RFR: 8350649: Class unloading accesses/resurrects dead Java mirror after JDK-8346567

[Aleksey Shipilev]

On Tue, 25 Feb 2025 13:00:52 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> See bug for description of the bug. Shenandoah seems to be the only GC that runs into this problem so far.
> 
> Before [JDK-8346567](https://bugs.openjdk.org/browse/JDK-8346567), we pulled class modifiers from the native `Klass*`, and so we bypassed this trouble. But now we take modifiers out of Java mirror, and this happens during unloading, which accesses/resurrects potentially dead mirror.
> 
> I think the solution is to keep storing a cached modifiers field in `Klass` instead of relying on Java mirror being accessible. Unfortunately, this patch undoes the removal of `u2` field from `Klass` done in [JDK-8346567](https://bugs.openjdk.org/browse/JDK-8346567).
> 
> Additional testing:
>  - [x] Linux x86_64 server fastdebug, original reproducer now passes
>  - [x] Linux x86_64 server fastdebug, `hotspot_gc_shenandoah`
>  - [x] Linux x86_64 server fastdebug, `jdk_jfr`
>  - [x] Linux x86_64 server fastdebug, `jdk_jfr` with `-XX:+UseShenandoahGC` now passes

So I think JFR can just call `compute_modifier_flags()` directly, without relying on Java mirror. I added the blurb around the method to point out it is safer to do from unloading paths. See new version.

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PR Comment: https://git.openjdk.org/jdk/pull/23775#issuecomment-2683162828