[concurrency-interest] Is Reference.reachabilityFence() needed in Reference constructor? (original) (raw)

Vitaly Davidovich vitalyd at gmail.com
Wed Oct 21 22:25:01 UTC 2015

Yes, if runtime stripes that instance before it is exposed anywhere (e.g. scalarizes the weakref)

If you look at escape analysis code ( http://hg.openjdk.java.net/jdk9/hs-comp/hotspot/file/a60bd3d34158/src/share/vm/opto/escape.cpp#l803), Reference and subclasses get special treatment and marked as GlobalEscape.

On Wed, Oct 21, 2015 at 1:08 PM, Aleksey Shipilev < aleksey.shipilev at oracle.com> wrote:

On 10/21/2015 06:53 PM, Andrew Haley wrote: > On 10/21/2015 04:44 PM, Aleksey Shipilev wrote: >> Of course, this does not tell another (scary) part of the story, what if >> the Reference itself is not discovered as strong ref by GC, e.g. when it >> isn't published on heap, or scalarized by compiler, etc. >> reachabilityFence, as currently implemented, extends the "liveness" >> scope of the local variable, but does not convey anything special to the >> GC/runtime otherwise. > > If the Reference itself is not reachable from a strong root then the > Reference is dead so we don't care what happens to it. But if the > Reference is put on a reachable ReferenceQueue, then it's fine.

Yes, if runtime stripes that instance before it is exposed anywhere (e.g. scalarizes the weakref), or those references are dead (e.g. the weakref is buried somewhere deep in garbage subgraph, and purged on sweep), then everything goes awry. But the thing is, a WeakReference is put on ReferenceQueue by the GC itself. In the object graph, ReferenceQueue does not normally reference weakrefs back. Before enqueueing the reference GC has to first discover that WeakReference -- but from where? In other words, "registering" on a reachable RefQueue does not make weakref to be reachable. Case in point: public class WhereIsWaldo { public static void main(String... args) throws Exception { for (int c = 0; true; c++) { new WhereIsWaldo().work(c); } } final ReferenceQueue rq = new ReferenceQueue<>(); void work(int id) throws Exception { hideWaldo(id); findWaldo(id); } WeakReference gwr; void hideWaldo(int id) { // Does not work, d'uh new WeakReference<>(new Object(), rq); // Does not work either :( WeakReference wr = new WeakReference<>(new Object(), rq); // This also does not help... once you leave the method, // the weakref is gone. Reference.reachabilityFence(wr); // This one helps, makes Waldo reachable // gwr = wr; } void findWaldo(int id) throws Exception { while (Thread.currentThread().isAlive()) { System.gc(); Reference ref = rq.remove(1000); if (ref != null) { return; } else { System.out.println("Where's Waldo #" + id + "?"); } } } }

Thanks, -Aleksey

More information about the core-libs-dev mailing list