On Fri, 6 Sep 2024 19:57:41 GMT, Brent Christian <bchri...@openjdk.org> wrote:
> From the bug description: > ForceGC would be improved by moving the Reference.reachabilityFence() calls > for 'obj' and 'ref'. > > Reference.reachabilityFence(obj) is currently placed after 'obj' has been set > to null, so effectively does nothing. It should occur before obj = null; > > For Reference.reachabilityFence(ref): 'ref' is a PhantomReference to 'obj', > and is registered with 'queue'. ForceGC.waitFor() later remove()s the > reference from the queue, as an indication that some GC and reference > processing has taken place (hopefully causing the BooleanSupplier to return > true). > > The code expects the PhantomReference to be cleared and be put on the queue. > But recall that a Reference refers to its queue, and not the other way > around. If a Reference becomes unreachable and is garbage collected, it will > never be enqueued. > > I argue that the VM/GC could determine that 'ref' is not used by waitFor() > and collect it before the call to queue.remove(). Moving > Reference.reachabilityFence(ref) after the for() loop would prevent this > scenario. > > While this is only a very minor deficiency in ForceGC, I believe it would be > good to ensure that the code behaves as expected. My understanding is that try/finally is needed to ensure the RF is guaranteed to be seen to be executed at the expected location. Otherwise the RF can in theory be moved around. ------------- PR Comment: https://git.openjdk.org/jdk/pull/20898#issuecomment-2340043873
