RFR: 8256517: (ref) Reference.clear during reference processing may lose notification [v2]

[Kim Barrett]

On Tue, 24 Nov 2020 02:59:50 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> Looks good. Just want to request that you also remove the following comment in zReferenceProcessor.cpp, as it's no longer true.
>> 
>> --- a/src/hotspot/share/gc/z/zReferenceProcessor.cpp
>> +++ b/src/hotspot/share/gc/z/zReferenceProcessor.cpp
>> @@ -184,12 +184,6 @@ bool ZReferenceProcessor::should_discover(oop reference, ReferenceType type) con
>>  }
>>  
>>  bool ZReferenceProcessor::should_drop(oop reference, ReferenceType type) const {
>> -  // This check is racing with a call to Reference.clear() from the application.
>> -  // If the application clears the reference after this check it will still end
>> -  // up on the pending list, and there's nothing we can do about that without
>> -  // changing the Reference.clear() API. This check is also racing with a call
>> -  // to Reference.enqueue() from the application, which is unproblematic, since
>> -  // the application wants the reference to be enqueued anyway.
>>    const oop referent = reference_referent(reference);
>>    if (referent == NULL) {
>>      // Reference has been cleared, by a call to Reference.enqueue()
>
>> Looks good. Just want to request that you also remove the following comment in zReferenceProcessor.cpp, as it's no longer true.
>> 
>> ```
>> --- a/src/hotspot/share/gc/z/zReferenceProcessor.cpp
>> +++ b/src/hotspot/share/gc/z/zReferenceProcessor.cpp
>> @@ -184,12 +184,6 @@ bool ZReferenceProcessor::should_discover(oop reference, ReferenceType type) con
>>  }
>>  
>>  bool ZReferenceProcessor::should_drop(oop reference, ReferenceType type) const {
>> -  // This check is racing with a call to Reference.clear() from the application.
>> -  // If the application clears the reference after this check it will still end
>> -  // up on the pending list, and there's nothing we can do about that without
>> -  // changing the Reference.clear() API. This check is also racing with a call
>> -  // to Reference.enqueue() from the application, which is unproblematic, since
>> -  // the application wants the reference to be enqueued anyway.
>>    const oop referent = reference_referent(reference);
>>    if (referent == NULL) {
>>      // Reference has been cleared, by a call to Reference.enqueue()
>> ```
> 
> Done.

I realized there was a theoretical problem with the new tests; they
should also be ensuring the Reference objects are in oldgen.  That's
fixed in the latest push.

-------------

PR: https://git.openjdk.java.net/jdk/pull/1376