RFR: 8366659: ObjectMonitor::wait() liveness problem with a suspension request [v23]

[Anton Artemov]

> 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: Modified the comment.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/27040/files
  - new: https://git.openjdk.org/jdk/pull/27040/files/95cf01f6..949db95e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27040&range=22
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27040&range=21-22

  Stats: 16 lines in 1 file changed: 10 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/27040.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27040/head:pull/27040

PR: https://git.openjdk.org/jdk/pull/27040