[jdk21u] Withdrawn: 8319137: release _object in ObjectMonitor dtor to avoid races

[Aleksey Shipilev]

On Fri, 24 Nov 2023 12:03:48 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> This effectively reverts [JDK-8256302](https://bugs.openjdk.org/browse/JDK-8256302), which introduced race condition in JDK 21. The patch does not apply cleanly for two reasons:
>  1. Deleted `release_object()` has a different form in JDK 21 due to slight WeakHandle API change. This is not relevant for this patch, just a backporting conflict. I removed `release_object()` by hand.
>  2. Without [JDK-8319896](https://bugs.openjdk.org/browse/JDK-8319896) we cannot do `JavaThread::cast` in the incoming changeset, because we can be called by a non-Java thread. I suspect JDK-8319896 would come with its own bugtail, so I would prefer not to backport it now. I have replaced the code back to original pre- JDK 21 form in a separate commit: [Modifications to fit JDK 21](https://github.com/openjdk/jdk21u/commit/17477d8fca6f4e8f9c8ae2732cfb42f0ce887897)
> 
> Additional testing:
>  - [x] MacOS AArch64 server fastdebug, `runtime/Monitor` passes
>  - [x] Linux AArch64 server fastdebug, `tier1 tier2 tier3 tier4`
>  - [x] Linux x86_64 server fastdebug, `tier1 tier2 tier3 tier4`

This pull request has been closed without being integrated.

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PR: https://git.openjdk.org/jdk21u/pull/401