Index: libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.guard/adopt_lock.pass.cpp
===================================================================
--- libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.guard/adopt_lock.pass.cpp
+++ libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.guard/adopt_lock.pass.cpp
@@ -18,6 +18,7 @@
 #include <cstdlib>
 #include <cassert>
 
+#include "make_test_thread.h"
 #include "test_macros.h"
 
 std::mutex m;
@@ -26,7 +27,10 @@
   {
     m.lock();
     std::lock_guard<std::mutex> lg(m, std::adopt_lock);
-    assert(m.try_lock() == false);
+    std::thread t = support::make_test_thread([](){
+      assert(m.try_lock() == false);
+    });
+    t.join();
   }
 
   m.lock();
Index: libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.guard/mutex.pass.cpp
===================================================================
--- libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.guard/mutex.pass.cpp
+++ libcxx/test/std/thread/thread.mutex/thread.lock/thread.lock.guard/mutex.pass.cpp
@@ -21,6 +21,7 @@
 #include <cstdlib>
 #include <cassert>
 
+#include "make_test_thread.h"
 #include "test_macros.h"
 
 std::mutex m;
@@ -28,7 +29,10 @@
 int main(int, char**) {
   {
     std::lock_guard<std::mutex> lg(m);
-    assert(m.try_lock() == false);
+    std::thread t = support::make_test_thread([](){
+      assert(m.try_lock() == false);
+    });
+    t.join();
   }
 
   m.lock();