RFR: 8261268: LOAD_INSTANCE placeholders unneeded for parallelCapable class loaders

[Coleen Phillimore]

On Tue, 9 Feb 2021 12:30:07 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> src/hotspot/share/classfile/systemDictionary.cpp line 594:
>> 
>>> 592:   // and has not yet finished.
>>> 593:   // In both cases the original caller will clean up the placeholder
>>> 594:   // entry on error.
>> 
>> Is this commentary really no longer applicable? While I don't fully understand it it does indicate the general conditions under which we will call handle_parallel_super_load.
>
> I'm in progress of investigating why this code exists.   The last two lines were false, and these lines seem to repeat the comments above handle_parallel_super_load.  Pointing out java.lang.instrument is even more befuddling.  There may have been a case once where two threads tried to load the same class in RedefineClasses.  I'm not sure why the LOAD_INSTANCE wouldn't have stopped them, just like it stops all classes from being duplicate loaded.  In any case, today RedefineClasses now locks loading the same class with the RedefineClasses_lock.

The second sentence of this comment seems correct but is repeated above handle_parallel_super_load. I was going to restore some of this but it doesn't add any information that isn't in the comment above this function.

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PR: https://git.openjdk.java.net/jdk/pull/2469