It's possible for deadlock to occur when an app thread holds a lock and then schedules a blocking compilation where the JVMCI compiler thread then tries to acquire the lock:

"JVMCI CompilerThread7" #12 daemon prio=9 os_prio=0 tid=0x00007f69540e7000 nid=0x45c0 waiting for monitor entry [0x00007f613897a000]
   java.lang.Thread.State: BLOCKED (on object monitor)
at java.util.Hashtable.get(Hashtable.java:362)
- waiting to lock <0x00007f6140051ae0> (a java.util.Hashtable)
at java.net.URL.getURLStreamHandler(URL.java:1128)
at java.net.URL.<init>(URL.java:592)
at java.net.URL.<init>(URL.java:483)
at java.net.URL.<init>(URL.java:432)
at sun.security.provider.NativePRNG.getEgdUrl(NativePRNG.java:109)
at sun.security.provider.NativePRNG.access$000(NativePRNG.java:74)
at sun.security.provider.NativePRNG$1.run(NativePRNG.java:140)
at sun.security.provider.NativePRNG$1.run(NativePRNG.java:128)
at java.security.AccessController.doPrivileged(Native Method)
at sun.security.provider.NativePRNG.initIO(NativePRNG.java:127)
at sun.security.provider.NativePRNG.<clinit>(NativePRNG.java:91)
at sun.security.provider.SunEntries.putEntries(SunEntries.java:96)
at sun.security.provider.Sun.<init>(Sun.java:56)
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:422)
at java.lang.Class.newInstance(Class.java:442)
at sun.security.jca.ProviderConfig$2.run(ProviderConfig.java:221)
at sun.security.jca.ProviderConfig$2.run(ProviderConfig.java:206)
at java.security.AccessController.doPrivileged(Native Method)
at sun.security.jca.ProviderConfig.doLoadProvider(ProviderConfig.java:206)
at sun.security.jca.ProviderConfig.getProvider(ProviderConfig.java:187)
- locked <0x00007f61470cf4d0> (a sun.security.jca.ProviderConfig)
at sun.security.jca.ProviderList.getProvider(ProviderList.java:233)
at sun.security.jca.ProviderList.getService(ProviderList.java:331)
at sun.security.jca.GetInstance.getInstance(GetInstance.java:157)
at java.security.cert.CertificateFactory.getInstance(CertificateFactory.java:183)
at sun.security.pkcs.PKCS7.parseSignedData(PKCS7.java:300)
at sun.security.pkcs.PKCS7.parse(PKCS7.java:186)
at sun.security.pkcs.PKCS7.parse(PKCS7.java:154)
at sun.security.pkcs.PKCS7.<init>(PKCS7.java:136)
at sun.security.util.SignatureFileVerifier.<init>(SignatureFileVerifier.java:95)
at java.util.jar.JarVerifier.processEntry(JarVerifier.java:297)
at java.util.jar.JarVerifier.update(JarVerifier.java:228)
at java.util.jar.JarFile.initializeVerifier(JarFile.java:383)
at java.util.jar.JarFile.getInputStream(JarFile.java:450)
- locked <0x00007f6147021310> (a java.util.jar.JarFile)
at sun.misc.URLClassPath$JarLoader$2.getInputStream(URLClassPath.java:940)
at sun.misc.Resource.cachedInputStream(Resource.java:77)
- locked <0x00007f6147022fb8> (a sun.misc.URLClassPath$JarLoader$2)
at sun.misc.Resource.getByteBuffer(Resource.java:160)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:454)
at java.net.URLClassLoader.access$100(URLClassLoader.java:73)
at java.net.URLClassLoader$1.run(URLClassLoader.java:368)
at java.net.URLClassLoader$1.run(URLClassLoader.java:362)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:361)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
- locked <0x00007f6147020da0> (a java.lang.Object)
at java.lang.ClassLoader.loadClass(ClassLoader.java:411)
- locked <0x00007f6147020cd0> (a java.lang.Object)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:348)
at com.oracle.graal.graphbuilderconf.MethodSubstitutionPlugin.resolveClass(MethodSubstitutionPlugin.java:172)
at com.oracle.graal.hotspot.meta.HotSpotGraphBuilderPlugins.registerAESPlugins(HotSpotGraphBuilderPlugins.java:296)
at com.oracle.graal.hotspot.meta.HotSpotGraphBuilderPlugins.create(HotSpotGraphBuilderPlugins.java:124)
at com.oracle.graal.hotspot.amd64.AMD64HotSpotBackendFactory.createGraphBuilderPlugins(AMD64HotSpotBackendFactory.java:141)
at com.oracle.graal.hotspot.amd64.AMD64HotSpotBackendFactory.createBackend(AMD64HotSpotBackendFactory.java:125)
at com.oracle.graal.hotspot.HotSpotGraalRuntime.<init>(HotSpotGraalRuntime.java:109)
at com.oracle.graal.hotspot.HotSpotGraalCompilerFactory.createCompiler(HotSpotGraalCompilerFactory.java:75)
at com.oracle.graal.hotspot.HotSpotGraalCompilerFactory.createCompiler(HotSpotGraalCompilerFactory.java:38)
at jdk.vm.ci.hotspot.HotSpotJVMCIRuntime.getCompiler(HotSpotJVMCIRuntime.java:184)
- locked <0x00007f614392d850> (a jdk.vm.ci.hotspot.HotSpotJVMCIRuntime)
at jdk.vm.ci.hotspot.HotSpotJVMCIRuntime.compileMethod(HotSpotJVMCIRuntime.java:228)

"main" #1 prio=5 os_prio=0 tid=0x00007f695400a000 nid=0x45b4 waiting on condition [0x00007f695b97e000]
   java.lang.Thread.State: RUNNABLE
at java.lang.String.equals(String.java:965)
at java.util.Hashtable.get(Hashtable.java:366)
- locked <0x00007f6140051ae0> (a java.util.Hashtable)
at java.net.URL.getURLStreamHandler(URL.java:1128)
at java.net.URL.<init>(URL.java:413)
at java.net.URL.<init>(URL.java:303)
at java.net.URL.<init>(URL.java:326)
at java.io.File.toURL(File.java:686)
at org.dacapo.harness.DacapoClassLoader.getJars(DacapoClassLoader.java:105)
at org.dacapo.harness.DacapoClassLoader.create(DacapoClassLoader.java:46)
at org.dacapo.harness.Benchmark.initialize(Benchmark.java:212)
at org.dacapo.harness.Benchmark.<init>(Benchmark.java:184)
at org.dacapo.harness.Benchmark.<init>(Benchmark.java:194)
at org.dacapo.harness.Pmd.<init>(Pmd.java:31)
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:422)
at org.dacapo.harness.TestHarness.runBenchmark(TestHarness.java:211)
at org.dacapo.harness.TestHarness.main(TestHarness.java:171)
at Harness.main(Harness.java:17)

To avoid this, we need to add a timeout to the wait performed by the app thread (i.e. in CompileBroker::wait_for_completion). The trickiest part is deciding which thread frees the CompileTask.