RFR: 8071507: (ref) Clear phantom reference as soft and weak references do

[Per Liden]

Hi Peter,

On 2015-12-04 13:35, Peter Levart wrote:
>
>
> On 12/04/2015 08:16 AM, Per Liden wrote:
>>
>> test/java/lang/ref/PhantomReferentClearing.java:
>>
>>   85         // Delete root -> O1, collect, verify P1 notified, P2 not
>> notified.
>>   86         O1 = null;
>>   87         System.gc();
>>   88         if (Q1.remove(ENQUEUE_TIMEOUT) == null) {
>>   89             throw new RuntimeException("P1 not notified by O1
>> deletion");
>>   90         } else if (Q2.remove(ENQUEUE_TIMEOUT) != null) {
>>   91             throw new RuntimeException("P2 notified by O1
>> deletion.");
>>   92         }
>>   93
>>   94         // Delete root -> O2, collect. P2 should be notified.
>>   95         O2 = null;
>>   96         System.gc();
>>   97         if (Q2.remove(ENQUEUE_TIMEOUT) == null) {
>>   98             throw new RuntimeException("P2 not notified by O2
>> deletion");
>>   99         }
>>
>> The calls to System.gc() isn't guaranteed to do what the test expects
>> here. As you know, System.gc(), might not actually do anything,
>> depending on which collector is used and the current circumstances (GC
>> locker held, a concurrent GC is already in process, etc). To make the
>> test robust I'd suggest you call System.gc() and check the queue in a
>> loop, and fail after some reasonable amount of time/iterations.
>
> Whether you poll the queue for some time T or you remove from queue with
> timeout of T, it doesn't matter. The Reference(s) are enqueued by a
> ReferenceHandler thread and the thread waiting in remove() will get
> notified when a reference gets enqueued...

I think you maybe missed my point. There's no guarantee that a call to 
System.gc() will cause reference processing to happen at all, and if it 
happens there's no guarantee that it will discover P1/P2 here. In this 
specific test it's very likely to happen, but that's a different story. 
I still don't think this test should rely on a behavior that isn't 
guaranteed by the spec.

cheers,
/Per

>
> Regards, Peter
>
>>
>> cheers,
>> /Per
>>
>>> http://cr.openjdk.java.net/~kbarrett/8071507/hotspot.05/
>>>
>>> Testing:
>>> jprt, aurora ad hoc (defaults, GC/Runtime nightly, JCK)
>>>
>