RFR: 8298853: JvmtiVTMSTransitionDisabler should support disabling one virtual thread transitions [v5]

[Serguei Spitsyn]

On Wed, 4 Jan 2023 04:54:27 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> Serguei Spitsyn has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   fix race between VTMS_transition_disable_for_one and start_VTMS_transition
>
> src/hotspot/share/prims/jvmtiThreadState.cpp line 482:
> 
>> 480:       _VTMS_transition_disable_for_all_count > 0) {
>> 481:     MonitorLocker ml(JvmtiVTMSTransition_lock, Mutex::_no_safepoint_check_flag);
>> 482:     ml.notify_all();
> 
> Checking the counts outside the lock seems really racy. Maybe it is safe in the original code but now we have two counters it is very hard to ascertain this is correct. Overall I find it very hard to see exactly how these counters get used.

This notification code is just an optimization. All related waiting fragments are with timeouts.
I agree this sync protocol is kind of complicated and also does not scale well. I'm still thinking on how to improve it.

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PR: https://git.openjdk.org/jdk/pull/11690