RFR: 8132306: java/lang/ref/ReferenceEnqueue.java fails with "RuntimeException: Error: poll() returned null; expected ref object"
kim.barrett at oracle.com
Thu Jul 30 19:54:11 UTC 2015
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.
>> r.isEnqueued() &&
>> q.poll() == null &&
>> 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.
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