RFR: 8299240: rank of JvmtiVTMSTransition_lock can be safepoint

[Serguei Spitsyn]

On Sat, 18 Feb 2023 01:30:27 GMT, Serguei Spitsyn <sspitsyn at openjdk.org> wrote:

>> src/hotspot/share/prims/jvmtiEnv.cpp line 952:
>> 
>>> 950:     }
>>> 951:     // protect thread_oop as a safepoint can be reached in disabler destructor
>>> 952:     self_tobj = Handle(current, thread_oop);
>> 
>> If you move this after line 953 then you can delete line 932 and just have:
>> 
>> Handle self_tobj = Handle(current, thread_oop);
>
> Thank you for looking at this PR, David!
> Please, note a disabler at L938. A safepont can be reached in its destructor.
> Also, see the comment at L952:
> 
>  937   {
>  938     JvmtiVTMSTransitionDisabler disabler(true);
>  939     ThreadsListHandle tlh(current);
>  940 
>  941     err = get_threadOop_and_JavaThread(tlh.list(), thread, &java_thread, &thread_oop);
>  942     if (err != JVMTI_ERROR_NONE) {
>  943       return err;
>  944     }
>  945 
>  946     // Do not use JvmtiVTMSTransitionDisabler in context of self suspend to avoid deadlocks.
>  947     if (java_thread != current) {
>  948       err = suspend_thread(thread_oop, java_thread, /* single_suspend */ true, nullptr);
>  949       return err;
>  950     }
>  951     // protect thread_oop as a safepoint can be reached in disabler destructor
>  952     self_tobj = Handle(current, thread_oop);
>  953   }

> it means there are now many places that can block for safepoints that previously wouldn't and it is very to hard to tell if that is okay or not.

We partially rely on testing and good test coverage here.

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PR: https://git.openjdk.org/jdk/pull/12550