diff --git a/sys/netinet/tcp_subr.c b/sys/netinet/tcp_subr.c
index 5f35ff90e7d0..97aec2561936 100644
--- a/sys/netinet/tcp_subr.c
+++ b/sys/netinet/tcp_subr.c
@@ -1,4 +1,4 @@
-/*	$NetBSD: tcp_subr.c,v 1.271 2017/07/29 05:08:48 maxv Exp $	*/
+/*	$NetBSD: tcp_subr.c,v 1.272 2018/01/19 07:53:01 ozaki-r Exp $	*/
 
 /*
  * Copyright (C) 1995, 1996, 1997, and 1998 WIDE Project.
@@ -91,7 +91,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: tcp_subr.c,v 1.271 2017/07/29 05:08:48 maxv Exp $");
+__KERNEL_RCSID(0, "$NetBSD: tcp_subr.c,v 1.272 2018/01/19 07:53:01 ozaki-r Exp $");
 
 #ifdef _KERNEL_OPT
 #include "opt_inet.h"
@@ -382,8 +382,6 @@ struct mowner tcp_sock_rx_mowner = MOWNER_INIT("tcp", "sock rx");
 struct mowner tcp_sock_tx_mowner = MOWNER_INIT("tcp", "sock tx");
 #endif
 
-callout_t tcp_slowtimo_ch;
-
 static int
 do_tcpinit(void)
 {
@@ -424,8 +422,7 @@ do_tcpinit(void)
 
 	vtw_earlyinit();
 
-	callout_init(&tcp_slowtimo_ch, CALLOUT_MPSAFE);
-	callout_reset(&tcp_slowtimo_ch, 1, tcp_slowtimo, NULL);
+	tcp_slowtimo_init();
 
 	return 0;
 }
diff --git a/sys/netinet/tcp_timer.c b/sys/netinet/tcp_timer.c
index fa2e1bccbf8f..6487b36e6ae2 100644
--- a/sys/netinet/tcp_timer.c
+++ b/sys/netinet/tcp_timer.c
@@ -1,4 +1,4 @@
-/*	$NetBSD: tcp_timer.c,v 1.92 2017/07/28 19:16:41 maxv Exp $	*/
+/*	$NetBSD: tcp_timer.c,v 1.93 2018/01/19 07:53:01 ozaki-r Exp $	*/
 
 /*
  * Copyright (C) 1995, 1996, 1997, and 1998 WIDE Project.
@@ -93,11 +93,12 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: tcp_timer.c,v 1.92 2017/07/28 19:16:41 maxv Exp $");
+__KERNEL_RCSID(0, "$NetBSD: tcp_timer.c,v 1.93 2018/01/19 07:53:01 ozaki-r Exp $");
 
 #ifdef _KERNEL_OPT
 #include "opt_inet.h"
 #include "opt_tcp_debug.h"
+#include "opt_net_mpsafe.h"
 #endif
 
 #include <sys/param.h>
@@ -108,6 +109,8 @@ __KERNEL_RCSID(0, "$NetBSD: tcp_timer.c,v 1.92 2017/07/28 19:16:41 maxv Exp $");
 #include <sys/protosw.h>
 #include <sys/errno.h>
 #include <sys/kernel.h>
+#include <sys/callout.h>
+#include <sys/workqueue.h>
 
 #include <net/if.h>
 
@@ -149,6 +152,15 @@ u_int	tcp_keepcnt = 0;		/* max idle probes */
 
 int	tcp_maxpersistidle = 0;		/* max idle time in persist */
 
+static callout_t	tcp_slowtimo_ch;
+#ifdef NET_MPSAFE
+static struct workqueue	*tcp_slowtimo_wq;
+static struct work	tcp_slowtimo_wk;
+#endif
+
+static void tcp_slowtimo_work(struct work *, void *);
+static void tcp_slowtimo(void *);
+
 /*
  * Time to delay the ACK.  This is initialized in tcp_init(), unless
  * its patched.
@@ -193,6 +205,21 @@ tcp_timer_init(void)
 		tcp_delack_ticks = TCP_DELACK_TICKS;
 }
 
