diff --git a/stl/inc/yvals.h b/stl/inc/yvals.h
index d3b61ec03af..9428b35aaeb 100644
--- a/stl/inc/yvals.h
+++ b/stl/inc/yvals.h
@@ -309,10 +309,9 @@ _EMIT_STL_WARNING(STL4001, "/clr:pure is deprecated and will be REMOVED.");
 #endif
 #endif // !defined(_CRTDATA2_IMPORT)
 
-#define _LOCK_LOCALE         0
-#define _LOCK_STREAM         2
-#define _LOCK_DEBUG          3
-#define _LOCK_AT_THREAD_EXIT 4
+#define _LOCK_LOCALE 0
+#define _LOCK_STREAM 2
+#define _LOCK_DEBUG  3
 
 #ifndef _STD_ATOMIC_ALWAYS_USE_CMPXCHG16B
 #if _STL_WIN32_WINNT >= _STL_WIN32_WINNT_WINBLUE && defined(_WIN64)
diff --git a/stl/src/xlock.cpp b/stl/src/xlock.cpp
index 226f8229f1c..1aea80d1e2d 100644
--- a/stl/src/xlock.cpp
+++ b/stl/src/xlock.cpp
@@ -13,7 +13,7 @@
 
 _STD_BEGIN
 
-constexpr int _Max_lock = 8; // must be power of two
+constexpr int _Max_lock = 8; // must be power of two; TRANSITION, ABI: may be less now
 
 #pragma warning(disable : 4074)
 #pragma init_seg(compiler)
@@ -121,14 +121,4 @@ void __cdecl _Lockit::_Lockit_dtor(int kind) noexcept { // unlock the mutex
         _Mtxunlock(&mtx[kind & (_Max_lock - 1)]);
     }
 }
-
-extern "C" {
-void _Lock_at_thread_exit_mutex() noexcept { // lock the at-thread-exit mutex
-    _Mtxlock(&mtx[_LOCK_AT_THREAD_EXIT]);
-}
-void _Unlock_at_thread_exit_mutex() noexcept { // unlock the at-thread-exit mutex
-    _Mtxunlock(&mtx[_LOCK_AT_THREAD_EXIT]);
-}
-} // extern "C"
-
 _STD_END
diff --git a/stl/src/xnotify.cpp b/stl/src/xnotify.cpp
index 1b6e8fcbdbf..96aebb485b2 100644
--- a/stl/src/xnotify.cpp
+++ b/stl/src/xnotify.cpp
@@ -2,6 +2,7 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 #include <cstdlib>
+#include <mutex>
 #include <xthreads.h>
 
 #include <Windows.h>
@@ -23,20 +24,19 @@ namespace {
     };
 
     _At_thread_exit_block _Thread_exit_data;
+
+    constinit std::mutex _Thread_exit_data_mutex;
 } // unnamed namespace
 
 extern "C" {
 
-void _Lock_at_thread_exit_mutex() noexcept;
-void _Unlock_at_thread_exit_mutex() noexcept;
-
 _CRTIMP2_PURE void __cdecl _Cnd_register_at_thread_exit(_Cnd_t cnd, _Mtx_t mtx, int* p) noexcept {
     // register condition variable and mutex for cleanup at thread exit
 
     // find block with available space
     _At_thread_exit_block* block = &_Thread_exit_data;
 
-    _Lock_at_thread_exit_mutex();
+    std::lock_guard _Lock{_Thread_exit_data_mutex};
     while (block != nullptr) { // loop through list of blocks
         if (block->num_used == _Nitems) { // block is full; move to next block and allocate
             if (block->next == nullptr) {
@@ -58,7 +58,6 @@ _CRTIMP2_PURE void __cdecl _Cnd_register_at_thread_exit(_Cnd_t cnd, _Mtx_t mtx,
             block = nullptr;
         }
     }
-    _Unlock_at_thread_exit_mutex();
 }
 
 _CRTIMP2_PURE void __cdecl _Cnd_unregister_at_thread_exit(_Mtx_t mtx) noexcept {
@@ -67,7 +66,7 @@ _CRTIMP2_PURE void __cdecl _Cnd_unregister_at_thread_exit(_Mtx_t mtx) noexcept {
     // find condition variables waiting for this thread to exit
     _At_thread_exit_block* block = &_Thread_exit_data;
 
-    _Lock_at_thread_exit_mutex();
+    std::lock_guard _Lock{_Thread_exit_data_mutex};
     while (block != nullptr) { // loop through list of blocks
         for (int i = 0; block->num_used != 0 && i < _Nitems; ++i) {
             if (block->data[i].mtx == mtx) { // release slot
@@ -78,7 +77,6 @@ _CRTIMP2_PURE void __cdecl _Cnd_unregister_at_thread_exit(_Mtx_t mtx) noexcept {
 
         block = block->next;
     }
-    _Unlock_at_thread_exit_mutex();
 }
 
 _CRTIMP2_PURE void __cdecl _Cnd_do_broadcast_at_thread_exit() noexcept {
@@ -88,7 +86,7 @@ _CRTIMP2_PURE void __cdecl _Cnd_do_broadcast_at_thread_exit() noexcept {
     _At_thread_exit_block* block       = &_Thread_exit_data;
     const unsigned int currentThreadId = _Thrd_id();
 
-    _Lock_at_thread_exit_mutex();
+    std::lock_guard _Lock{_Thread_exit_data_mutex};
     while (block != nullptr) { // loop through list of blocks
         for (int i = 0; block->num_used != 0 && i < _Nitems; ++i) {
             if (block->data[i].mtx != nullptr && block->data[i].id._Id == currentThreadId) { // notify and release slot
@@ -104,7 +102,6 @@ _CRTIMP2_PURE void __cdecl _Cnd_do_broadcast_at_thread_exit() noexcept {
 
         block = block->next;
     }
-    _Unlock_at_thread_exit_mutex();
 }
 
 } // extern "C"