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

[Daniel Fuchs]

Hi Kim,

I agree with your proposed fix.

I see that you have added a comment for future maintainers.
Thanks for that - as the implication of the ordering between
the two volatile writes is not immediately perceptible to
the casual reader.

You have good eyes - I hadn't spotted the race condition, but
it is definitely there. I have tried to see if the proposed
reordering could have other side effects but couldn't find any:
The reference might now temporarily appear unenqueued though
it's already at the head of the queue - but since 'head' is
private in ReferenceQueue and access to the head reference
are adequately protected by 'lock' then it looks like this
is not going to be an issue.

+1

Thanks for finding the cause of the failure and the devising the fix!

best regards,

-- daniel

On 29/07/15 09:57, Kim Barrett wrote:
> Please review this fix of a race condition in
> j.l.r.Reference/ReferenceQueue.  See comments in the bug report for a
> description of the race.  The race is being fixed by reordering a pair
> of volatile assignments.
>
> CR:
> https://bugs.openjdk.java.net/browse/JDK-8132306
>
> Webrev:
> http://cr.openjdk.java.net/~kbarrett/8132306/webrev.00/
>
> Testing:
> jprt with default and hotspot testsets
>
> Locally tested race in original code by insertion of sleep in enqueue and
> verifying ReferenceEnqueue regression test fails consistently as
> described in the bug report.
>
> Locally tested fixed code by insertion of sleeps at various points and
> verifying ReferenceEnqueue regression test still passes.
>