diff --git a/lib/libc/gen/posix_spawn.c b/lib/libc/gen/posix_spawn.c
index d405517ba68..ee2ce05ff31 100644
--- a/lib/libc/gen/posix_spawn.c
+++ b/lib/libc/gen/posix_spawn.c
@@ -325,6 +325,11 @@ do_posix_spawn(pid_t *pid, const char *path,
 	 * ideal vfork(2) if we get an EINVAL from rfork -- this should only
 	 * happen with newer libc on older kernel that doesn't accept
 	 * RFSPAWN.
+	 *
+	 * Combination of vfork() (or its equivalent rfork() form) and
+	 * a special property of the libthr rtld locks ensure that
+	 * rtld is operational in the child.  In particular, libthr
+	 * rtld locks do not store owner' tid into the lock word.
 	 */
 #ifdef _RFORK_THREAD_STACK_SIZE
 	/*