Index: include/__threading_support
===================================================================
--- include/__threading_support
+++ include/__threading_support
@@ -40,14 +40,30 @@
 #define _LIBCPP_THREAD_ABI_VISIBILITY inline _LIBCPP_INLINE_VISIBILITY
 #endif
 
+#if defined(__FreeBSD__) && !defined(_LIBCPP_DISABLE_THREAD_SAFETY_ATTRIBUTE) && \
+    !defined(_LIBCPP_ENABLE_THREAD_SAFETY_ATTRIBUTE)
+# define _LIBCPP_ENABLE_THREAD_SAFETY_ATTRIBUTE
+#endif
+
+#ifndef _LIBCPP_THREAD_SAFETY_ATTRIBUTE
+# if defined(__clang__) && __has_attribute(acquire_capability) && \
+     defined(_LIBCPP_ENABLE_THREAD_SAFETY_ATTRIBUTE)
+#  define _LIBCPP_THREAD_SAFETY_ATTRIBUTE(x) __attribute__((x))
+# else
+#  define _LIBCPP_THREAD_SAFETY_ATTRIBUTE(x)
+# endif
+#endif
+
 _LIBCPP_BEGIN_NAMESPACE_STD
 
 #if defined(_LIBCPP_HAS_THREAD_API_PTHREAD)
 // Mutex
-typedef pthread_mutex_t __libcpp_mutex_t;
+typedef pthread_mutex_t __libcpp_mutex_t
+_LIBCPP_THREAD_SAFETY_ATTRIBUTE(capability("mutex"));
 #define _LIBCPP_MUTEX_INITIALIZER PTHREAD_MUTEX_INITIALIZER
 
-typedef pthread_mutex_t __libcpp_recursive_mutex_t;
+typedef pthread_mutex_t __libcpp_recursive_mutex_t
+_LIBCPP_THREAD_SAFETY_ATTRIBUTE(capability("mutex"));
 
 // Condition Variable
 typedef pthread_cond_t __libcpp_condvar_t;
@@ -71,10 +87,12 @@
 #define _LIBCPP_TLS_DESTRUCTOR_CC
 #else
 // Mutex
-typedef SRWLOCK __libcpp_mutex_t;
+typedef SRWLOCK __libcpp_mutex_t
+_LIBCPP_THREAD_SAFETY_ATTRIBUTE(capability("mutex"));
 #define _LIBCPP_MUTEX_INITIALIZER SRWLOCK_INIT
 
-typedef CRITICAL_SECTION __libcpp_recursive_mutex_t;
+typedef CRITICAL_SECTION __libcpp_recursive_mutex_t
+_LIBCPP_THREAD_SAFETY_ATTRIBUTE(capability("mutex"));
 
 // Condition Variable
 typedef CONDITION_VARIABLE __libcpp_condvar_t;
@@ -103,25 +121,31 @@
 int __libcpp_recursive_mutex_init(__libcpp_recursive_mutex_t *__m);
 
 _LIBCPP_THREAD_ABI_VISIBILITY
-int __libcpp_recursive_mutex_lock(__libcpp_recursive_mutex_t *__m);
+int __libcpp_recursive_mutex_lock(__libcpp_recursive_mutex_t *__m)
+_LIBCPP_THREAD_SAFETY_ATTRIBUTE(acquire_capability(*__m));
 
 _LIBCPP_THREAD_ABI_VISIBILITY
-bool __libcpp_recursive_mutex_trylock(__libcpp_recursive_mutex_t *__m);
+bool __libcpp_recursive_mutex_trylock(__libcpp_recursive_mutex_t *__m)
+_LIBCPP_THREAD_SAFETY_ATTRIBUTE(try_acquire_capability(true, *__m));
 
 _LIBCPP_THREAD_ABI_VISIBILITY
-int __libcpp_recursive_mutex_unlock(__libcpp_recursive_mutex_t *__m);
+int __libcpp_recursive_mutex_unlock(__libcpp_recursive_mutex_t *__m)
+_LIBCPP_THREAD_SAFETY_ATTRIBUTE(release_capability(*__m));
 
 _LIBCPP_THREAD_ABI_VISIBILITY
 int __libcpp_recursive_mutex_destroy(__libcpp_recursive_mutex_t *__m);
 
 _LIBCPP_THREAD_ABI_VISIBILITY
-int __libcpp_mutex_lock(__libcpp_mutex_t *__m);
+int __libcpp_mutex_lock(__libcpp_mutex_t *__m)
+_LIBCPP_THREAD_SAFETY_ATTRIBUTE(acquire_capability(*__m));
 
 _LIBCPP_THREAD_ABI_VISIBILITY
-bool __libcpp_mutex_trylock(__libcpp_mutex_t *__m);
+bool __libcpp_mutex_trylock(__libcpp_mutex_t *__m)
+_LIBCPP_THREAD_SAFETY_ATTRIBUTE(try_acquire_capability(true, *__m));
 
 _LIBCPP_THREAD_ABI_VISIBILITY
-int __libcpp_mutex_unlock(__libcpp_mutex_t *__m);
+int __libcpp_mutex_unlock(__libcpp_mutex_t *__m)
+_LIBCPP_THREAD_SAFETY_ATTRIBUTE(release_capability(*__m));
 
 _LIBCPP_THREAD_ABI_VISIBILITY
 int __libcpp_mutex_destroy(__libcpp_mutex_t *__m);
@@ -134,11 +158,13 @@
 int __libcpp_condvar_broadcast(__libcpp_condvar_t* __cv);
 
 _LIBCPP_THREAD_ABI_VISIBILITY
-int __libcpp_condvar_wait(__libcpp_condvar_t* __cv, __libcpp_mutex_t* __m);
+int __libcpp_condvar_wait(__libcpp_condvar_t* __cv, __libcpp_mutex_t* __m)
+_LIBCPP_THREAD_SAFETY_ATTRIBUTE(requires_capability(*__m));
 
 _LIBCPP_THREAD_ABI_VISIBILITY
 int __libcpp_condvar_timedwait(__libcpp_condvar_t *__cv, __libcpp_mutex_t *__m,
-                               timespec *__ts);
+                               timespec *__ts)
+_LIBCPP_THREAD_SAFETY_ATTRIBUTE(requires_capability(*__m));
 
 _LIBCPP_THREAD_ABI_VISIBILITY
 int __libcpp_condvar_destroy(__libcpp_condvar_t* __cv);