RFR: 8295673: Deprecate legacy parallel class loading workaround for non-parallel-capable class loaders

[Mandy Chung]

On Mon, 24 Oct 2022 12:16:54 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

> This change adds an option **EnableWaitForParallelLoad** to enable the legacy behavior where the VM will manage synchronization for multiple threads loading the same class using a non-parallel capable class loader that have released the class loader lock.  The VM will break the class loader lock for parallel threads trying to load the class, and wait for the first thread that initiated loading the class to complete.  The subsequent threads will use the result of the first thread, rather than get a LinkageError: duplicate class definition for loading the class without synchronization.
> Releasing the class loader lock was a common workaround for class loaders that used a non-hierarchical delegation scheme to avoid deadlock, before parallel capable class loading was added. 
> https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/ClassLoader.html#registerAsParallelCapable()
> 
> Tested with tier1-6 and internal applications.

Marked as reviewed by mchung (Reviewer).

Adding core-libs-dev so that more developers know this behavioral change. 

*Background:*

Prior to JDK 7, custom class loaders using non-hierarchical class delegation model were prone to deadlock (see [1] for details).   This workaround was added in the VM in JDK 6 to allow parallel class loading for non-parallel-capable class loaders to avoid deadlocks.  The VM also supports a common workaround at the time that some customers explicitly issue a wait() on the class loader lock.

Such workaround was kept to allow those customers to migrate to using multi-thread-safe parallel-capable class loaders.  It has been 11 years since Java 7 and it's expected that those deadlock-prone custom class loaders should have migrated to the parallel-capable class loaders. 

If customers have class loaders depending on this legacy behavior, they can set `-XX:+EnableWaitForParallelLoad` flag to enable the legacy behavior and migrate to parallel-capable class loaders as soon as possible.  The `EnableWaitForParallelLoad` VM flag will be removed in a future release.

Refers to CSR [JDK-8295848](https://bugs.openjdk.org/browse/JDK-8295848) for details.

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PR: https://git.openjdk.org/jdk/pull/10832