This has been observed once in 9-dev CI run.

Please refer to the test output below:
command: testng CompletionSuggestionTest
reason: User specified action: run testng CompletionSuggestionTest
Mode: agentvm
Agent id: 10
Additional exports to unnamed modules from @modules: jdk.compiler/com.sun.tools.javac.api jdk.compiler/com.sun.tools.javac.main jdk.jdeps/com.sun.tools.javap
Additional opens to unnamed modules from @modules: jdk.jshell/jdk.jshell
Timeout signalled after 600 seconds
Timeout information:
Running jstack on process 35471
2017-06-17 10:38:41
Full thread dump Java HotSpot(TM) 64-Bit Server VM (9-ea+175-mach5-b2486 mixed mode):

"Attach Listener" #204 daemon prio=9 os_prio=64 tid=0x000000000234e000 nid=0xe0 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"output reader" #203 daemon prio=5 os_prio=64 tid=0x0000000003082800 nid=0xdf runnable [0xffff80ffb70fb000]
   java.lang.Thread.State: RUNNABLE
at java.net.SocketInputStream.socketRead0(java.base@9-ea/Native Method)
at java.net.SocketInputStream.socketRead(java.base@9-ea/SocketInputStream.java:116)
at java.net.SocketInputStream.read(java.base@9-ea/SocketInputStream.java:171)
at java.net.SocketInputStream.read(java.base@9-ea/SocketInputStream.java:141)
at java.net.SocketInputStream.read(java.base@9-ea/SocketInputStream.java:224)
at java.io.FilterInputStream.read(java.base@9-ea/FilterInputStream.java:83)
at jdk.jshell.execution.DemultiplexInput.run(jdk.jshell@9-ea/DemultiplexInput.java:58)

"event-handler" #202 daemon prio=5 os_prio=64 tid=0x0000000000e99000 nid=0xde in Object.wait() [0xffff80ffb77fe000]
   java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(java.base@9-ea/Native Method)
- waiting on <no object reference available>
at java.lang.Object.wait(java.base@9-ea/Object.java:516)
at com.sun.tools.jdi.EventQueueImpl.removeUnfiltered(jdk.jdi@9-ea/EventQueueImpl.java:189)
- waiting to re-lock in wait() <0x00000000f9b81630> (a com.sun.tools.jdi.EventQueueImpl)
at com.sun.tools.jdi.EventQueueImpl.remove(jdk.jdi@9-ea/EventQueueImpl.java:96)
at com.sun.tools.jdi.EventQueueImpl.remove(jdk.jdi@9-ea/EventQueueImpl.java:82)
at jdk.jshell.execution.JdiEventHandler.run(jdk.jshell@9-ea/JdiEventHandler.java:79)
at java.lang.Thread.run(java.base@9-ea/Thread.java:844)

"JDI Target VM Interface" #200 daemon prio=5 os_prio=64 tid=0x000000000433a800 nid=0xdd runnable [0xffff80ffb72fd000]
   java.lang.Thread.State: RUNNABLE
at java.net.SocketInputStream.socketRead0(java.base@9-ea/Native Method)
at java.net.SocketInputStream.socketRead(java.base@9-ea/SocketInputStream.java:116)
at java.net.SocketInputStream.read(java.base@9-ea/SocketInputStream.java:171)
at java.net.SocketInputStream.read(java.base@9-ea/SocketInputStream.java:141)
at java.net.SocketInputStream.read(java.base@9-ea/SocketInputStream.java:224)
at com.sun.tools.jdi.SocketConnection.readPacket(jdk.jdi@9-ea/SocketTransportService.java:414)
- locked <0x00000000f9b80c78> (a java.lang.Object)
at com.sun.tools.jdi.TargetVM.run(jdk.jdi@9-ea/TargetVM.java:118)
at java.lang.Thread.run(java.base@9-ea/Thread.java:844)

"JDI Internal Event Handler" #201 daemon prio=5 os_prio=64 tid=0x0000000001e07000 nid=0xdc in Object.wait() [0xffff80ffb6ffa000]
   java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(java.base@9-ea/Native Method)
- waiting on <no object reference available>
at java.lang.Object.wait(java.base@9-ea/Object.java:516)
at com.sun.tools.jdi.EventQueueImpl.removeUnfiltered(jdk.jdi@9-ea/EventQueueImpl.java:189)
- waiting to re-lock in wait() <0x00000000f9b81318> (a com.sun.tools.jdi.EventQueueImpl)
at com.sun.tools.jdi.EventQueueImpl.removeInternal(jdk.jdi@9-ea/EventQueueImpl.java:124)
at com.sun.tools.jdi.InternalEventHandler.run(jdk.jdi@9-ea/InternalEventHandler.java:54)
at java.lang.Thread.run(java.base@9-ea/Thread.java:844)

