From 4d0da18ab6e83549e81074e15011cf8a4fbd4ea9 Mon Sep 17 00:00:00 2001
From: David Holmes <dholmes@openjdk.org>
Date: Wed, 8 Oct 2025 20:28:21 +0000
Subject: [PATCH] 8369250: Assess and remedy any unsafe usage of the Semaphore
 used by NonJavaThread::List

Reviewed-by: kbarrett, stefank
---
 src/hotspot/share/runtime/nonJavaThread.cpp | 21 ++++++++++++---------
 src/hotspot/share/runtime/nonJavaThread.hpp | 12 +++++++++---
 src/hotspot/share/runtime/threads.cpp       |  3 +++
 3 files changed, 24 insertions(+), 12 deletions(-)

diff --git a/src/hotspot/share/runtime/nonJavaThread.cpp b/src/hotspot/share/runtime/nonJavaThread.cpp
index 2fb4c2dce02..cb0c3f8910d 100644
--- a/src/hotspot/share/runtime/nonJavaThread.cpp
+++ b/src/hotspot/share/runtime/nonJavaThread.cpp
@@ -50,15 +50,19 @@ public:
   List() : _head(nullptr), _protect() {}
 };
 
-NonJavaThread::List NonJavaThread::_the_list;
+DeferredStatic<NonJavaThread::List> NonJavaThread::_the_list;
+
+void NonJavaThread::init() {
+  _the_list.initialize();
+}
 
 NonJavaThread::Iterator::Iterator() :
-  _protect_enter(_the_list._protect.enter()),
-  _current(AtomicAccess::load_acquire(&_the_list._head))
+  _protect_enter(_the_list->_protect.enter()),
+  _current(AtomicAccess::load_acquire(&_the_list->_head))
 {}
 
 NonJavaThread::Iterator::~Iterator() {
-  _the_list._protect.exit(_protect_enter);
+  _the_list->_protect.exit(_protect_enter);
 }
 
 void NonJavaThread::Iterator::step() {
@@ -76,8 +80,8 @@ void NonJavaThread::add_to_the_list() {
   MutexLocker ml(NonJavaThreadsList_lock, Mutex::_no_safepoint_check_flag);
   // Initialize BarrierSet-related data before adding to list.
   BarrierSet::barrier_set()->on_thread_attach(this);
-  AtomicAccess::release_store(&_next, _the_list._head);
-  AtomicAccess::release_store(&_the_list._head, this);
+  AtomicAccess::release_store(&_next, _the_list->_head);
+  AtomicAccess::release_store(&_the_list->_head, this);
 }
 
 void NonJavaThread::remove_from_the_list() {
@@ -85,7 +89,7 @@ void NonJavaThread::remove_from_the_list() {
     MutexLocker ml(NonJavaThreadsList_lock, Mutex::_no_safepoint_check_flag);
     // Cleanup BarrierSet-related data before removing from list.
     BarrierSet::barrier_set()->on_thread_detach(this);
-    NonJavaThread* volatile* p = &_the_list._head;
+    NonJavaThread* volatile* p = &_the_list->_head;
     for (NonJavaThread* t = *p; t != nullptr; p = &t->_next, t = *p) {
       if (t == this) {
         *p = _next;
@@ -97,7 +101,7 @@ void NonJavaThread::remove_from_the_list() {
   // allowed, so do it while holding a dedicated lock.  Outside and distinct
   // from NJTList_lock in case an iteration attempts to lock it.
   MutexLocker ml(NonJavaThreadsListSync_lock, Mutex::_no_safepoint_check_flag);
-  _the_list._protect.synchronize();
+  _the_list->_protect.synchronize();
   _next = nullptr;                 // Safe to drop the link now.
 }
 
@@ -344,4 +348,3 @@ void WatcherThread::print_on(outputStream* st) const {
   Thread::print_on(st);
   st->cr();
 }
-
diff --git a/src/hotspot/share/runtime/nonJavaThread.hpp b/src/hotspot/share/runtime/nonJavaThread.hpp
index b2ac6a31737..aaf0dcdfa2d 100644
--- a/src/hotspot/share/runtime/nonJavaThread.hpp
+++ b/src/hotspot/share/runtime/nonJavaThread.hpp
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2021, 2023, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2021, 2025, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -26,12 +26,18 @@
 #define SHARE_RUNTIME_NONJAVATHREAD_HPP
 
 #include "runtime/thread.hpp"
+#include "utilities/deferredStatic.hpp"
 
 class NonJavaThread: public Thread {
-  NonJavaThread* volatile _next;
+  friend class Threads;
 
   class List;
-  static List _the_list;
+  static DeferredStatic<List> _the_list;
+
+  // Deferred static initialization
+  static void init();
+
+  NonJavaThread* volatile _next;
 
   void add_to_the_list();
   void remove_from_the_list();
diff --git a/src/hotspot/share/runtime/threads.cpp b/src/hotspot/share/runtime/threads.cpp
index d098343c354..ffe1a86cda5 100644
--- a/src/hotspot/share/runtime/threads.cpp
+++ b/src/hotspot/share/runtime/threads.cpp
@@ -446,6 +446,9 @@ jint Threads::create_vm(JavaVMInitArgs* args, bool* canTryAgain) {
   // Check version
   if (!is_supported_jni_version(args->version)) return JNI_EVERSION;
 
+  // Deferred "static" initialization
+  NonJavaThread::init();
+
   // Initialize library-based TLS
   ThreadLocalStorage::init();