diff --git a/man2/setreuid.2 b/man2/setreuid.2
index 94ca82aaf..de6d49267 100644
--- a/man2/setreuid.2
+++ b/man2/setreuid.2
@@ -1,4 +1,5 @@
 .\" Copyright (c) 1983, 1991 The Regents of the University of California.
+.\" and Copyright (C) 2009, 2010, 2014, 2015, Michael Kerrisk <mtk.manpages@gmail.com>
 .\" All rights reserved.
 .\"
 .\" %%%LICENSE_START(BSD_4_CLAUSE_UCB)
@@ -191,6 +192,23 @@ The glibc
 and
 .BR setregid ()
 wrapper functions transparently deal with the variations across kernel versions.
+.\"
+.SS C library/kernel ABI differences
+At the kernel level, user IDs and group IDs are a per-thread attribute.
+However, POSIX requires that all threads in a process
+share the same credentials.
+The NPTL threading implementation handles the POSIX requirements by
+providing wrapper functions for
+the various system calls that change process UIDs and GIDs.
+These wrapper functions (including those for
+.BR setreuid ()
+and
+.BR setregid ())
+employ a signal-based technique to ensure
+that when one thread changes credentials,
+all of the other threads in the process also change their credentials.
+For details, see
+.BR nptl (7).
 .SH SEE ALSO
 .BR getgid (2),
 .BR getuid (2),