Fatal errors when running JCK tests with JDK15/16 debug build

[Doerr, Martin]

Hi,

we have seen the following fatal error more than 50 times since 2020-05-25 in various JCK tests vm/jvmti.
fatal error: String conversion failure: [check] ExitLock destroyed
-->    [check] ExitLock exited
(followed by garbage output)

8166358: Re-enable String verification in java_lang_String::create_from_str()
was pushed at that date which introduced the call to fatal.
Stack (example from linuxppc64le, but also observed on x86 and aarch64):

V  [libjvm.so+0xee242c]  java_lang_String::create_from_str(char const*, Thread*) [clone .part.158]+0x51c

V  [libjvm.so+0xee2530]  java_lang_String::create_oop_from_str(char const*, Thread*)+0x40

V  [libjvm.so+0x1026a30]  jni_NewStringUTF+0x1e0

C  [libjckjvmti.so+0x3ce4c]  logWrite+0x5c

C  [libjckjvmti.so+0x3cd20]  lprintf+0x170

C  [libjckjvmti.so+0x485b8]  gast00104_agent_proc+0x254

V  [libjvm.so+0x1218f0c]  JvmtiAgentThread::call_start_function()+0x24c

V  [libjvm.so+0x193a8fc]  JavaThread::thread_main_inner()+0x32c

V  [libjvm.so+0x19418a0]  Thread::call_run()+0x160

V  [libjvm.so+0x15c9d0c]  thread_native_entry(Thread*)+0x18c

C  [libpthread.so.0+0x9b48]  start_thread+0x108

(Problem could have been there before but without this fatal message.)

The messages are generated by:
tests/vm/jvmti/GetAllStackTraces/gast001/gast00104/gast00104.c

This looks like a race condition. The message changes while the VM creates a String object from it. Has anybody seen this before?
Is it a test problem? I'm not familiar with the lprintf calls in the test.

Best regards,
Martin