tty: Make ldisc input flow control concurrency-friendly

Although line discipline receiving is single-producer/single-consumer,
using tty->receive_room to manage flow control creates unnecessary
critical regions requiring additional lock use.

Instead, introduce the optional .receive_buf2() ldisc method which
returns the # of bytes actually received. Serialization is guaranteed
by the caller.

In turn, the line discipline should schedule the buffer work item
whenever space becomes available; ie., when there is room to receive
data and receive_room() previously returned 0 (the buffer work
item stops processing if receive_buf2() returns 0). Note the
'no room' state need not be atomic despite concurrent use by two
threads because only the buffer work thread can set the state and
only the read() thread can clear the state.

Add n_tty_receive_buf2() as the receive_buf2() method for N_TTY.
Provide a public helper function, tty_ldisc_receive_buf(), to use
when directly accessing the receive_buf() methods.

Line disciplines not using input flow control can continue to set
tty->receive_room to a fixed value and only provide the receive_buf()
method.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 45 insertions, 27 deletions

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 4bf0fc0843d7..eddeb7889e62 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -79,6 +79,9 @@ struct n_tty_data {
 	unsigned long overrun_time;
 	int num_overrun;
 
+	/* non-atomic */
+	bool no_room;
+
 	unsigned char lnext:1, erasing:1, raw:1, real_raw:1, icanon:1;
 	unsigned char echo_overrun:1;
 
@@ -114,25 +117,10 @@ static inline int tty_put_user(struct tty_struct *tty, unsigned char x,
 	return put_user(x, ptr);
 }
 
-/**
- *	n_tty_set_room	-	receive space
- *	@tty: terminal
- *
- *	Updates tty->receive_room to reflect the currently available space
- *	in the input buffer, and re-schedules the flip buffer work if space
- *	just became available.
- *
- *	Locks: Concurrent update is protected with read_lock
- */
-
-static int set_room(struct tty_struct *tty)
+static int receive_room(struct tty_struct *tty)
 {
 	struct n_tty_data *ldata = tty->disc_data;
 	int left;
-	int old_left;
-	unsigned long flags;
-
-	raw_spin_lock_irqsave(&ldata->read_lock, flags);
 
 	if (I_PARMRK(tty)) {
 		/* Multiply read_cnt by 3, since each byte might take up to
@@ -150,18 +138,27 @@ static int set_room(struct tty_struct *tty)
 	 */
 	if (left <= 0)
 		left = ldata->icanon && !ldata->canon_data;
-	old_left = tty->receive_room;
-	tty->receive_room = left;
 
-	raw_spin_unlock_irqrestore(&ldata->read_lock, flags);
-
-	return left && !old_left;
+	return left;
 }
 
+/**
+ *	n_tty_set_room	-	receive space
+ *	@tty: terminal
+ *
+ *	Re-schedules the flip buffer work if space just became available.
+ *
+ *	Locks: Concurrent update is protected with read_lock
+ */
+
 static void n_tty_set_room(struct tty_struct *tty)
 {
+	struct n_tty_data *ldata = tty->disc_data;
+
 	/* Did this open up the receive buffer? We may need to flip */
-	if (set_room(tty)) {
+	if (unlikely(ldata->no_room) && receive_room(tty)) {
+		ldata->no_room = 0;
+
 		WARN_RATELIMIT(tty->port->itty == NULL,
 				"scheduling with invalid itty\n");
 		/* see if ldisc has been killed - if so, this means that
@@ -1408,8 +1405,8 @@ static void n_tty_write_wakeup(struct tty_struct *tty)
  *	calls one at a time and in order (or using flush_to_ldisc)
  */
 
-static void n_tty_receive_buf(struct tty_struct *tty, const unsigned char *cp,
-			      char *fp, int count)
+static void __receive_buf(struct tty_struct *tty, const unsigned char *cp,
+			  char *fp, int count)
 {
 	struct n_tty_data *ldata = tty->disc_data;
 	const unsigned char *p;
@@ -1464,8 +1461,6 @@ static void n_tty_receive_buf(struct tty_struct *tty, const unsigned char *cp,
 			tty->ops->flush_chars(tty);
 	}
 
-	set_room(tty);
-
 	if ((!ldata->icanon && (ldata->read_cnt >= ldata->minimum_to_wake)) ||
 		L_EXTPROC(tty)) {
 		kill_fasync(&tty->fasync, SIGIO, POLL_IN);
@@ -1480,7 +1475,7 @@ static void n_tty_receive_buf(struct tty_struct *tty, const unsigned char *cp,
 	 */
 	while (1) {
 		tty_set_flow_change(tty, TTY_THROTTLE_SAFE);
-		if (tty->receive_room >= TTY_THRESHOLD_THROTTLE)
+		if (receive_room(tty) >= TTY_THRESHOLD_THROTTLE)
 			break;
 		if (!tty_throttle_safe(tty))
 			break;
@@ -1488,6 +1483,28 @@ static void n_tty_receive_buf(struct tty_struct *tty, const unsigned char *cp,
 	__tty_set_flow_change(tty, 0);
 }
 
+static void n_tty_receive_buf(struct tty_struct *tty, const unsigned char *cp,
+			      char *fp, int count)
+{
+	__receive_buf(tty, cp, fp, count);
+}
+
+static int n_tty_receive_buf2(struct tty_struct *tty, const unsigned char *cp,
+			      char *fp, int count)
+{
+	struct n_tty_data *ldata = tty->disc_data;
+	int room;
+
+	tty->receive_room = room = receive_room(tty);
+	if (!room)
+		ldata->no_room = 1;
+	count = min(count, room);
+	if (count)
+		__receive_buf(tty, cp, fp, count);
+
+	return count;
+}
+
 int is_ignored(int sig)
 {
 	return (sigismember(&current->blocked, sig) ||
@@ -2203,6 +2220,7 @@ struct tty_ldisc_ops tty_ldisc_N_TTY = {
 	.receive_buf     = n_tty_receive_buf,
 	.write_wakeup    = n_tty_write_wakeup,
 	.fasync		 = n_tty_fasync,
+	.receive_buf2	 = n_tty_receive_buf2,
 };
 
 /**