RFR: 8298853: JvmtiVTMSTransitionDisabler should support disabling one virtual thread transitions

[Serguei Spitsyn]

On Fri, 16 Dec 2022 02:16:36 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> Now the `JvmtiVTMSTransitionDisabler` mechanism supports disabling VTMS transitions for all virtual threads only. It should also support disabling transitions for any specific virtual thread as well. This will improve scalability of the JVMTI functions operating on target virtual threads as the functions can be executed concurrently without blocking each other execution when target virtual threads are different.
>> New constructor `JvmtiVTMSTransitionDisabler(jthread vthread)` is added which has jthread parameter of the target virtual thread.
>> 
>> Testing:
>>   mach5 jobs are TBD (preliminary testing was completed):
>>    - all JVMTI, JDWP, JDI and JDB tests have to be run
>>    - Kitchensink
>>    - tier5
>
> src/hotspot/share/prims/jvmtiThreadState.cpp line 304:
> 
>> 302: 
>> 303:   ThreadBlockInVM tbivm(thread);
>> 304:   MonitorLocker ml(JvmtiVTMSTransition_lock, Mutex::_no_safepoint_check_flag);
> 
> Aside: this pattern looks very odd. Why not just lock with the safepoint check?

@dholmes-ora
Thank you for looking at this fix and for the comments.
I will on replying and resolving them.

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