RFR(s): 8235912: JvmtiBreakpoint remove oops_do and metadata_do

[Robbin Ehn]

Hi Coleen,

> It's in the ClassLoaderData initialization.  I don't know what it means, you'd 
> have to ask someone who knows.  I don't think it matters though.

It means that if G1 concurrently scans this oop and we over-write the
uninitialized oop, the uninitialized oop would be saved in the SATB-barrier.
The oopStorage contains strong roots which are done in a safeopint.

Thanks, Robbin

> 
> Coleen
>>
>>>
>>> You can leave out using OopHandle.  I have a patch to add the missing 
>>> functionality and add it to your code.   Actually, I was looking to see how 
>>> much OopHandle is used to see if it's helping anything and there is a lot of 
>>> code using it.  Most of it is to hide oop* in ClassLoaderData.
>>>
>>> This change otherwise looks great.
>>
>> Thanks, Robbin
>>
>>> Thanks,
>>> Coleen
>>>
>>>
>>>> Thanks for having a look, Robbin
>>>>
>>>> On 12/16/19 1:32 PM, coleen.phillimore at oracle.com wrote:
>>>>>
>>>>> I have to think about this.   Could there be breakpoints in old emcp 
>>>>> methods that we do not remove?   The metadata_do function is trying to keep 
>>>>> old Methods from being deleted while there are still references to them.
>>>>>
>>>>> http://cr.openjdk.java.net/~rehn/8235912/v1/webrev/src/hotspot/share/prims/jvmtiImpl.hpp.udiff.html 
>>>>>
>>>>>
>>>>> + oop* _class_holder; // keeps _method memory from being deallocated
>>>>>
>>>>>
>>>>> We created the class OopHandle to encapsulate strong oopStorage references, 
>>>>> although it's missing oop_store.  Can you use that?
>>>>
>>>>
>>>>>
>>>>> Coleen
>>>>>
>>>>> On 12/16/19 4:47 AM, Robbin Ehn wrote:
>>>>>> Hi all, please review.
>>>>>>
>>>>>> From issue, https://bugs.openjdk.java.net/browse/JDK-8235912:
>>>>>>
>>>>>> JvmtiBreakpoints are walked via VMThread oops_do (the breakpoint is in a 
>>>>>> vm operation) before they are installed in the safeopint and after they 
>>>>>> have been installed, walked with JvmtiCurrentBreakpoints::oops_do().
>>>>>> By putting the class holder inside oopStorage there is no need for this.
>>>>>>
>>>>>> JvmtiCurrentBreakpoints::metadata_do is not needed because redefine 
>>>>>> classes actually removes the breakpoints before updating them (so there is 
>>>>>> no breakpoints to update).
>>>>>> We can just remove metadata_do.
>>>>>>
>>>>>>
>>>>>> I also removed some unused code.
>>>>>>
>>>>>> Changeset:
>>>>>> http://cr.openjdk.java.net/~rehn/8235912/v1/webrev/
>>>>>>
>>>>>> Passes several runs of nsk jvmti/jdi and t1-7.
>>>>>>
>>>>>> Thanks, Robbin
>>>>>
>>>
>