diff --git a/test/libcxx/test/config.py b/test/libcxx/test/config.py
index 21e54bb7f..06d3a7bdd 100644
--- a/test/libcxx/test/config.py
+++ b/test/libcxx/test/config.py
@@ -226,6 +226,7 @@ class Configuration(object):
                 'unsupported value for "cxx_stdlib_under_test": %s'
                 % self.cxx_stdlib_under_test)
         if self.cxx_stdlib_under_test == 'libstdc++':
+            self.config.available_features.add('libstdc++')
             # Manually enable the experimental and filesystem tests for libstdc++
             # if the options aren't present.
             # FIXME this is a hack.
diff --git a/test/std/thread/thread.mutex/thread.lock.algorithm/lock.pass.cpp b/test/std/thread/thread.mutex/thread.lock.algorithm/lock.pass.cpp
index eac7600b5..9e20144e8 100644
--- a/test/std/thread/thread.mutex/thread.lock.algorithm/lock.pass.cpp
+++ b/test/std/thread/thread.mutex/thread.lock.algorithm/lock.pass.cpp
@@ -10,6 +10,11 @@
 // XFAIL: libcpp-no-exceptions
 // UNSUPPORTED: libcpp-has-no-threads
 
+// This test hangs forever when built against libstdc++. In order to allow
+// validation of the test suite against other STLs we have to mark it
+// unsupported.
+// UNSUPPORTED: libstdc++
+
 // <mutex>
 
 // template <class L1, class L2, class... L3>