RFR: 8315373: Change VirtualThread to unmount after freezing, re-mount before thawing

[Alan Bateman]

On Fri, 1 Sep 2023 10:47:25 GMT, Markus Grönlund <mgronlun at openjdk.org> wrote:

>> Just to add that  Patricio suggested today to run the stress tests with -Xint and that does lead to triggering the assert quickly when the thread is sampled in native. There are several native methods that are @IntrinsicCandidate that are invoked after the thread identity has changed to the virtual thread and before the continuation is mounted. Probably less likely on the unmount as the only native method is the one that reverts the thread identity.
>
> Ok. If the thread can run native code as part of the mount / unmount, and the sampler can see the intermediary state there too, the check is needed.

Thanks. One other thing that I see when more testing with generational ZGC is that ZPageAllocator::alloc_page will record an event and this can happen during a mount transition. So I think JfrStackTrace::record also needs to test if the current thread is a virtual thread without a continuation mounted.

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PR Review Comment: https://git.openjdk.org/jdk/pull/15492#discussion_r1312910082