Index: lib/CMakeLists.txt
===================================================================
--- lib/CMakeLists.txt
+++ lib/CMakeLists.txt
@@ -42,6 +42,8 @@
     set(LIBFILE "libclang_rt.asan_osx_dynamic.dylib")
   elseif("${LLVM_USE_SANITIZER}" STREQUAL "Undefined")
     set(LIBFILE "libclang_rt.ubsan_osx_dynamic.dylib")
+  elseif("${LLVM_USE_SANITIZER}" STREQUAL "Thread")
+    set(LIBFILE "libclang_rt.tsan_osx_dynamic.dylib")
   else()
     message(WARNING "LLVM_USE_SANITIZER=${LLVM_USE_SANITIZER} is not supported on OS X")
   endif()
Index: test/libcxx/test/target_info.py
===================================================================
--- test/libcxx/test/target_info.py
+++ test/libcxx/test/target_info.py
@@ -115,7 +115,7 @@
         return False
 
     def add_sanitizer_features(self, sanitizer_type, features):
-        if san == 'Undefined':
+        if sanitizer_type == 'Undefined':
             features.add('sanitizer-new-delete')
 
 
Index: test/std/thread/thread.mutex/thread.once/thread.once.callonce/race.pass.cpp
===================================================================
--- test/std/thread/thread.mutex/thread.once/thread.once.callonce/race.pass.cpp
+++ test/std/thread/thread.mutex/thread.once/thread.once.callonce/race.pass.cpp
@@ -0,0 +1,48 @@
+//===----------------------------------------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// UNSUPPORTED: libcpp-has-no-threads
+
+// <mutex>
+
+// struct once_flag;
+
+// template<class Callable, class ...Args>
+//   void call_once(once_flag& flag, Callable&& func, Args&&... args);
+
+// This test is supposed to be run with ThreadSanitizer and verifies that
+// call_once propertly synchronizes user state, a data race that was fixed
+// in https://reviews.llvm.org/D24028.
+
+#include <mutex>
+#include <thread>
+#include <cassert>
+
+std::once_flag flg0;
+long global = 0;
+
+void init0()
+{
+    ++global;
+}
+
+void f0()
+{
+    std::call_once(flg0, init0);
+    assert(global == 1);
+}
+
+int main()
+{
+    std::thread t0(f0);
+    std::thread t1(f0);
+    t0.join();
+    t1.join();
+    assert(global == 1);
+}