RFR: 8316397: StackTrace/Suspended/GetStackTraceSuspendedStressTest.java failed with: SingleStep event is NOT expected [v4]

[David Holmes]

On Tue, 4 Mar 2025 00:46:47 GMT, Patricio Chilano Mateo <pchilanomate at openjdk.org> wrote:

>> For use in ` JvmtiExport::at_single_stepping_point`.
>
>> Do we still have a "suspension disabler" that would prevent the suspend from being processed by the mutex blocking?
>>
> We have `JavaThread::_is_disable_suspend` for JVMTI, but that only makes the target ignore the `ThreadSelfSuspensionHandshake` (second part of the suspend mechanism), it doesn't prevent the `SuspendThreadHandshake` (first part) from being executed. The suspender considers the target as suspended after the first handshake is done. (Seems this `_is_disable_suspend` should have prevented the first part instead but I'm guessing it wasn't as straightforward without introducing deadlocks). But also note we have the same issue for other events so we need a general solution.

Let me see if I have the sequence of events correct:
- we have a thread leaving Java to enter the VM, in this case to post a single-step event, which is not suspended
- when trying to acquire the JvmtiThreadState_lock the thread blocks and becomes handshake/safepoint-safe and is then suspended
- the thread acquires the lock and goes to post the event but because it has now been suspended it is an error to do so

So what we need is a way to disable suspension so that the transient/temporary switch to _thread_blocked does not allow a suspend request to be processed. I think that is preferable to trying to find where to add a place to check for suspension and actually suspend it.

But how is any of this new?

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

PR Review Comment: https://git.openjdk.org/jdk/pull/23490#discussion_r1978474572