The following test failed in the JDK16 CI:

vmTestbase/gc/ArrayJuggle/Juggle18/TestDescription.java

Here's a snippet from the log file:

----------System.out:(45/2665)----------
Stress time: 30 seconds
Stress iterations factor: 1
Stress threads factor: 1
Stress runs factor: 1
Max memory: 2063597568
Sleep time: 500
Iterations: 0
Number of threads: 8
Run GC thread: false
Run mem diag thread: false
Run forever: false
Garbage producer: nsk.share.gc.gp.array.LongArrayProducer@71362508
Memory strategy: high
Object count: 100
Object size: 12381585
Starting Thread[gc.ArrayJuggle.Juggle01.Juggle01$Juggler@79fd10dd,5,MainThreadGroup]
Starting Thread[gc.ArrayJuggle.Juggle01.Juggle01$Juggler@4844c962,5,MainThreadGroup]
Starting Thread[gc.ArrayJuggle.Juggle01.Juggle01$Juggler@59ae434f,5,MainThreadGroup]
Starting Thread[gc.ArrayJuggle.Juggle01.Juggle01$Juggler@258f282,5,MainThreadGroup]
Starting Thread[gc.ArrayJuggle.Juggle01.Juggle01$Juggler@62e69388,5,MainThreadGroup]
Starting Thread[gc.ArrayJuggle.Juggle01.Juggle01$Juggler@c87496f,5,MainThreadGroup]
Starting Thread[gc.ArrayJuggle.Juggle01.Juggle01$Juggler@2798c78b,5,MainThreadGroup]
Starting Thread[gc.ArrayJuggle.Juggle01.Juggle01$Juggler@6291fd18,5,MainThreadGroup]
For random generator using seed: -6399519651592787741
To re-run test with same seed value please add "-Djdk.test.lib.random.seed=-6399519651592787741" to command line.
# To suppress the following error report, specify this argument
# after -XX: or in .hotspotrc: SuppressErrorAt=/g1ConcurrentMarkThread.cpp:167
#
# A fatal error has been detected by the Java Runtime Environment:
#
# Internal Error (/System/Volumes/Data/mesos/work_dir/slaves/4076d11c-c6ed-4d07-84c1-4ab8d55cd975-S176973/frameworks/1735e8a2-a1db-478c-8104-60c8b0af87dd-0196/executors/c5c819c7-549f-49fe-9ba7-2d8b29efc828/runs/19bb77df-56b0-47be-9e73-fd5d8eb93268/workspace/open/src/hotspot/share/gc/g1/g1ConcurrentMarkThread.cpp:167), pid=15463, tid=12291
# assert(!in_progress()) failed: should have been cleared
#
# JRE version: Java(TM) SE Runtime Environment (16.0+19) (fastdebug build 16-ea+19-958)
# Java VM: Java HotSpot(TM) 64-Bit Server VM (fastdebug 16-ea+19-958, mixed mode, sharing, tiered, compressed oops, g1 gc, bsd-amd64)
# Core dump will be written. Default location: core.15463
#
# An error report file with more information is saved as:
# /mesos/work_dir/slaves/4076d11c-c6ed-4d07-84c1-4ab8d55cd975-S106665/frameworks/1735e8a2-a1db-478c-8104-60c8b0af87dd-0196/executors/aedda1ba-f71c-4eb4-aab1-c8b949f88e26/runs/880ab4aa-daef-4f16-9821-cd6779be9595/testoutput/test-support/jtreg_open_test_hotspot_jtreg_vmTestbase_vm_gc_juggle/scratch/0/hs_err_pid15463.log
[thread 27395 also had an error]
#
# If you would like to submit a bug report, please visit:
# https://bugreport.java.com/bugreport/crash.jsp
#
Timeout refired 480 times
----------System.err:(0/0)----------

Here's the crashing thread's stack:

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

Current thread (0x00007fb61950f6e0): ConcurrentGCThread "G1 Main Marker" [stack: 0x00007000068cc000,0x00007000069cc000] [id=12291]

Stack: [0x00007000068cc000,0x00007000069cc000], sp=0x00007000069cba20, free space=1022k
Native frames: (J=compiled Java code, A=aot compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [libjvm.dylib+0x1086565] VMError::report_and_die(int, char const*, char const*, __va_list_tag*, Thread*, unsigned char*, void*, void*, char const*, int, unsigned long)+0x6d5
V [libjvm.dylib+0x1086c9b] VMError::report_and_die(Thread*, void*, char const*, int, char const*, char const*, __va_list_tag*)+0x3b
V [libjvm.dylib+0x5739af] report_vm_error(char const*, int, char const*, char const*, ...)+0x13f
V [libjvm.dylib+0x6a6a84] G1ConcurrentMarkThread::wait_for_next_cycle()+0x34
V [libjvm.dylib+0x6a6a02] G1ConcurrentMarkThread::run_service()+0x1d2
V [libjvm.dylib+0x55046b] ConcurrentGCThread::run()+0x2b
V [libjvm.dylib+0xfec4c7] Thread::call_run()+0x1b7
V [libjvm.dylib+0xdaf1bf] thread_native_entry(Thread*)+0x15f
C [libsystem_pthread.dylib+0x3661] _pthread_body+0x154
C [libsystem_pthread.dylib+0x350d] _pthread_body+0x0
C [libsystem_pthread.dylib+0x2bf9] thread_start+0xd