Index: lib/libthr/pthread.map
===================================================================
--- lib/libthr/pthread.map
+++ lib/libthr/pthread.map
@@ -296,6 +296,14 @@
 	_thread_size_key;
 	_thread_state_running;
 	_thread_state_zoombie;
+
+	/* To avoid TSan false-positives */
+	__tsan_mutex_create;
+	__tsan_mutex_destroy;
+	__tsan_mutex_pre_lock;
+	__tsan_mutex_post_lock;
+	__tsan_mutex_pre_unlock;
+	__tsan_mutex_post_unlock;
 };
 
 FBSD_1.1 {
Index: lib/libthr/thread/thr_malloc.c
===================================================================
--- lib/libthr/thread/thr_malloc.c
+++ lib/libthr/thread/thr_malloc.c
@@ -56,6 +56,13 @@
 	}
 	pagesizes = pagesizes_d;
 	_thr_umutex_init(&thr_malloc_umtx);
+	/*
+	 * TODO: should probably annotate other internal locks too (maybe even
+	 * move it down to the internal API). However, annotating only the
+	 * malloc lock appears to silence (almost?) all false-positives, so
+	 * this can likely wait (and also has a lower performance overhead).
+	 */
+	__tsan_mutex_create(&thr_malloc_umtx, __tsan_mutex_write_reentrant);
 }
 
 static void
@@ -65,12 +72,14 @@
 
 	if (curthread == NULL)
 		return;
+	__tsan_mutex_pre_lock(&thr_malloc_umtx, 0);
 	curthread->locklevel++;
 	curtid = TID(curthread);
 	if ((uint32_t)thr_malloc_umtx.m_owner == curtid)
 		thr_malloc_umtx_level++;
 	else
 		_thr_umutex_lock(&thr_malloc_umtx, curtid);
+	__tsan_mutex_post_lock(&thr_malloc_umtx, 0, 1);
 }
 
 static void
@@ -79,12 +88,14 @@
 
 	if (curthread == NULL)
 		return;
+	__tsan_mutex_pre_unlock(&thr_malloc_umtx, 0);
 	if (thr_malloc_umtx_level > 0)
 		thr_malloc_umtx_level--;
 	else
 		_thr_umutex_unlock(&thr_malloc_umtx, TID(curthread));
 	curthread->locklevel--;
 	_thr_ast(curthread);
+	__tsan_mutex_post_unlock(&thr_malloc_umtx, 0);
 }
 
 void *
@@ -140,13 +151,15 @@
 void
 __thr_malloc_prefork(struct pthread *curthread)
 {
-
+	__tsan_mutex_pre_lock(&thr_malloc_umtx, 0);
 	_thr_umutex_lock(&thr_malloc_umtx, TID(curthread));
+	__tsan_mutex_post_lock(&thr_malloc_umtx, 0, 1);
 }
 
 void
 __thr_malloc_postfork(struct pthread *curthread)
 {
-
+	__tsan_mutex_pre_unlock(&thr_malloc_umtx, 0);
 	_thr_umutex_unlock(&thr_malloc_umtx, TID(curthread));
+	__tsan_mutex_post_unlock(&thr_malloc_umtx, 0);
 }
Index: lib/libthr/thread/thr_private.h
===================================================================
--- lib/libthr/thread/thr_private.h
+++ lib/libthr/thread/thr_private.h
@@ -1099,6 +1099,15 @@
 int __Tthr_mutex_lock(pthread_mutex_t *);
 int __Tthr_mutex_trylock(pthread_mutex_t *);
 
+/* These are required to avoid TSan false-positives for libthr-internal locks */
+#define __tsan_mutex_write_reentrant (1 << 1)
+void __tsan_mutex_create(void *, unsigned) __weak_symbol;
+void __tsan_mutex_destroy(void *, unsigned) __weak_symbol;
+void __tsan_mutex_pre_lock(void *, unsigned) __weak_symbol;
+void __tsan_mutex_post_lock(void *, unsigned, int) __weak_symbol;
+int __tsan_mutex_pre_unlock(void *, unsigned) __weak_symbol;
+void __tsan_mutex_post_unlock(void *, unsigned) __weak_symbol;
+
 __END_DECLS
 __NULLABILITY_PRAGMA_POP
 
Index: lib/libthr/thread/thr_umtx.c
===================================================================
--- lib/libthr/thread/thr_umtx.c
+++ lib/libthr/thread/thr_umtx.c
@@ -374,3 +374,36 @@
 	if (_thr_rwlock_unlock(rwlock))
 		PANIC("unlock error");
 }
+
+/*
+ * To avoid TSan false-positives we provide weak versions of the TSan
+ * callbacks that are used to annotate mutexes. Without this we get tons of
+ * false-positives each time __thr_malloc, etc. is used.
+ * See tsan_interface.h inside compiler-rt.
+ */
+void __weak_symbol
+__tsan_mutex_create(void *addr __unused, unsigned flags __unused)
+{
+}
+void __weak_symbol
+__tsan_mutex_destroy(void *addr __unused, unsigned flags __unused)
+{
+}
+void __weak_symbol
+__tsan_mutex_pre_lock(void *addr __unused, unsigned flags __unused)
+{
+}
+void __weak_symbol
+__tsan_mutex_post_lock(void *addr __unused, unsigned flags __unused,
+    int recursion __unused)
+{
+}
+int __weak_symbol
+__tsan_mutex_pre_unlock(void *addr __unused, unsigned flags __unused)
+{
+	return (0);
+}
+void __weak_symbol
+__tsan_mutex_post_unlock(void *addr __unused, unsigned flags __unused)
+{
+}