diff --git a/libc/private/bionic_futex.h b/libc/private/bionic_futex.h
index bfc3520..5602af7 100644
--- a/libc/private/bionic_futex.h
+++ b/libc/private/bionic_futex.h
@@ -28,6 +28,7 @@
 #ifndef _BIONIC_FUTEX_H
 #define _BIONIC_FUTEX_H
 
+#include <linux/compiler.h> /* needed for __user in non-uapi futex.h */
 #include <linux/futex.h>
 #include <sys/cdefs.h>
 
diff --git a/libc/private/bionic_tls.h b/libc/private/bionic_tls.h
index 094c71c..ff13fdb 100644
--- a/libc/private/bionic_tls.h
+++ b/libc/private/bionic_tls.h
@@ -51,7 +51,9 @@
   TLS_SLOT_THREAD_ID,
   TLS_SLOT_ERRNO,
 
-  /* This slot is used when starting a new thread, before any code that needs errno runs. */
+  /* This slot in the child's TLS is used to synchronize the parent and child
+   * during thread initialization. The child finishes with this mutex before
+   * running any code that can set errno, so we can reuse the errno slot. */
   TLS_SLOT_START_MUTEX = TLS_SLOT_ERRNO,
 
   /* These two aren't used by bionic itself, but allow the graphics code to