RFR: 8261268: LOAD_INSTANCE placeholders unneeded for parallelCapable class loaders

[Coleen Phillimore]

On Tue, 9 Feb 2021 12:27:12 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> src/hotspot/share/classfile/placeholders.cpp line 107:
>> 
>>> 105: // Doubly-linked list of Threads per action for class/classloader pair
>>> 106: // Class circularity support: links in thread before loading superclass
>>> 107: // bootstrap loader support:  links in a thread before load_instance_class
>> 
>> I don't think the loader is what the comment is referring to. The reference to bootstrapsearchpath relates to this comment in SD::resolve_instance_class_or_null:
>>     // add placeholder entry to record loading instance class
>>     // Five cases:
>>     // All cases need to prevent modifying bootclasssearchpath
>>     // in parallel with a classload of same classname
>
> I don't see any code that does this though.  The code used to compute_loader_lock_object() and return NULL in JVMTI where it was modifying the bootclass search path.  I wonder if that code should have a wait on the placeholder.  I'll investigate.

ClassLoader::load_class reads the bootstrapsearchpath lock free.  If it's modified after it's read, then the new entry isn't found.  I don't think there's a requirement for locking.

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