On Wed, 21 Jan 2026 13:26:33 GMT, Anton Artemov <[email protected]> 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. src/hotspot/share/runtime/objectMonitor.cpp line 1933: > 1931: // Post monitor waited event. Note that this is past-tense, we are > done waiting. > 1932: // An event could have been enabled after notification, need to > check the state. > 1933: if (JvmtiExport::should_post_monitor_waited() && node.TState != > ObjectWaiter::TS_ENTER) { I don't think this is correct - if the state is TS_ENTER then where do we post the event? I think this should be: if (JvmtiExport::should_post_monitor_waited()) { if (node.TState != ObjectWaiter::TS_ENTER) { // Process suspend requests now if any, before posting the event. ThreadBlockInVM tbvm(current, true); } JvmtiExport::post_monitor_waited(current, this, ret == OS_TIMEOUT); } ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/27040#discussion_r2715266992
