On Tue, Aug 8, 2017 at 5:21 AM, Peter Levart <peter.lev...@gmail.com> 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 >> http://cr.openjdk.java.net/~martin/webrevs/openjdk10/jsr166-integration/ >> >> (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? > Yeah, I don't think we have any really good answer for this. I've lobbied for "safe publication of non-final fields as if final" for a long time. Even making the field volatile is not enough in theory (but is in practice) to make the uninitialized field unobservable. In theory a fence in the constructor is not enough because you need a fence before every read as well. Maybe if you replace every write with a release write and every read with an acquire read you're OK. But you also want to keep the "final" for software engineering purposes, since it's "mostly" final.
