RFR: 8366659: ObjectMonitor::wait() liveness problem with a suspension request [v20]

[Daniel D. Daugherty]

On Fri, 9 Jan 2026 12:05:37 GMT, Anton Artemov <aartemov at openjdk.org> wrote:

>> Hi, please consider the following changes:
>> 
>> If suspension is allowed when a thread is re-entering an object monitor (OM), then a following liveness issues can happen in the `ObjectMonitor::wait()` method.
>> 
>> The waiting thread is made to be a successor and is unparked. Upon a suspension request, the thread will suspend itself whilst clearing the successor. The OM will be left unlocked (not grabbed by any thread), while the other threads are parked until a thread grabs the OM and the exits it. The suspended thread is on the entry-list and can be selected as a successor again. None of other threads can be woken up to grab the OM until the suspended thread has been resumed and successfully releases the OM.
>> 
>> This can happen in three places where the successor could be suspended: 
>> 
>> 1:
>> https://github.com/openjdk/jdk/blob/6322aaba63b235cb6c73d23a932210af318404ec/src/hotspot/share/runtime/objectMonitor.cpp#L1897
>> 
>> 2:
>> https://github.com/openjdk/jdk/blob/6322aaba63b235cb6c73d23a932210af318404ec/src/hotspot/share/runtime/objectMonitor.cpp#L1149
>> 
>> 3:
>> https://github.com/openjdk/jdk/blob/6322aaba63b235cb6c73d23a932210af318404ec/src/hotspot/share/runtime/objectMonitor.cpp#L1951
>> 
>> The issues are addressed by not allowing suspension in case 1, and by handling the suspension request at a later stage, after the thread has grabbed the OM in `reenter_internal()` in case 2. In case of a suspension request, the thread exits the OM and enters it again once resumed. 
>> 
>> Case 3 is handled by not transferring a thread to the `entry_list` in `notify_internal()` in case the corresponding JVMTI event is allowed. Instead, a tread is unparked and let run. Since it is not on the `entry_list`, it will not be chosen as a successor and it is no harm to suspend it if needed when posting the event. 
>> 
>> Possible issue of posting a `waited` event while still be suspended is addressed by adding a suspension check just before the posting of event.
>> 
>> Tests are added.
>> 
>> Tested in tiers 1 - 7.
>
> Anton Artemov has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 44 commits:
> 
>  - 8366659: Fixed year in the copyright.
>  - Merge remote-tracking branch 'origin/master' into JDK-8366659-OM-wait-suspend-deadlock
>  - 8366659: Removed ClearSuccOnSuspend
>  - 8366659: Fixed liveness problem.
>  - Merge remote-tracking branch 'origin/master' into JDK-8366659-OM-wait-suspend-deadlock
>  - 8366659: Fixed build problem.
>  - 8366659: Fixed build issue.
>  - 8366659: Changed the way how notify_internal works if JVMTI monitor waited event allowed.
>  - Merge remote-tracking branch 'origin/master' into JDK-8366659-OM-wait-suspend-deadlock
>  - 8366659: Fixed semi-broken tests
>  - ... and 34 more: https://git.openjdk.org/jdk/compare/a01283a5...21b83214

See the original issue from JDK1.3.1 here:
[JDK-4413752](https://bugs.openjdk.org/browse/JDK-4413752) Linux: suspended thread blocks raw ObjectMonitor entry

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PR Comment: https://git.openjdk.org/jdk/pull/27040#issuecomment-3762029149