RFR: 8135322: ConstantPool::release_C_heap_structures not run in some circumstances

[Andreas Eriksson]

Hi,

When running some additional testing I had a test crash.
Looks like sometimes data we need to run a full deallocate_contents call 
when we are in ~ClassLoaderData is already freed (seems to depend on 
what GC we are running, and what we have in the _deallocate_list).

Doing the free_deallocate_list call in 
ClassLoaderDataGraph::do_unloading instead will most likely work, since 
it's run at an earlier point in the GC.
Or I could do what I mentioned in the initial email, only run 
release_C_heap_structures() on constant pools in the list that do not 
have a corresponding InstanceKlass.

I'm leaning towards, doing the free_deallocate_list call in 
ClassLoaderDataGraph::do_unloading, when we detect that the 
ClassLoaderData is dead.
I'll send out a new review once I've had some time to test that fix.

Thanks,
Andreas

On 2016-04-01 02:15, serguei.spitsyn at oracle.com wrote:
> Andreas,
>
> I agree with Coleen, it is a good find and a good and safe fix.
>
> Thanks!
> Serguei
>
>
> On 3/30/16 16:06, Coleen Phillimore wrote:
>>
>> Andreas,
>>
>> Good find!  This seems like a safe fix.  It's unfortunate to have to 
>> walk the deallocate list, but I think it's short enough and less work 
>> in comparison to having to call free_C_heap_structures on all the 
>> other classes in the dead class loader data.   And 
>> deallocate_contents removes the scratch_class from the CLD _klasses 
>> list so won't walk it twice.
>>
>> This is really good.
>>
>> Coleen
>>
>> On 3/30/16 8:12 AM, Andreas Eriksson wrote:
>>> Hi,
>>>
>>> Please review this fix for 8135322: 
>>> ConstantPool::release_C_heap_structures not run in some circumstances.
>>>
>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8135322
>>> Webrev: http://cr.openjdk.java.net/~aeriksso/8135322/webrev.00
>>>
>>> The way this occurs is:
>>>
>>> 1. A ConstantPool, /merge_cp/, without a corresponding InstanceKlass is
>>>    created during class redefinition.
>>> 2. /Merge_cp/ is added to ClassLoaderData::_deallocate_list, but is not
>>>    cleaned up by ClassLoaderDataGraph::do_unloading because the holding
>>>    ClassLoaderData is already dead.
>>> 3. ClassLoaderData::~ClassLoaderData is called, which usually runs
>>>    InstanceKlass::release_C_heap_structures, which in turn will call
>>>    ConstantPool::release_C_heap_structures. But since there is no
>>>    corresponding InstanceKlass for /merge_cp/ it will never release its
>>>    C heap structures.
>>> 4. For JDK 8 this means we leak a Monitor (ConstantPool::_lock). For
>>>    JDK 9 the Monitor has been removed, but we will miss doing a call to
>>>    ConstantPool::unreference_symbols, which is probably not good 
>>> either.
>>>
>>>
>>> This change adds a call to free everything in the _deallocate_list 
>>> first thing in ClassLoaderData::~ClassLoaderData.
>>>
>>> Note:
>>> Running deallocate_contents() on everything in the list might be 
>>> doing unnecessary work:
>>> What we really would like to do is run release_C_heap_structures() 
>>> on constant pools in the list that do not have a corresponding 
>>> InstanceKlass. Not sure it's worth the effort to do the filtering, 
>>> since I believe the _deallocate_list is usually not that long.
>>>
>>> Note 2:
>>> After fixing in JDK 9 I'll backport this to JDK 8, where it has a 
>>> more immediately visible effect.
>>>
>>> Regards,
>>> Andreas
>>>
>>
>