RFR: 8328665: serviceability/jvmti/vthread/PopFrameTest failed with a timeout [v3]

[Serguei Spitsyn]

On Wed, 27 Mar 2024 05:12:53 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

>> Serguei Spitsyn has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:
>> 
>>  - “Merge”
>>  - review: updated test with one more call to notifyAtBreakpoint to reset the native state
>>  - 8328665: serviceability/jvmti/vthread/PopFrameTest failed with a timeout
>
> test/hotspot/jtreg/serviceability/jvmti/vthread/PopFrameTest/PopFrameTest.java line 147:
> 
>> 145:             }
>> 146:             log("Main #B.2: got expected JVMTI_ERROR_NONE");
>> 147:             notifyAtBreakpoint(); // needed to reset the native state
> 
> This doesn't make much sense to me.  The purpose of `notifyAtBreakpoint()` is to unblock the Breakpoint event handler, which is waiting for this notify. However, the event handler can only ever be entered once, yet we have a previous `noitfyBreakpoint()` above and a 3rd `notifyAtBreakpoint()` below. It seems that this call and the one below are no-ops. I don't see how bp_sync_reached ever gets set true again after the first `notifyAtBreakpoint()` call is made.

Thank you for the comment, Chris. You are right.
I've already figured my mistake/confusion (shared it on our team meeting) but tested my update which has been just pushed. My analysis in the bug report is incorrect, I'll update it soon. In fact, I don't really know what was the root cause of this deadlock because I can't reproduce the issue. Also, it is not easy to figure out by the code observation. The only idea I have is that it was a spurious wakeup on the `wait()` at line 52 in the Breakpoint handler of the native agent. I've added a couple of checks for robustness and added more tracing to get more details if this happen again.

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

PR Review Comment: https://git.openjdk.org/jdk/pull/18419#discussion_r1540566744