Index: share/man/man9/timeout.9
===================================================================
RCS file: /cvs/src/share/man/man9/timeout.9,v
retrieving revision 1.44
diff -u -p -r1.44 timeout.9
--- share/man/man9/timeout.9	22 Sep 2016 12:55:24 -0000	1.44
+++ share/man/man9/timeout.9	17 Nov 2017 04:19:31 -0000
@@ -38,6 +38,7 @@
 .Nm timeout_add_ts ,
 .Nm timeout_add_bt ,
 .Nm timeout_del ,
+.Nm timeout_barrier ,
 .Nm timeout_pending ,
 .Nm timeout_initialized ,
 .Nm timeout_triggered ,
@@ -54,6 +55,8 @@
 .Fn timeout_add "struct timeout *to" "int ticks"
 .Ft int
 .Fn timeout_del "struct timeout *to"
+.Ft void
+.Fn timeout_barrier "struct timeout *to"
 .Ft int
 .Fn timeout_pending "struct timeout *to"
 .Ft int
@@ -153,6 +156,11 @@ will cancel the timeout in the argument
 If the timeout has already executed or has never been added
 the call will have no effect.
 .Pp
+.Fn timeout_barrier
+ensures that any current execution of the timeout in the argument
+.Fa to
+has completed before returning.
+.Pp
 The
 .Fn timeout_pending
 macro can be used to check if a timeout is scheduled to run.
@@ -217,6 +225,9 @@ context.
 can be called during autoconf, from process context, or from any
 interrupt context at or below
 .Dv IPL_CLOCK .
+.Pp
+.Fn timeout_barrier
+can be called from process context.
 .Pp
 When the timeout runs, the
 .Fa fn
Index: sys/sys/timeout.h
===================================================================
RCS file: /cvs/src/sys/sys/timeout.h,v
retrieving revision 1.26
diff -u -p -r1.26 timeout.h
--- sys/sys/timeout.h	22 Sep 2016 12:55:24 -0000	1.26
+++ sys/sys/timeout.h	17 Nov 2017 04:19:31 -0000
@@ -99,6 +99,7 @@ int timeout_add_msec(struct timeout *, i
 int timeout_add_usec(struct timeout *, int);
 int timeout_add_nsec(struct timeout *, int);
 int timeout_del(struct timeout *);
+void timeout_barrier(struct timeout *);
 
 void timeout_startup(void);
 void timeout_adjust_ticks(int);
Index: sys/kern/kern_timeout.c
===================================================================
RCS file: /cvs/src/sys/kern/kern_timeout.c,v
retrieving revision 1.50
diff -u -p -r1.50 kern_timeout.c
--- sys/kern/kern_timeout.c	3 Oct 2016 11:54:29 -0000	1.50
+++ sys/kern/kern_timeout.c	17 Nov 2017 04:19:31 -0000
@@ -324,6 +324,45 @@ timeout_del(struct timeout *to)
 	return (ret);
 }
 
+void	timeout_proc_barrier(void *);
+
+void
+timeout_barrier(struct timeout *to)
+{
+	if (!ISSET(to->to_flags, TIMEOUT_NEEDPROCCTX)) {
+		KERNEL_LOCK();
+		splx(splsoftclock());
+		KERNEL_UNLOCK();
+	} else {
+		int wait = 1;
+		struct timeout barrier;
+		struct sleep_state sls;
+
+		timeout_set(&barrier, timeout_proc_barrier, &wait);
+
+		mtx_enter(&timeout_mutex);
+		CIRCQ_INSERT(&to->to_list, &timeout_proc);
+		mtx_leave(&timeout_mutex);
+
+		wakeup_one(&timeout_proc);
+
+		while (wait) {
+			sleep_setup(&sls, &wait, PSWP, "tmobar");
+			sleep_finish(&sls, wait);
+		}
+	}
+}
+
+void
+timeout_proc_barrier(void *arg)
+{
+	int *wait = arg;
+
+	*wait = 0;
+
+	wakeup_one(wait);
+}
+
 /*
  * This is called from hardclock() once every tick.
  * We return !0 if we need to schedule a softclock.