"SameVMThread" #47 prio=5 os_prio=64 tid=0x00000000013f6000 nid=0x43 in Object.wait() [0xffff80ffb6bfc000]
   java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(java.base@9-ea/Native Method)
- waiting on <no object reference available>
at java.lang.Object.wait(java.base@9-ea/Object.java:516)
at jdk.jshell.execution.PipeInputStream.read(jdk.jshell@9-ea/PipeInputStream.java:53)
- waiting to re-lock in wait() <0x00000000f9b5a338> (a jdk.jshell.execution.PipeInputStream)
at jdk.jshell.execution.PipeInputStream.read(jdk.jshell@9-ea/PipeInputStream.java:75)
- waiting to re-lock in wait() <0x00000000f9b5a338> (a jdk.jshell.execution.PipeInputStream)
at java.io.ObjectInputStream$PeekInputStream.read(java.base@9-ea/ObjectInputStream.java:2709)
at java.io.ObjectInputStream$PeekInputStream.readFully(java.base@9-ea/ObjectInputStream.java:2725)
at java.io.ObjectInputStream$BlockDataInputStream.readShort(java.base@9-ea/ObjectInputStream.java:3222)
at java.io.ObjectInputStream.readStreamHeader(java.base@9-ea/ObjectInputStream.java:866)
at java.io.ObjectInputStream.<init>(java.base@9-ea/ObjectInputStream.java:343)
at jdk.jshell.execution.Util.remoteInputOutput(jdk.jshell@9-ea/Util.java:201)
at jdk.jshell.execution.JdiDefaultExecutionControl.create(jdk.jshell@9-ea/JdiDefaultExecutionControl.java:124)
at jdk.jshell.execution.JdiExecutionControlProvider.generate(jdk.jshell@9-ea/JdiExecutionControlProvider.java:148)
at jdk.jshell.spi.ExecutionControl.generate(jdk.jshell@9-ea/ExecutionControl.java:179)
at jdk.jshell.spi.ExecutionControl.generate(jdk.jshell@9-ea/ExecutionControl.java:296)
at jdk.jshell.execution.FailOverExecutionControlProvider.generate(jdk.jshell@9-ea/FailOverExecutionControlProvider.java:103)
at jdk.jshell.spi.ExecutionControl.generate(jdk.jshell@9-ea/ExecutionControl.java:179)
at jdk.jshell.spi.ExecutionControl.generate(jdk.jshell@9-ea/ExecutionControl.java:296)
at jdk.jshell.JShell.<init>(jdk.jshell@9-ea/JShell.java:136)
at jdk.jshell.JShell$Builder.build(jdk.jshell@9-ea/JShell.java:405)
at KullaTesting.setUp(KullaTesting.java:197)
at KullaTesting.setUp(KullaTesting.java:169)
at CompletionSuggestionTest.setUp(CompletionSuggestionTest.java:650)
at jdk.internal.reflect.GeneratedMethodAccessor10.invoke(Unknown Source)
at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(java.base@9-ea/DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(java.base@9-ea/Method.java:564)
at org.testng.internal.MethodInvocationHelper.invokeMethod(MethodInvocationHelper.java:85)
at org.testng.internal.Invoker.invokeConfigurationMethod(Invoker.java:510)
at org.testng.internal.Invoker.invokeConfigurations(Invoker.java:211)
at org.testng.internal.Invoker.invokeMethod(Invoker.java:585)
at org.testng.internal.Invoker.invokeTestMethod(Invoker.java:821)
at org.testng.internal.Invoker.invokeTestMethods(Invoker.java:1131)
at org.testng.internal.TestMethodWorker.invokeTestMethods(TestMethodWorker.java:124)
at org.testng.internal.TestMethodWorker.run(TestMethodWorker.java:108)
at org.testng.TestRunner.privateRun(TestRunner.java:773)
at org.testng.TestRunner.run(TestRunner.java:623)
at org.testng.SuiteRunner.runTest(SuiteRunner.java:357)
at org.testng.SuiteRunner.runSequentially(SuiteRunner.java:352)
at org.testng.SuiteRunner.privateRun(SuiteRunner.java:310)
at org.testng.SuiteRunner.run(SuiteRunner.java:259)
at org.testng.SuiteRunnerWorker.runSuite(SuiteRunnerWorker.java:52)
at org.testng.SuiteRunnerWorker.run(SuiteRunnerWorker.java:86)
at org.testng.TestNG.runSuitesSequentially(TestNG.java:1185)
at org.testng.TestNG.runSuitesLocally(TestNG.java:1110)
at org.testng.TestNG.run(TestNG.java:1018)
at com.sun.javatest.regtest.agent.TestNGRunner.main(TestNGRunner.java:94)
at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(java.base@9-ea/Native Method)
at jdk.internal.reflect.NativeMethodAccessorImpl.invoke(java.base@9-ea/NativeMethodAccessorImpl.java:62)
at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(java.base@9-ea/DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(java.base@9-ea/Method.java:564)
at com.sun.javatest.regtest.agent.MainActionHelper$SameVMRunnable.run(MainActionHelper.java:230)
at java.lang.Thread.run(java.base@9-ea/Thread.java:844)

