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List:       openjdk-serviceability-dev
Subject:    Re: RFR: 8293613: need to properly handle and hide tmp VTMS transitions [v4]
From:       Serguei Spitsyn <sspitsyn () openjdk ! org>
Date:       2022-09-28 4:45:22
Message-ID: jM-CSK5y4gjxEV_Vf5ne4QXvEGVO09cfFnBcG_AEjuY=.bcf4b36e-3db9-4432-b57f-984aee5c5161 () github ! com
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On Thu, 22 Sep 2022 09:16:28 GMT, Serguei Spitsyn <sspitsyn@openjdk.org> wrote:

> > There are several places in VirtualThread class implementation where virtual \
> > threads are being mounted or unmounted, so there is a transition of the \
> > JavaThread identity from carrier thread to virtual thread and back. The execution \
> > state in such transitions is inconsistent, and so, has to be invisible to JVM TI \
> > agents. Such transitions are named as VTMS (virtual thread mount state) \
> > transitions, and there is a JVM TI mechanism which supports them. Besides normal \
> > VTMS transitions there are also a couple of performance-critical contexts (in \
> > `VirtualThread` methods: `scheduleUnpark()`, `cancel()` and `unpark()`) which can \
> > be named as temporary VTMS transitions. Execution state of such temporary VTMS \
> > transitions has to be also invisible to the JVM TI agent which is implemented in \
> > the current update. 
> > There are a couple of details of this fix to highlight:
> > -  A JavaThread which is in temporary VTMS transition is marked with a special \
> > bit `_is_in_tmp_VTMS_transition`. It is done with the native notification method \
> >                 `toggleJvmtiTmpVTMSTrans()`.
> > - A couple of lambda form classes can be created in context of temporary VTMS \
> >                 transitions, so their `ClassLoad`, `ClassPrepare` and `CFLH` \
> >                 events are ignored.
> > - Suspending threads in temporary transition is allowed.
> > 
> > The fix was tested in Loom repository by using `JTREG_MAIN_WRAPPER=Virtual` mode \
> > of execution which forces all main threads to be run as virtual. All `JVM TI` and \
> > `JDI` tests were run on all platforms. It includes `Kitchensink` and `JCK` tests. \
> > Additionally, the fix was tested in the jdk 20 repository. It includes `JVM TI` \
> > and `JDI` tests and tiers 1-6.
> 
> Serguei Spitsyn has updated the pull request incrementally with one additional \
> commit since the last revision: 
> 1. addressed review comments from Chris; added VirtualThread.java update from Alan

Thank you for review, Leonid!

-------------

PR: https://git.openjdk.org/jdk/pull/10321


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