RFC (csr): 8225142: JVMTI spec incorrectly states that PopFrame can not be called on the current thread

[David Holmes]

On 1/06/2019 3:09 pm, serguei.spitsyn at oracle.com wrote:
> Hi David,
> 
> Thank you for looking at it!
> 
> 
> On 5/31/19 9:47 PM, David Holmes wrote:
>> Hi Serguei,
>>
>> Sorry but I'm rather confused - what does it mean to do a "popframe" 
>> on the current thread?
> 
> I was confused too.
> We have several JVMTI tests that calls PopFrame in event callbacks.
>   For instance (see also: popframe006, popframe007, popframe008):
>     test/hotspot/jtreg/vmTestbase/nsk/jvmti/PopFrame/popframe010
> 
> This test gets a breakpoint event and in the callback:
>   - clears the breakpoint
>   - enables single stepping
>   - calls the PopFrame
> 
> This seems to be working.

Okay that makes sense. I hadn't thought about callbacks executing in the 
current thread and causing it to back itself up.

>> The whole point of popframe is to suspend a target thread when it is 
>> executing a specific method, and then call popframe to "back up" the 
>> thread to the point where it will call that method again when resumed. 
>> How does that work with the current thread?
> 
> Probably, the most interesting question is:
>   How will the target thread continue its execution?
> 
> As I understand, it will be single stepping.

I would hope so in the scenario you describe. But presumably the 
callback could just clear the breakpoint and do a popframe and so 
execution would continue as before?

Cheers,
David
------

> In such a case, the thread is executed in the interp-only mode.
> 
> Thanks,
> Serguei
> 
>>
>> Thanks,
>> David
>>
>> On 1/06/2019 1:59 pm, serguei.spitsyn at oracle.com wrote:
>>> Hi All,
>>>
>>> Please comment or.and review the following CSR.
>>>
>>> The CSR:
>>> https://bugs.openjdk.java.net/browse/JDK-8225142
>>>
>>> The bug:
>>> https://bugs.openjdk.java.net/browse/JDK-8205126
>>>
>>> Updated JVMTI spec:
>>> http://cr.openjdk.java.net/~sspitsyn/webrevs/2019/8205126-jvmti-spec-popframe.1/jvmti.html 
>>>
>>>
>>>
>>> Specdiff:
>>> http://cr.openjdk.java.net/~sspitsyn/webrevs/2019/8205126-jvmti-spec-popframe.1/jvmti-specdiff/ 
>>>
>>>
>>>
>>> Summary:
>>>    The JVMTI PopFrame() spec does not match the implementation.
>>>    It says the  specified thread can not be the current thread.
>>>    The fix aligns:
>>>     - spec with implementaion
>>>     - PopFrame spec with ForceEarlyReturn spec
>>>
>>> Thanks,
>>> Serguei
>