"Thread-0" #28 daemon prio=5 os_prio=64 tid=0x0000000001341000 nid=0x30 waiting on condition [0xffff80ffb71fc000]
   java.lang.Thread.State: WAITING (parking)
at jdk.internal.misc.Unsafe.park(java.base@9-ea/Native Method)
- parking to wait for <0x00000000d0b1e5b8> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
at java.util.concurrent.locks.LockSupport.park(java.base@9-ea/LockSupport.java:194)
at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(java.base@9-ea/AbstractQueuedSynchronizer.java:2062)
at java.util.concurrent.LinkedBlockingQueue.take(java.base@9-ea/LinkedBlockingQueue.java:435)
at java.util.concurrent.ThreadPoolExecutor.getTask(java.base@9-ea/ThreadPoolExecutor.java:1086)
at java.util.concurrent.ThreadPoolExecutor.runWorker(java.base@9-ea/ThreadPoolExecutor.java:1146)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(java.base@9-ea/ThreadPoolExecutor.java:635)
at java.lang.Thread.run(java.base@9-ea/Thread.java:844)

"process reaper" #21 daemon prio=10 os_prio=64 tid=0x0000000001773000 nid=0x29 runnable [0xffff80ffbb2ae000]
   java.lang.Thread.State: RUNNABLE
at java.lang.ProcessHandleImpl.waitForProcessExit0(java.base@9-ea/Native Method)
at java.lang.ProcessHandleImpl.lambda$completion$2(java.base@9-ea/ProcessHandleImpl.java:136)
at java.lang.ProcessHandleImpl$$Lambda$60/1711473265.run(java.base@9-ea/Unknown Source)
at java.util.concurrent.ThreadPoolExecutor.runWorker(java.base@9-ea/ThreadPoolExecutor.java:1161)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(java.base@9-ea/ThreadPoolExecutor.java:635)
at java.lang.Thread.run(java.base@9-ea/Thread.java:844)

"pool-1-thread-1" #11 daemon prio=5 os_prio=64 tid=0x0000000000ba4800 nid=0x20 waiting on condition [0xffff80ffb9ef3000]
   java.lang.Thread.State: TIMED_WAITING (parking)
at jdk.internal.misc.Unsafe.park(java.base@9-ea/Native Method)
- parking to wait for <0x00000000d013d5f0> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
at java.util.concurrent.locks.LockSupport.parkNanos(java.base@9-ea/LockSupport.java:234)
at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.awaitNanos(java.base@9-ea/AbstractQueuedSynchronizer.java:2104)
at java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(java.base@9-ea/ScheduledThreadPoolExecutor.java:1131)
at java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(java.base@9-ea/ScheduledThreadPoolExecutor.java:848)
at java.util.concurrent.ThreadPoolExecutor.getTask(java.base@9-ea/ThreadPoolExecutor.java:1086)
at java.util.concurrent.ThreadPoolExecutor.runWorker(java.base@9-ea/ThreadPoolExecutor.java:1146)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(java.base@9-ea/ThreadPoolExecutor.java:635)
at java.lang.Thread.run(java.base@9-ea/Thread.java:844)

"Service Thread" #10 daemon prio=9 os_prio=64 tid=0x0000000000afd800 nid=0x1e runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Common-Cleaner" #9 daemon prio=8 os_prio=64 tid=0x0000000000960000 nid=0x1d in Object.wait() [0xffff80ffba1f6000]
   java.lang.Thread.State: TIMED_WAITING (on object monitor)
at java.lang.Object.wait(java.base@9-ea/Native Method)
- waiting on <no object reference available>
at java.lang.ref.ReferenceQueue.remove(java.base@9-ea/ReferenceQueue.java:151)
- waiting to re-lock in wait() <0x00000000d013dac8> (a java.lang.ref.ReferenceQueue$Lock)
at jdk.internal.ref.CleanerImpl.run(java.base@9-ea/CleanerImpl.java:148)
at java.lang.Thread.run(java.base@9-ea/Thread.java:844)
at jdk.internal.misc.InnocuousThread.run(java.base@9-ea/InnocuousThread.java:122)

"Sweeper thread" #8 daemon prio=9 os_prio=64 tid=0x0000000000891800 nid=0x1c runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C1 CompilerThread2" #7 daemon prio=9 os_prio=64 tid=0x0000000000887800 nid=0x1b waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE
   No compile task

