4 files changed, 37 insertions, 6 deletions

diff --git a/sys/kern/tty.c b/sys/kern/tty.c
index d73683c196652..ee9595871d759 100644
--- a/sys/kern/tty.c
+++ b/sys/kern/tty.c
@@ -438,15 +438,28 @@ ttydev_write(struct cdev *dev, struct uio *uio, int ioflag)
 
 	if (tp->t_termios.c_lflag & TOSTOP) {
 		error = tty_wait_background(tp, curthread, SIGTTOU);
-		if (error) {
-			tty_unlock(tp);
-			return (error);
-		}
+		if (error)
+			goto done;
 	}
 
-	error = ttydisc_write(tp, uio, ioflag);
-	tty_unlock(tp);
+	if (ioflag & IO_NDELAY && tp->t_flags & TF_BUSY_OUT) {
+		/* Allow non-blocking writes to bypass serialization. */
+		error = ttydisc_write(tp, uio, ioflag);
+	} else {
+		/* Serialize write() calls. */
+		while (tp->t_flags & TF_BUSY_OUT) {
+			error = tty_wait(tp, &tp->t_bgwait);
+			if (error)
+				goto done;
+		}
+ 
+ 		tp->t_flags |= TF_BUSY_OUT;
+		error = ttydisc_write(tp, uio, ioflag);
+ 		tp->t_flags &= ~TF_BUSY_OUT;
+		cv_broadcast(&tp->t_bgwait);
+	}
 
+done:	tty_unlock(tp);
 	return (error);
 }
 
@@ -1880,6 +1893,11 @@ static struct {
 	{ TF_ZOMBIE,		'Z' },
 	{ TF_HOOK,		's' },
 
+	/* Keep these together -> 'bi' and 'bo'. */
+	{ TF_BUSY,		'b' },
+	{ TF_BUSY_IN,		'i' },
+	{ TF_BUSY_OUT,		'o' },
+
 	{ 0,			'\0'},
 };
 
diff --git a/sys/sys/tty.h b/sys/sys/tty.h
index 218fd214cfd0f..893509395e443 100644
--- a/sys/sys/tty.h
+++ b/sys/sys/tty.h
@@ -83,6 +83,9 @@ struct tty {
 #define	TF_BYPASS	0x04000	/* Optimized input path. */
 #define	TF_ZOMBIE	0x08000	/* Modem disconnect received. */
 #define	TF_HOOK		0x10000	/* TTY has hook attached. */
+#define	TF_BUSY_IN	0x20000	/* Process busy in read() -- not supported. */
+#define	TF_BUSY_OUT	0x40000	/* Process busy in write(). */
+#define	TF_BUSY		(TF_BUSY_IN|TF_BUSY_OUT)
 	unsigned int	t_revokecnt;	/* (t) revoke() count. */
 
 	/* Buffering mechanisms. */
diff --git a/usr.sbin/pstat/pstat.8 b/usr.sbin/pstat/pstat.8
index b92abd8ae783d..5e3760cecb7ea 100644
--- a/usr.sbin/pstat/pstat.8
+++ b/usr.sbin/pstat/pstat.8
@@ -206,6 +206,11 @@ block mode input routine in use
 connection lost
 .It s
 i/o being snooped
+.It b
+busy in
+.Xr read 2
+or
+.Xr write 2
 .El
 .Pp
 The
diff --git a/usr.sbin/pstat/pstat.c b/usr.sbin/pstat/pstat.c
index 2cb52fec88128..8272eee38bd65 100644
--- a/usr.sbin/pstat/pstat.c
+++ b/usr.sbin/pstat/pstat.c
@@ -315,6 +315,11 @@ static struct {
 	{ TF_ZOMBIE,		'Z' },
 	{ TF_HOOK,		's' },
 
+	/* Keep these together -> 'bi' and 'bo'. */
+	{ TF_BUSY,		'b' },
+	{ TF_BUSY_IN,		'i' },
+	{ TF_BUSY_OUT,		'o' },
+
 	{ 0,			'\0'},
 };