+void
+tcp_slowtimo_init(void)
+{
+#ifdef NET_MPSAFE
+	int error;
+
+	error = workqueue_create(&tcp_slowtimo_wq, "tcp_slowtimo",
+	    tcp_slowtimo_work, NULL, PRI_SOFTNET, IPL_SOFTNET, WQ_MPSAFE);
+	if (error != 0)
+		panic("%s: workqueue_create failed (%d)\n", __func__, error);
+#endif
+	callout_init(&tcp_slowtimo_ch, CALLOUT_MPSAFE);
+	callout_reset(&tcp_slowtimo_ch, 1, tcp_slowtimo, NULL);
+}
+
 /*
  * Callout to process delayed ACKs for a TCPCB.
  */
@@ -229,8 +256,8 @@ tcp_delack(void *arg)
  * Updates the timers in all active tcb's and
  * causes finite state machine actions if timers expire.
  */
-void
-tcp_slowtimo(void *arg)
+static void
+tcp_slowtimo_work(struct work *wk, void *arg)
 {
 
 	mutex_enter(softnet_lock);
@@ -241,6 +268,17 @@ tcp_slowtimo(void *arg)
 	callout_schedule(&tcp_slowtimo_ch, hz / PR_SLOWHZ);
 }
 
+static void
+tcp_slowtimo(void *arg)
+{
+
+#ifdef NET_MPSAFE
+	workqueue_enqueue(tcp_slowtimo_wq, &tcp_slowtimo_wk, NULL);
+#else
+	tcp_slowtimo_work(NULL, NULL);
+#endif
+}
+
 /*
  * Cancel all timers for TCP tp.
  */
diff --git a/sys/netinet/tcp_timer.h b/sys/netinet/tcp_timer.h
index 629f8e0f7f36..fd209cd93a3c 100644
--- a/sys/netinet/tcp_timer.h
+++ b/sys/netinet/tcp_timer.h
@@ -1,4 +1,4 @@
-/*	$NetBSD: tcp_timer.h,v 1.28 2011/05/24 18:37:52 gdt Exp $	*/
+/*	$NetBSD: tcp_timer.h,v 1.29 2018/01/19 07:53:01 ozaki-r Exp $	*/
 
 /*-
  * Copyright (c) 2001, 2005 The NetBSD Foundation, Inc.
@@ -190,6 +190,7 @@ extern int tcp_ttl;			/* time to live for TCP segs */
 extern const int tcp_backoff[];
 
 void	tcp_timer_init(void);
+void	tcp_slowtimo_init(void);
 #endif
 
 #endif /* !_NETINET_TCP_TIMER_H_ */
diff --git a/sys/netinet/tcp_var.h b/sys/netinet/tcp_var.h
index 542450d05850..faf0a5ec92e4 100644
--- a/sys/netinet/tcp_var.h
+++ b/sys/netinet/tcp_var.h
@@ -1,4 +1,4 @@
-/*	$NetBSD: tcp_var.h,v 1.181 2017/11/15 09:55:22 ozaki-r Exp $	*/
+/*	$NetBSD: tcp_var.h,v 1.182 2018/01/19 07:53:01 ozaki-r Exp $	*/
 
 /*
  * Copyright (C) 1995, 1996, 1997, and 1998 WIDE Project.
@@ -928,8 +928,6 @@ void	 tcp_setpersist(struct tcpcb *);
 int	 tcp_signature_compute(struct mbuf *, struct tcphdr *, int, int,
 	    int, u_char *, u_int);
 #endif
-void	 tcp_slowtimo(void *);
-extern callout_t tcp_slowtimo_ch;
 void	 tcp_fasttimo(void);
 struct mbuf *
 	 tcp_template(struct tcpcb *);