RFR: 8364343: Virtual Thread transition management needs to be independent of JVM TI [v3]

[Patricio Chilano Mateo]

On Sat, 22 Nov 2025 08:37:39 GMT, Serguei Spitsyn <sspitsyn at openjdk.org> wrote:

>> src/hotspot/share/runtime/mountUnmountDisabler.cpp line 147:
>> 
>>> 145:       MonitorLocker ml(VTMSTransition_lock);
>>> 146:       while (is_start_transition_disabled(current, vth())) {
>>> 147:         ml.wait(200);
>> 
>> I see a lot of timed-waits throughout this code. Is that because we poll rather than synchronizing properly? All this potential busy-waiting is surely going to cause performance glitches.
>
> The timeouts are for reliability purposes only. Technically, they are not needed and can be removed after this code becomes stable. The `wait()` calls are inside while loop which rechecks the loop-ending conditions.

I tried to minimize the changes with respect to the current code so I kept the timed-waits. As Serguei points out we should be able to remove this particular one. As for the ones executed by the disablers, we could make them poll for the transition bits in a loop with backoff, similar to how we do it in safepoint and handshake cases. But I agree with Serguei we should do it in a separate bug once the code is stable.

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PR Review Comment: https://git.openjdk.org/jdk/pull/28361#discussion_r2561204423