"C2 CompilerThread1" #6 daemon prio=9 os_prio=64 tid=0x0000000000885800 nid=0x1a waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE
   No compile task

"C2 CompilerThread0" #5 daemon prio=9 os_prio=64 tid=0x0000000000883800 nid=0x19 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE
   No compile task

"Signal Dispatcher" #4 daemon prio=9 os_prio=64 tid=0x0000000000881800 nid=0x18 runnable [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"Finalizer" #3 daemon prio=8 os_prio=64 tid=0x000000000086d000 nid=0x17 in Object.wait() [0xffff80ffba7fc000]
   java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(java.base@9-ea/Native Method)
- waiting on <no object reference available>
at java.lang.ref.ReferenceQueue.remove(java.base@9-ea/ReferenceQueue.java:151)
- waiting to re-lock in wait() <0x00000000d013e6d0> (a java.lang.ref.ReferenceQueue$Lock)
at java.lang.ref.ReferenceQueue.remove(java.base@9-ea/ReferenceQueue.java:172)
at java.lang.ref.Finalizer$FinalizerThread.run(java.base@9-ea/Finalizer.java:216)

"Reference Handler" #2 daemon prio=10 os_prio=64 tid=0x0000000000856800 nid=0x16 waiting on condition [0xffff80ffba8fd000]
   java.lang.Thread.State: RUNNABLE
at java.lang.ref.Reference.waitForReferencePendingList(java.base@9-ea/Native Method)
at java.lang.ref.Reference.processPendingReferences(java.base@9-ea/Reference.java:174)
at java.lang.ref.Reference.access$000(java.base@9-ea/Reference.java:44)
at java.lang.ref.Reference$ReferenceHandler.run(java.base@9-ea/Reference.java:138)

"main" #1 prio=5 os_prio=64 tid=0x0000000000432800 nid=0x2 in Object.wait() [0xffff80ffbf50e000]
   java.lang.Thread.State: WAITING (on object monitor)
at java.lang.Object.wait(java.base@9-ea/Native Method)
- waiting on <no object reference available>
at java.lang.Thread.join(java.base@9-ea/Thread.java:1353)
- waiting to re-lock in wait() <0x00000000d1fc8958> (a java.lang.Thread)
at java.lang.Thread.join(java.base@9-ea/Thread.java:1427)
at com.sun.javatest.regtest.agent.MainActionHelper.runClass(MainActionHelper.java:133)
at com.sun.javatest.regtest.agent.AgentServer.doMain(AgentServer.java:238)
at com.sun.javatest.regtest.agent.AgentServer.run(AgentServer.java:184)
at com.sun.javatest.regtest.agent.AgentServer.main(AgentServer.java:60)

"VM Thread" os_prio=64 tid=0x0000000000850000 nid=0x15 runnable

"GC Thread#0" os_prio=64 tid=0x0000000000448800 nid=0x3 runnable

"GC Thread#1" os_prio=64 tid=0x0000000000449800 nid=0x4 runnable

"GC Thread#2" os_prio=64 tid=0x000000000044b000 nid=0x5 runnable

"GC Thread#3" os_prio=64 tid=0x000000000044c800 nid=0x6 runnable

"GC Thread#4" os_prio=64 tid=0x000000000044e000 nid=0x7 runnable

"GC Thread#5" os_prio=64 tid=0x000000000044f800 nid=0x8 runnable

"GC Thread#6" os_prio=64 tid=0x0000000000451000 nid=0x9 runnable

"G1 Main Marker" os_prio=64 tid=0x0000000000483800 nid=0x12 runnable

"G1 Marker#0" os_prio=64 tid=0x0000000000485000 nid=0x13 runnable

"G1 Marker#1" os_prio=64 tid=0x0000000000486800 nid=0x14 runnable

"G1 Refine#0" os_prio=64 tid=0x000000000045c800 nid=0x10 runnable

"G1 Refine#1" os_prio=64 tid=0x000000000045b000 nid=0xf runnable

"G1 Refine#2" os_prio=64 tid=0x0000000000459000 nid=0xe runnable

"G1 Refine#3" os_prio=64 tid=0x0000000000457800 nid=0xd runnable

"G1 Refine#4" os_prio=64 tid=0x0000000000456000 nid=0xc runnable

"G1 Refine#5" os_prio=64 tid=0x0000000000454800 nid=0xb runnable

"G1 Refine#6" os_prio=64 tid=0x0000000000453000 nid=0xa runnable

"G1 Young RemSet Sampling" os_prio=64 tid=0x000000000045e000 nid=0x11 runnable

"VM Periodic Task Thread" os_prio=64 tid=0x0000000000b21000 nid=0x1f waiting on condition

JNI global references: 1820