diff --git a/include/linux/rtmutex.h b/include/linux/rtmutex.h
index 281d8fd..8d522ff 100644
--- a/include/linux/rtmutex.h
+++ b/include/linux/rtmutex.h
@@ -16,6 +16,8 @@
 #include <linux/plist.h>
 #include <linux/spinlock_types.h>
 
+extern int max_lock_depth; /* for sysctl */
+
 /**
  * The rt_mutex structure
  *