diff --git a/lib/isc/include/isc/rwlock.h b/lib/isc/include/isc/rwlock.h
index 24934ea984..07cf846ec4 100644
--- a/lib/isc/include/isc/rwlock.h
+++ b/lib/isc/include/isc/rwlock.h
@@ -28,8 +28,10 @@ typedef enum {
 } isc_rwlocktype_t;
 
 typedef struct isc_rwlock {
+	/* Unlocked. */
 	unsigned int		magic;
 	isc_mutex_t		lock;
+	/* Locked by lock. */
 	isc_condition_t		readable;
 	isc_condition_t		writeable;
 	isc_rwlocktype_t	type;