RFR: 8366659: ObjectMonitor::wait() liveness problem with a suspension request [v27]

[David Holmes]

On Wed, 21 Jan 2026 13:26:33 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 incrementally with one additional commit since the last revision:
> 
>   8366659: Addressed reviewers' comments.

There are still issues with `was_notified` and I don't think the possibility of the event enabling changing has been fully handled.

src/hotspot/share/runtime/objectMonitor.cpp line 1907:

> 1905:     // then we'll acquire the lock and then re-fetch a fresh TState value.
> 1906:     // That is, we fail toward safety.
> 1907:     was_notified = true;

You can't just go back to this as it is wrong. Your are now marking an initial interrupt case as "was_notified"

src/hotspot/share/runtime/objectMonitor.cpp line 2066:

> 2064:       }
> 2065:     } else {
> 2066:       if (!JvmtiExport::should_post_monitor_waited()) {

What if this has changed value since we checked it above?

-------------

Changes requested by dholmes (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/27040#pullrequestreview-3690267548
PR Review Comment: https://git.openjdk.org/jdk/pull/27040#discussion_r2715060072
PR Review Comment: https://git.openjdk.org/jdk/pull/27040#discussion_r2715066067