RFR (S) 8218851: JVM crash in custom classloader stress test, JDK 12 & 13

[coleen.phillimore at oracle.com]

On 2/15/19 8:41 AM, David Holmes wrote:
> On 15/02/2019 11:35 pm, coleen.phillimore at oracle.com wrote:
>> On 2/15/19 8:26 AM, David Holmes wrote:
>>> On 15/02/2019 10:04 pm, coleen.phillimore at oracle.com wrote:
>>>> On 2/14/19 8:54 PM, David Holmes wrote:
>>>>> Hi Coleen,
>>>>>
>>>>> On 15/02/2019 2:58 am, coleen.phillimore at oracle.com wrote:
>>>>>> Summary: Handle NULL and unloaded constraint class in loader 
>>>>>> constraint table, also cope with unloaded but not cleaned out 
>>>>>> klass in loader constraint entries.
>>>>>
>>>>> Pardon my ignorance but how can an unloaded class still be 
>>>>> involved in a loader constraint?
>>>>
>>>> The class can be not loaded yet, because we apparently add the 
>>>> klass to the loader constraint table before the is_loaded flag is 
>>>> set.  It can be already unloaded with concurrent class unloading 
>>>> because the table hasn't been cleaned out yet.   I put interesting 
>>>> bits of the stack trace in the bug report.
>>>
>>> That sounds really racy. So if we do the check before is_loaded is 
>>> set it passes or fails? And if we do the check a millisecond later 
>>> when is_loaded is set, it passes or fails? Surely there has to be 
>>> some atomicity here.
>>
>> The loader constraint table is examined under the 
>> SystemDictionary_lock, and the is_loaded flag on an InstanceKlass is 
>> also.   The loader constraint table checks the constraints whether 
>> it's fully loaded or not, because it just compares the _klass.  This 
>> is the existing code. The error message is new and didn't have a NULL 
>> check for the return for the find_constrained_klass().  This could 
>> have failed without ZGC, except we didn't get it to reproduce.
>>
>> There are two issues here, I'm not sure which one you are worried about.
>
> I'm not sure either. I guess I don't understand how NULL can be 
> appearing in this context. I know you explained it but I don't see how 
> it makes sense conceptually. This just seems racy.
>
> I have to call it a night. :)

Ok. I can explain on slack tomorrow for you if you want.
Thanks,
Coleen

>
> Cheers,
> David
>
>> thanks,
>> Coleen
>>
>>>
>>> David
>>>> thanks,
>>>> Coleen
>>>>
>>>>>
>>>>> Thanks,
>>>>> David
>>>>>
>>>>>> Tested overnight with the application that caused the crash. 
>>>>>> Tested with hs tier1-6.   Tested JCKs with ZGC.
>>>>>>
>>>>>> open webrev at 
>>>>>> http://cr.openjdk.java.net/~coleenp/2019/8218851.01/webrev
>>>>>> bug link https://bugs.openjdk.java.net/browse/JDK-8218851
>>>>>>
>>>>>> Thanks,
>>>>>> Coleen
>>>>
>>