Index: share/man/man9/mutex.9
===================================================================
RCS file: /cvsroot/src/share/man/man9/mutex.9,v
retrieving revision 1.25
diff -p -u -r1.25 mutex.9
--- share/man/man9/mutex.9	7 Jul 2012 21:27:56 -0000	1.25
+++ share/man/man9/mutex.9	9 Jan 2013 01:07:41 -0000
@@ -1,6 +1,6 @@
 .\"	$NetBSD: mutex.9,v 1.25 2012/07/07 21:27:56 rmind Exp $
 .\"
-.\" Copyright (c) 2007, 2009 The NetBSD Foundation, Inc.
+.\" Copyright (c) 2007, 2009, 2013 The NetBSD Foundation, Inc.
 .\" All rights reserved.
 .\"
 .\" This code is derived from software contributed to The NetBSD Foundation
@@ -27,7 +27,7 @@
 .\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 .\" POSSIBILITY OF SUCH DAMAGE.
 .\"
-.Dd July 7, 2012
+.Dd January 9, 2013
 .Dt MUTEX 9
 .Os
 .Sh NAME
@@ -130,11 +130,22 @@ and interrupt handlers.
 .Pp
 The
 .Fa ipl
-argument is used to pass a system interrupt priority level (IPL)
-that will block all interrupt handlers that may try to acquire the mutex.
-.Pp
-LWPs that own spin mutexes may not sleep, and therefore must not
-try to acquire adaptive mutexes or other sleep locks.
+argument is used by
+.Fn mutex_enter
+to set the CPU's interrupt priority level (IPL) to block interrupts
+whose handlers may try to acquire the mutex while it is held.
+This avoids deadlock when a LWP holding a mutex is interrupted and the
+interrupt handler tries to acquire the mutex, since the LWP cannot
+continue until the interrupt has been handled.
+.Pp
+Consequently, LWPs that own spin mutexes may not sleep, and therefore
+must not try to acquire adaptive mutexes or other sleep locks.
+.Pp
+Note that although
+.Fn mutex_enter
+blocks interrupts on the current CPU, an interrupt handler must still
+take the mutex itself in order to exclude LWPs and interrupt handlers
+on other CPUs.
 .Pp
 A processor will always busy-wait when attempting to acquire
 a spin mutex that is already held.