8346306: Unattached thread can cause crash during VM exit if it calls wait_if_vm_exited

Reviewed-by: coleenp, ccheung

Author: David Holmes

src/hotspot/share/prims/jvm.cpp

@@ -3319,8 +3319,9 @@ JVM_END
 // VM Raw monitors (not to be confused with JvmtiRawMonitors) are a simple mutual exclusion
 // lock (not actually monitors: no wait/notify) that is exported by the VM for use by JDK
 // library code. They may be used by JavaThreads and non-JavaThreads and do not participate
-// in the safepoint protocol, thread suspension, thread interruption, or anything of that
-// nature. JavaThreads will be "in native" when using this API from JDK code.
+// in the safepoint protocol, thread suspension, thread interruption, or most things of that
+// nature, except JavaThreads will be blocked by VM_Exit::block_if_vm_exited if the VM has
+// shutdown. JavaThreads will be "in native" when using this API from JDK code.
 
 
 JNIEXPORT void* JNICALL JVM_RawMonitorCreate(void) {

src/hotspot/share/runtime/vmOperations.cpp

@@ -611,12 +611,16 @@ void VM_Exit::doit() {
 
 
 void VM_Exit::wait_if_vm_exited() {
-  if (_vm_exited &&
-      Thread::current_or_null() != _shutdown_thread) {
-    // _vm_exited is set at safepoint, and the Threads_lock is never released
-    // so we will block here until the process dies.
-    Threads_lock->lock();
-    ShouldNotReachHere();
+  if (_vm_exited) {
+    // Need to check for an unattached thread as only attached threads
+    // can acquire the lock.
+    Thread* current = Thread::current_or_null();
+    if (current != nullptr && current != _shutdown_thread) {
+      // _vm_exited is set at safepoint, and the Threads_lock is never released
+      // so we will block here until the process dies.
+      Threads_lock->lock();
+      ShouldNotReachHere();
+    }
   }
 }