While investigating JDK-8260684 I've been repeatedly running a modified version of vmTestbase/gc/gctests/PhantomReference/phantom002/TestDescription.java. (I don't think the modifications have anything to do with these failures, but I haven't tried an unmodified version.)

This is using jdk17 through JDK-8268316 (so HEAD as I write this).

Occasionally (1-2 times per thousand) it fails with the assert below on Windows with -XX:+UseZGC. I've not tried on other platforms.

#
# A fatal error has been detected by the Java Runtime Environment:
#
# Internal Error (t:\workspace\open\src\hotspot\share\gc\z\zBarrier.cpp:41), pid=44060, tid=10996
# assert(ZAddress::is_marked(addr)) failed: Should be marked
#
# JRE version: Java(TM) SE Runtime Environment (17.0) (fastdebug build 17-internal+0-LTS-2021-06-20-0926154.kim.barrett.jdk)
# Java VM: Java HotSpot(TM) 64-Bit Server VM (fastdebug 17-internal+0-LTS-2021-06-20-0926154.kim.barrett.jdk, mixed mode, sharing, tiered, compressed class ptrs, z gc, windows-amd64)
# Core dump will be written. Default location: T:\testoutput\test-support\jtreg_open_test_hotspot_jtreg_vmTestbase_gc_gctests_PhantomReference_phantom002\scratch\0\hs_err_pid44060.mdmp

Current thread (0x000001371efd8760): JavaThread "gc.gctests.PhantomReference.phantom001.phantom001$Test@4cc92486" [_thread_in_Java, id=10996, stack(0x0000002db6d00000,0x0000002db6e00000)]

Stack: [0x0000002db6d00000,0x0000002db6e00000]
Native frames: (J=compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [jvm.dll+0xadd001] os::platform_print_native_stack+0xf1 (os_windows_x86.cpp:235)
V [jvm.dll+0xd05395] VMError::report+0x1005 (vmError.cpp:739)
V [jvm.dll+0xd06d1e] VMError::report_and_die+0x7fe (vmError.cpp:1549)
V [jvm.dll+0xd07454] VMError::report_and_die+0x64 (vmError.cpp:1330)
V [jvm.dll+0x4c7937] report_vm_error+0xb7 (debug.cpp:282)
V [jvm.dll+0xd4f734] ZBarrier::should_mark_through<0>+0x44 (zBarrier.cpp:41)
V [jvm.dll+0xd502b7] ZBarrier::load_barrier_on_oop_slow_path+0x67 (zBarrier.cpp:130)
V [jvm.dll+0xd580b6] ZBarrierSetRuntime::load_barrier_on_oop_field_preloaded+0x46 (zBarrierSetRuntime.cpp:31)
C 0x0000013514ca9b8b