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

[Coleen Phillimore]

On Fri, 4 Nov 2022 02:07:25 GMT, David Holmes <dholmes 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.
>
> src/hotspot/share/classfile/placeholders.cpp line 137:
> 
>> 135:   assert(action != PlaceholderTable::LOAD_INSTANCE || seen == NULL,
>> 136:          "Only one LOAD_INSTANCE allowed at a time");
>> 137: 
> 
> Unclear why this is removed? If disabling the new flag causes this fail then shouldn't the new flag form part of the condition?

If you want, it seemed unnecessary clutter for the assert.

> src/hotspot/share/classfile/systemDictionary.cpp line 597:
> 
>> 595:           double_lock_wait(current, lockObject);
>> 596:         } else {
>> 597:           return NULL;
> 
> Not clear why we return NULL here rather than just falling through and retrying?

If we don't return we'll actually do a wait and will hang.  Worse, we'll be in a tight loop holding both the ClassLoader and SystemDictionary_lock, that the other thread can't get.

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