On Tue, 20 Jan 2026 09:56:09 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: Fixed whitespace. I last reviewed v19 and I've re-reviewed all the changes since then as one diff between v19 and v25. This looks good to me and I've posted just a couple of comments. src/hotspot/share/runtime/objectMonitor.cpp line 1869: > 1867: > 1868: int ret = OS_OK; > 1869: bool was_notified = true; L2021: // Monitor notify has precedence over thread interrupt. With this change to `was_notified` default value, I don't think we can say that monitor notify has precedence over thread interrupt anymore. src/hotspot/share/runtime/objectMonitor.cpp line 1933: > 1931: // Post monitor waited event. Note that this is past-tense, we are > done waiting. > 1932: // A thread which should post monitor_waited event is never in > TS_ENTER state. > 1933: if (JvmtiExport::should_post_monitor_waited()) { Is it worth asserting that `node.TState != ObjectWaiter::TS_ENTER` inside the if-statement? ------------- Marked as reviewed by dcubed (Reviewer). PR Review: https://git.openjdk.org/jdk/pull/27040#pullrequestreview-3683757507 PR Review Comment: https://git.openjdk.org/jdk/pull/27040#discussion_r2709683636 PR Review Comment: https://git.openjdk.org/jdk/pull/27040#discussion_r2709638312
