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

Per Liden per.liden at oracle.com
Mon Dec 7 07:26:41 UTC 2015

On 2015-12-04 19:16, Kim Barrett wrote:
> On Dec 4, 2015, at 2:16 AM, Per Liden <per.liden at oracle.com> wrote:
>> On 2015-12-02 19:37, Kim Barrett wrote:
>>> Please review this change to PhantomReference processing, changing the
>>> GC-based notification to automatically clear the referent.
>>> […]
>>> CR:
>>> https://bugs.openjdk.java.net/browse/JDK-8071507
>>> Webrevs:
>>> http://cr.openjdk.java.net/~kbarrett/8071507/jdk.05/
>> 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.
> I'd rather not clutter and uglify this and other tests to deal with a
> problem we don't (so far as I can tell) presently actually have.  If a
> problem does arise, the form of that problem will help guide what the
> solution needs to be.  I suspect adding something to WhiteBox would be
> the proper approach, so that it can be shared with other tests of this
> form.

We do actually have this problem today, i.e. there are cases where our 
collectors will not do a "full GC" as a result of a call to System.gc(). 
One example would be if the GC-locker is active. However, it's seems 
very unlikely that this particular test would provoking such a condition.

Our current WhiteBox API for FullGC is prone to the same issue. Maybe it 
would be a good idea to strengthen that API and used that in the future 
for tests like this.

I understand if you don't want to take on this problem here and now, 
given that we have other test which is very similar to this one, and 
they seem to have worked so far without issues.


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