreads();
> 1108:   static int MAX_RECHECK_INTERVAL = 1000;

Is this a constant?  This is the same as the enter case, should there be only one?

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PR Review Comment: https://git.openjdk.org/jdk/pull/27597#discussion_r2403055591