diff --git a/libexec/rtld-elf/rtld_lock.h b/libexec/rtld-elf/rtld_lock.h
index fdbdc9917075..c88b558b72b8 100644
--- a/libexec/rtld-elf/rtld_lock.h
+++ b/libexec/rtld-elf/rtld_lock.h
@@ -1,90 +1,103 @@
 /*-
  * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
  *
  * Copyright 2003 Alexander Kabaev.
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
  *    documentation and/or other materials provided with the distribution.
  *
  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
  * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
  * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
  * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
  * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
  * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  *
  * $FreeBSD$
  */
 
 #ifndef _RTLD_LOCK_H_
 #define	_RTLD_LOCK_H_
 
 #define	RTLI_VERSION_ONE	0x01
 #define	RTLI_VERSION		0x02
 
 #define	MAX_RTLD_LOCKS	8
 
+/*
+ * This structure is part of the ABI between rtld and threading
+ * libraries, like libthr and even libc_r.  Its layout is fixed and
+ * can be changed only by appending new fields at the end, with the
+ * bump of RTLI_VERSION.
+ */
 struct RtldLockInfo
 {
+	/*
+	 * Valid if the object calling _rtld_thread_init() exported
+	 * symbol _pli_rtli_version.  Otherwise assume RTLI_VERSION_ONE.
+	 */
 	unsigned int rtli_version;
+
 	void *(*lock_create)(void);
 	void  (*lock_destroy)(void *);
 	void  (*rlock_acquire)(void *);
 	void  (*wlock_acquire)(void *);
 	void  (*lock_release)(void *);
 	int   (*thread_set_flag)(int);
 	int   (*thread_clr_flag)(int);
 	void  (*at_fork)(void);
+
+	/* Version 2 fields */
 	char *(*dlerror_loc)(void);
 	int  *(*dlerror_seen)(void);
 	int   dlerror_loc_sz;
 };
 
 #if defined(IN_RTLD) || defined(PTHREAD_KERNEL)
 
 void _rtld_thread_init(struct RtldLockInfo *) __exported;
 void _rtld_atfork_pre(int *) __exported;
 void _rtld_atfork_post(int *) __exported;
 
 #endif /* IN_RTLD || PTHREAD_KERNEL */
 
 #ifdef IN_RTLD
 
 struct rtld_lock;
 typedef struct rtld_lock *rtld_lock_t;
 
 extern rtld_lock_t	rtld_bind_lock;
 extern rtld_lock_t	rtld_libc_lock;
 extern rtld_lock_t	rtld_phdr_lock;
 
 extern struct RtldLockInfo lockinfo;
 
 #define	RTLD_LOCK_UNLOCKED	0
 #define	RTLD_LOCK_RLOCKED	1
 #define	RTLD_LOCK_WLOCKED	2
 
 struct Struct_RtldLockState;
 typedef struct Struct_RtldLockState RtldLockState;
 
 void	rlock_acquire(rtld_lock_t, RtldLockState *);
 void 	wlock_acquire(rtld_lock_t, RtldLockState *);
 void	lock_release(rtld_lock_t, RtldLockState *);
 void	lock_upgrade(rtld_lock_t, RtldLockState *);
 void	lock_restart_for_upgrade(RtldLockState *);
 
 void	dlerror_dflt_init(void);
 
 #endif	/* IN_RTLD */
 
 #endif