On Mon, 23 Nov 2020 01:43:39 GMT, Kim Barrett <kbarr...@openjdk.org> wrote:
> Please review this change to Reference.clear() to address several issues. > > (JDK-8240696) For GCs using a SATB barrier, simply assigning the referent > field to null may extend the lifetime of the referent value. > > (JDK-8240696) For GCs with concurrent reference processing, clearing the > referent field during reference processing may discard the expected > notification. > > Both of these are addressed by introducing a private native helper function > for clearing the referent, rather than using an ordinary in-Java field > assignment. Tests have been added for both of these issues. This required > adding a new breakpoint in reference processing for ZGC. > > Of course, finalization adds some complexity to the problem. We deal with > that by having FinalReference override clear. The implementation is > provided by a new package-private method in Reference. (There are a number > of alternatives, all of them clumsy; finalization is annoying that way.) > > While dealing with FinalReference clearing it was noted that the recent > JDK-8256106 and JDK-8256370 have some problems. FinalizerHistogram was not > updated to call the new Reference.getInactive(), instead still calling get() > on FinalReferences, with the JDK-8256106 problems. Fixing that showed the > assertion for inactive FinalReference added by JDK-8256370 used the wrong > test. > > Rather than tracking down and changing all get() and clear() calls on final > references and changing them to use getInactive and a new similar clear > function, I've changed FinalReference to override get and clear, which call > the helper functions in Reference. I've also renamed getInactive to be more > explanatory and less convenient to call directly, and similarly named the > helper for clear. This means that get/clear should never be called on an > active FinalReference. That's already never done, and would have problems > if it were. > > Testing: > mach5 tier1-6 > Local (linux-x64) tier1 using Shenandoah. > New TestReferenceClearDuringMarking fails for G1 without these changes. > New TestReferenceClearDuringReferenceProcessing fails for ZGC without these > changes. Looks good, except one small nit in one of the test configs. test/hotspot/jtreg/gc/TestReferenceClearDuringReferenceProcessing.java line 28: > 26: /* @test > 27: * @bug 8256517 > 28: * @requires vm.gc.Z Please add | vm.gc.Shenandoah here. ------------- Changes requested by rkennke (Reviewer). PR: https://git.openjdk.java.net/jdk/pull/1376
