On 08/08/2017 02:21 PM, Peter Levart wrote:
Hi Martin,

Just a purely theoretical question...

On 08/08/2017 02:06 AM, Martin Buchholz wrote:
Need to fix

    1. JDK-8185830 <https://bugs.openjdk.java.net/browse/JDK-8185830>

ConcurrentSkipListSet.clone() fails with UnsupportedOperationException

(It would be nice if we could submit a jdk9 backport now instead of waiting)

Regarding ConcurrentSkipListSet.clone()... The 'm' field is final presumably to allow "safe" publication of ConcurrentSkipListSet objects to other threads via data races.

clone() O.T.O.H. uses Field.setAccessible(true) on a final field, followed with Field.set(), which amounts to Unsafe.putObjectVolatile(). Is this equivalent to volatile write? Is it possible for a normal write of a reference to a cloned ConcurrentSkipListSet (i.e. publication via data race) to bubble up before the volatile write of 'm' field inside the clone and consequently allow an observer of a reference to the clone to modify the backing map of the original ConcurrentSkipListSet?

Regards, Peter

I know it can't be done any better than that, because ConcurrentSkipListSet is not final. If it was final, clone() could use private constructor to construct a clone. But the same can be done at least for objects of ConcurrentSkipListSet runtime class:

if (getClass() == ConcurrentSkipListSet.class) {
    return new ConcurrentSkipListSet(clonedMap);
} else {
    // proceed as is


An alternative would be to use explicit fence before the end of clone()

Regards, Peter

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