RFR: 8132306: java/lang/ref/ReferenceEnqueue.java fails with "RuntimeException: Error: poll() returned null; expected ref object"

[Kim Barrett]

On Jul 30, 2015, at 8:09 AM, David Holmes <david.holmes at oracle.com> wrote:
> 
> On 30/07/2015 9:57 PM, Peter Levart wrote:
>> 'r' has been enqueued.
>> 
>> Thread-1:
>> 
>> r.isEnqueued() &&
>> q.poll() == null &&
>> r.isEnqueued()
>> 
>> Thread-2:
>> 
>> q.poll();
>> 
>> 
>> Sequence of actions:
>> 
>> T1: r.isEnqueued() ==> true
>> 
>> T2: q.poll() executed to the following point (see HERE) and 'r' was the
>> last element in the queue ('head' has been assigned to null):
> 
> Yeah thanks - just realized it is that darned unsynchronized "fast-path" again. What a mess.
> 
> It a kind of inverse of the original problem.
> 
> Original: don't update reference state to enqueued before the queue is updated
> This one: don't update the queue state to empty before the reference state shows it is de-queued.
> 
> So yes the fix here is to move "r.queue = null" to before the assignment to head.
> 
> Bring on the next race ;-)

I agree with everything David said above.  Bleh!

So I think I can either:

1. Go ahead with my change + Peter's change.

2. Give this back to core-libs while I step carefully away :-)

I *think* option (1) is at least an improvement.  But I completely
missed Peter's race, despite having specifically looked for problems
there, so take my opinion with an appropriate quantity of salt.