8305670: Performance regression in LockSupport.unpark with lots of idle threads

Co-authored-by: Robbin Ehn <rehn@openjdk.org>
Reviewed-by: rehn, dholmes

src/hotspot/share/prims/unsafe.cpp

@@ -783,21 +783,22 @@ UNSAFE_ENTRY(void, Unsafe_Park(JNIEnv *env, jobject unsafe, jboolean isAbsolute,
 
 UNSAFE_ENTRY(void, Unsafe_Unpark(JNIEnv *env, jobject unsafe, jobject jthread)) {
   if (jthread != nullptr) {
-    ThreadsListHandle tlh;
-    JavaThread* thr = nullptr;
-    oop java_thread = nullptr;
-    (void) tlh.cv_internal_thread_to_JavaThread(jthread, &thr, &java_thread);
-    if (java_thread != nullptr) {
-      // This is a valid oop.
-      if (thr != nullptr) {
-        // The JavaThread is alive.
-        Parker* p = thr->parker();
-        HOTSPOT_THREAD_UNPARK((uintptr_t) p);
-        p->unpark();
-      }
+    oop thread_oop = JNIHandles::resolve_non_null(jthread);
+    // Get the JavaThread* stored in the java.lang.Thread object _before_
+    // the embedded ThreadsListHandle is constructed so we know if the
+    // early life stage of the JavaThread* is protected. We use acquire
+    // here to ensure that if we see a non-nullptr value, then we also
+    // see the main ThreadsList updates from the JavaThread* being added.
+    FastThreadsListHandle ftlh(thread_oop, java_lang_Thread::thread_acquire(thread_oop));
+    JavaThread* thr = ftlh.protected_java_thread();
+    if (thr != nullptr) {
+      // The still live JavaThread* is protected by the FastThreadsListHandle
+      // so it is safe to access.
+      Parker* p = thr->parker();
+      HOTSPOT_THREAD_UNPARK((uintptr_t) p);
+      p->unpark();
     }
-  } // ThreadsListHandle is destroyed here.
-
+  } // FastThreadsListHandle is destroyed here.
 } UNSAFE_END
 
 UNSAFE_ENTRY(jint, Unsafe_GetLoadAverage0(JNIEnv *env, jobject unsafe, jdoubleArray loadavg, jint nelem)) {

src/hotspot/share/runtime/threadSMR.cpp

@@ -839,6 +839,20 @@ bool ThreadsListHandle::cv_internal_thread_to_JavaThread(jobject jthread,
   return true;
 }
 
+FastThreadsListHandle::FastThreadsListHandle(oop thread_oop, JavaThread* java_thread) : _protected_java_thread(nullptr) {
+  assert(thread_oop != nullptr, "must be");
+  if (java_thread != nullptr) {
+    // We captured a non-nullptr JavaThread* before the _tlh was created
+    // so that covers the early life stage of the target JavaThread.
+    _protected_java_thread = java_lang_Thread::thread(thread_oop);
+    assert(_protected_java_thread == nullptr || _tlh.includes(_protected_java_thread), "must be");
+    // If we captured a non-nullptr JavaThread* after the _tlh was created
+    // then that covers the end life stage of the target JavaThread and we
+    // we know that _tlh protects the JavaThread*. The underlying atomic
+    // load is sufficient (no acquire necessary here).
+  }
+}
+
 void ThreadsSMRSupport::add_thread(JavaThread *thread){
   ThreadsList *new_list = ThreadsList::add_thread(get_java_thread_list(), thread);
   if (EnableThreadSMRStatistics) {

src/hotspot/share/runtime/threadSMR.hpp

@@ -333,6 +333,29 @@ public:
   }
 };
 
+// This stack allocated FastThreadsListHandle implements the special case
+// where we want to quickly determine if a JavaThread* is protected by the
+// embedded ThreadsListHandle.
+//
+class FastThreadsListHandle : public StackObj {
+  JavaThread* _protected_java_thread;
+  ThreadsListHandle _tlh;
+
+public:
+  // The 'java_thread' parameter to the constructor must be provided
+  // by a java_lang_Thread::thread_acquire(thread_oop) call which gets
+  // us the JavaThread* stored in the java.lang.Thread object _before_
+  // the embedded ThreadsListHandle is constructed. We use acquire there
+  // to ensure that if we see a non-nullptr value, then we also see the
+  // main ThreadsList updates from the JavaThread* being added.
+  //
+  FastThreadsListHandle(oop thread_oop, JavaThread* java_thread);
+
+  JavaThread* protected_java_thread() {
+    return _protected_java_thread;
+  }
+};
+
 // This stack allocated JavaThreadIterator is used to walk the
 // specified ThreadsList using the following style:
 //