RFR: 8308614: Enabling JVMTI ClassLoad event slows down vthread creation by factor 10 [v3]

[Daniel D. Daugherty]

On Sat, 18 Nov 2023 12:22:10 GMT, Serguei Spitsyn <sspitsyn at openjdk.org> wrote:

>> src/hotspot/share/prims/jvmtiEventController.cpp line 374:
>> 
>>> 372:   // Protects any existing JavaThread's from being terminated while it is set.
>>> 373:   // The FJP carrier thread compensating mechanism can create carrier threads concurrently.
>>> 374:   ThreadsListHandle tlh(current);
>> 
>> A ThreadsListHandle does not prevent a JavaThread from being terminated. It
>> prevents a JavaThread from exiting and being freed. The JavaThread is able to
>> set the terminated state on itself, but will not be able to complete exiting while
>> it is on a ThreadsListHandle. There is a subtle difference.
>> 
>> There's a `target` JavaThread that is fetched from a `JvmtiThreadState` object
>> and that `target` JavaThread is only protected by this `tlh` if `target` is included
>> in the ThreadsList that was captured by this `tlh`. In all likelihood, there should be
>> a ThreadsListHandle farther up the stack that's protecting the JavaThread from
>> which the `JvmtiThreadState` object was extracted and passed to this function.
>> 
>> As for carrier threads, if they are created _after_ this `tlh` was created, then this
>> `tlh` cannot protect them because they won't be on this `tlh`'s ThreadsList.
>
> Thank you for the comment, Dan!
> Agreed, the comment needs to be corrected in two aspects.
> I tried to simplify it but failed to do it correctly.
> It is interesting that there is a `ThreadsListHandle` farther up the stack but it does not help sometimes.
> It is observed that a `JavaThread` (of a carrier thread) referenced from the  `JvmtiThreadState` object can be just created, so we need a `ThreadsListHandle` to avoid possible asserts. With this fix in place I do not see the asserts fired anymore.

@sspitsyn - Please point me at the code where these JavaThreads are newly created?

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PR Review Comment: https://git.openjdk.org/jdk/pull/16686#discussion_r1398317396