The following test failed in the JDK21 CI:

serviceability/jvmti/vthread/FollowReferences/VThreadStackRefTest.java#default

Here's a snippet from the log file:

#section:main
----------messages:(6/398)----------
command: main -Djdk.virtualThreadScheduler.parallelism=1 -agentlib:VThreadStackRefTest VThreadStackRefTest
reason: User specified action: run main/othervm/native -Djdk.virtualThreadScheduler.parallelism=1 -agentlib:VThreadStackRefTest VThreadStackRefTest
started: Thu Jun 08 03:32:05 UTC 2023
Mode: othervm [/othervm specified]
finished: Thu Jun 08 03:32:32 UTC 2023
elapsed time (seconds): 26.98
----------configuration:(0/0)----------
----------System.out:(41/2859)----------
created class VThreadStackRefTest$VThreadUnmountedReferenced
created class VThreadStackRefTest$VThreadUnmountedEnded
created class VThreadStackRefTest$VThreadMountedReferenced
created class VThreadStackRefTest$VThreadMountedJNIReferenced
created class VThreadStackRefTest$PThreadReferenced
threads:
  - vthreadUnmounted: VirtualThread[#32]/waiting
  - vthreadEnded: VirtualThread[#35]/terminated
  - vthreadMounted: VirtualThread[#36]/runnable@ForkJoinPool-1-worker-1
  - pthread: Thread[#37,Thread-1,5,MainThreadGroup]
test classes:
  (0) class VThreadStackRefTest$VThreadUnmountedReferenced
  (1) class VThreadStackRefTest$VThreadUnmountedJNIReferenced
  (2) class VThreadStackRefTest$VThreadMountedReferenced
  (3) class VThreadStackRefTest$VThreadMountedJNIReferenced
  (4) class VThreadStackRefTest$PThreadReferenced
  (5) class VThreadStackRefTest$VThreadUnmountedEnded
Thread VirtualThread[#32]/waiting: unmounted
Thread VirtualThread[#36]/runnable@ForkJoinPool-1-worker-1: mounted
JNI local: index = 3, thread_id = 36
Stack local: index = 2, thread_id = 36
Stack local: index = 4, thread_id = 37
#
# A fatal error has been detected by the Java Runtime Environment:
#
# Internal Error (/opt/mach5/mesos/work_dir/slaves/741e9afd-8c02-45c3-b2e2-9db1450d0832-S14090/frameworks/1735e8a2-a1db-478c-8104-60c8b0af87dd-0196/executors/12aad8ed-cf3c-41cb-8164-344f33d1386e/runs/683ff040-6fd8-48dd-9782-2bb011040acf/workspace/open/src/hotspot/share/oops/compressedOops.inline.hpp:140), pid=14722, tid=14742
# assert(check_alignment(result)) failed: address not aligned: 0x00000008baadbabe
#
# JRE version: Java(TM) SE Runtime Environment (21.0+26) (fastdebug build 21-ea+26-LTS-2277)
# Java VM: Java HotSpot(TM) 64-Bit Server VM (fastdebug 21-ea+26-LTS-2277, compiled mode, sharing, compressed class ptrs, z gc, linux-amd64)
# Problematic frame:
# V [libjvm.so+0x5ed590] CompressedKlassPointers::decode_not_null(unsigned int, unsigned char*)+0xa0
#
# Core dump will be written. Default location: Core dumps may be processed with "/opt/core.sh %p" (or dumping to /opt/mach5/mesos/work_dir/slaves/cd627e65-f015-4fb1-a1d2-b6c9b8127f98-S1109/frameworks/1735e8a2-a1db-478c-8104-60c8b0af87dd-0196/executors/423bf862-ffee-460e-82a8-41c38006fe63/runs/fdcf1912-6ed8-40cd-b14a-9b71de02efa6/testoutput/test-support/jtreg_open_test_hotspot_jtreg_hotspot_loom/scratch/1/core.14722)
#
# An error report file with more information is saved as:
# /opt/mach5/mesos/work_dir/slaves/cd627e65-f015-4fb1-a1d2-b6c9b8127f98-S1109/frameworks/1735e8a2-a1db-478c-8104-60c8b0af87dd-0196/executors/423bf862-ffee-460e-82a8-41c38006fe63/runs/fdcf1912-6ed8-40cd-b14a-9b71de02efa6/testoutput/test-support/jtreg_open_test_hotspot_jtreg_hotspot_loom/scratch/1/hs_err_pid14722.log
#
# If you would like to submit a bug report, please visit:
# https://bugreport.java.com/bugreport/crash.jsp
#
----------System.err:(0/0)----------
----------rerun:(40/6613)*----------


Here's the crashing thread's stack:

--------------- T H R E A D ---------------

Current thread (0x00007fd4e413d890): VMThread "VM Thread" [id=14742, stack(0x00007fd4d894c000,0x00007fd4d8a4c000) (1024K)]

Stack: [0x00007fd4d894c000,0x00007fd4d8a4c000], sp=0x00007fd4d8a48ee0, free space=1011k
Native frames: (J=compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [libjvm.so+0x5ed590] CompressedKlassPointers::decode_not_null(unsigned int, unsigned char*)+0xa0 (compressedOops.inline.hpp:140)
V [libjvm.so+0x11b7281] CallbackWrapper::CallbackWrapper(JvmtiTagMap*, oop)+0xa1 (compressedOops.inline.hpp:134)
V [libjvm.so+0x11ba031] CallbackInvoker::invoke_advanced_stack_ref_callback(jvmtiHeapReferenceKind, long, long, int, _jmethodID*, long, int, oop)+0xd1 (jvmtiTagMap.cpp:1736)
V [libjvm.so+0x11bab27] JNILocalRootsClosure::do_oop(oop*)+0x167 (jvmtiTagMap.cpp:2001)
V [libjvm.so+0x14103da] nmethod::oops_do(OopClosure*, bool)+0x29a (nmethod.cpp:1773)
V [libjvm.so+0x11b66ff] StackRefCollector::do_frame(vframe*)+0x11f (nmethod.hpp:551)
V [libjvm.so+0x11b6834] StackRefCollector::process_frames(vframe*)+0x34 (jvmtiTagMap.cpp:2353)
V [libjvm.so+0x11bb464] VM_HeapWalkOperation::collect_vthread_stack_refs(oop)+0x434 (jvmtiTagMap.cpp:2886)
V [libjvm.so+0x11bbdc7] VM_HeapWalkOperation::visit(oop)+0x5e7 (jvmtiTagMap.cpp:2910)
V [libjvm.so+0x11b6b1e] VM_HeapWalkOperation::doit()+0x2ce (jvmtiTagMap.cpp:2958)
V [libjvm.so+0x1872459] VM_Operation::evaluate()+0x139 (vmOperations.cpp:71)
V [libjvm.so+0x189515b] VMThread::evaluate_operation(VM_Operation*)+0x10b (vmThread.cpp:281)
V [libjvm.so+0x1895ccf] VMThread::inner_execute(VM_Operation*)+0x3bf (vmThread.cpp:435)
V [libjvm.so+0x1895eb4] VMThread::loop()+0x84 (vmThread.cpp:502)
V [libjvm.so+0x1895fb7] VMThread::run()+0x87 (vmThread.cpp:175)
V [libjvm.so+0x1793f8a] Thread::call_run()+0xba (thread.cpp:217)
V [libjvm.so+0x149505c] thread_native_entry(Thread*)+0x11c (os_linux.cpp:775)

Since we're crashing during a heap walk operation, I'm starting this
bug off in hotspot/gc for initial triage.