8235913: ThreadStop should be a handshake

Reviewed-by: dholmes, mdoerr

src/hotspot/share/runtime/thread.cpp

@@ -526,9 +526,25 @@ void Thread::start(Thread* thread) {
   }
 }
 
+class InstallAsyncExceptionClosure : public HandshakeClosure {
+  Handle _throwable; // The Throwable thrown at the target Thread
+public:
+  InstallAsyncExceptionClosure(Handle throwable) : HandshakeClosure("InstallAsyncException"), _throwable(throwable) {}
+
+  void do_thread(Thread* thr) {
+    JavaThread* target = (JavaThread*)thr;
+    // Note that this now allows multiple ThreadDeath exceptions to be
+    // thrown at a thread.
+    // The target thread has run and has not exited yet.
+    target->send_thread_stop(_throwable());
+  }
+};
+
 void Thread::send_async_exception(oop java_thread, oop java_throwable) {
-  VM_ThreadStop vm_stop(java_thread, java_throwable);
-  VMThread::execute(&vm_stop);
+  Handle throwable(Thread::current(), java_throwable);
+  JavaThread* target = java_lang_Thread::thread(java_thread);
+  InstallAsyncExceptionClosure vm_stop(throwable);
+  Handshake::execute(&vm_stop, target);
 }
 
 
@@ -2383,9 +2399,8 @@ void JavaThread::handle_special_runtime_exit_condition(bool check_asyncs) {
 }
 
 void JavaThread::send_thread_stop(oop java_throwable)  {
-  assert(Thread::current()->is_VM_thread(), "should be in the vm thread");
-  assert(Threads_lock->is_locked(), "Threads_lock should be locked by safepoint code");
-  assert(SafepointSynchronize::is_at_safepoint(), "all threads are stopped");
+  ResourceMark rm;
+  assert(Thread::current()->is_VM_thread() || Thread::current() == this, "should be in the vm thread");
 
   // Do not throw asynchronous exceptions against the compiler thread
   // (the compiler thread should not be a Java thread -- fix in 1.4.2)