Integration of the following enhancement triggered/introduced new failure mode with a deadlock:
JDK-8299414: JVMTI FollowReferences should support references from VirtualThread stack

This thread is (stuck?) in transition:

debugee.stdout> [516.812s][error][jvmti] ForkJoinPool-1-worker-4(ForkJoinPool-1-worker-4) exiting: 0 is_susp: 0 is_thread_susp: 0 is_vthread_susp: 0 is_VTMS_transition_disabler: 0, is_in_VTMS_transition = 1
debugee.stdout>
debugee.stdout> [516.812s][error][jvmti] java.lang.Thread java.lang.Thread.currentCarrierThread():-1
debugee.stdout> [516.812s][error][jvmti] void java.lang.VirtualThread.mount():353
debugee.stdout> [516.812s][error][jvmti] boolean java.lang.VirtualThread.yieldContinuation():444
debugee.stdout> [516.812s][error][jvmti] void java.lang.VirtualThread.park():592
debugee.stdout> [516.812s][error][jvmti] void java.lang.System$2.parkVirtualThread():2649
debugee.stdout> [516.812s][error][jvmti] void jdk.internal.misc.VirtualThreads.park():54
debugee.stdout> [516.812s][error][jvmti] void java.util.concurrent.locks.LockSupport.park():369
debugee.stdout> [516.812s][error][jvmti] void sun.nio.ch.Poller.poll2(int, long, java.util.function.BooleanSupplier):139
debugee.stdout> [516.812s][error][jvmti] void sun.nio.ch.Poller.poll(int, long, java.util.function.BooleanSupplier):102
debugee.stdout> [516.812s][error][jvmti] void sun.nio.ch.Poller.poll(int, int, long, java.util.function.BooleanSupplier):87
debugee.stdout> [516.812s][error][jvmti] void sun.nio.ch.NioSocketImpl.park(java.io.FileDescriptor, int, long):175
debugee.stdout> [516.812s][error][jvmti] void sun.nio.ch.NioSocketImpl.park(java.io.FileDescriptor, int):201
debugee.stdout> [516.812s][error][jvmti] int sun.nio.ch.NioSocketImpl.implRead(byte[], int, int):309
debugee.stdout> [516.812s][error][jvmti] int sun.nio.ch.NioSocketImpl.read(byte[], int, int):346
debugee.stdout> [516.812s][error][jvmti] int sun.nio.ch.NioSocketImpl$1.read(byte[], int, int):796
debugee.stdout> [516.812s][error][jvmti] int java.net.Socket$SocketInputStream.read(byte[], int, int):1099
debugee.stdout> [516.812s][error][jvmti] int java.net.Socket$SocketInputStream.read():1093
debugee.stdout> [516.812s][error][jvmti] int java.io.ObjectInputStream$PeekInputStream.peek():2893
debugee.stdout> [516.812s][error][jvmti] int java.io.ObjectInputStream$BlockDataInputStream.peek():3220
debugee.stdout> [516.812s][error][jvmti] byte java.io.ObjectInputStream$BlockDataInputStream.peekByte():3230
debugee.stdout> [516.812s][error][jvmti] java.lang.Object java.io.ObjectInputStream.readObject0(java.lang.Class, boolean):1713
debugee.stdout> [516.812s][error][jvmti] java.lang.Object java.io.ObjectInputStream.readObject(java.lang.Class):540
debugee.stdout> [516.812s][error][jvmti] java.lang.Object java.io.ObjectInputStream.readObject():498
debugee.stdout> [516.812s][error][jvmti] java.lang.Object nsk.share.jpda.SocketConnection.doReadObject():581
debugee.stdout> [516.812s][error][jvmti] java.lang.Object nsk.share.jpda.SocketConnection.readObject():518
debugee.stdout> [516.812s][error][jvmti] java.lang.String nsk.share.jpda.SocketIOPipe.readln():193
debugee.stdout> [516.812s][error][jvmti] void nsk.share.jpda.AbstractDebuggeeTest.doTest():309


This thread is waiting for the thread in transition to disable VTMS transitions:

Current thread (0x0000ffff8c339cc0): JavaThread "JDWP Transport Listener: dt_socket" daemon [_thread_in_vm, id=3016377, stack(0x0000ffff65600000,0x0000ffff65800000) (2048K)]

Stack: [0x0000ffff65600000,0x0000ffff65800000], sp=0x0000ffff657fe130, free space=2040k
Native frames: (J=compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [libjvm.so+0x10a20b0] JvmtiVTMSTransitionDisabler::VTMS_transition_disable_for_all()+0x210 (jvmtiThreadState.cpp:358)
V [libjvm.so+0x10929bc] JvmtiTagMap::follow_references(int, Klass*, _jobject*, jvmtiHeapCallbacks const*, void const*)+0x16c (jvmtiTagMap.cpp:3030)
V [libjvm.so+0x103ae00] JvmtiEnv::FollowReferences(int, _jclass*, _jobject*, jvmtiHeapCallbacks const*, void const*)+0x1f0 (jvmtiEnv.cpp:2105)
V [libjvm.so+0xfe5d78] jvmti_FollowReferences+0x118 (jvmtiEnter.cpp:1715)
C [libjdwp.so+0x31464] classInstances+0xb0 (util.c:2548)
C [libjdwp.so+0x8648] instances+0x98 (ReferenceTypeImpl.c:272)
C [libjdwp.so+0x13c64] debugLoop_run+0x2b4 (debugLoop.c:162)
C [libjdwp.so+0x290e4] attachThread+0x54 (transport.c:392)
V [libjvm.so+0x1071d84] JvmtiAgentThread::call_start_function()+0xc4 (jvmtiImpl.cpp:89)
V [libjvm.so+0xda870c] JavaThread::thread_main_inner()+0xec (javaThread.cpp:719)
V [libjvm.so+0x15f7054] Thread::call_run()+0xb0 (thread.cpp:217)
V [libjvm.so+0x1353b80] thread_native_entry(Thread*)+0x130 (os_linux.cpp:775)
C [libpthread.so.0+0x7908] start_thread+0x188