n_tty: Fix stuck throttled driver

As noted in the following comment:

  /* FIXME: there is a tiny race here if the receive room check runs
     before the other work executes and empties the buffer (upping
     the receiving room and unthrottling. We then throttle and get
     stuck. This has been observed and traced down by Vincent Pillet/
     We need to address this when we sort out out the rx path locking */

Use new safe throttle/unthrottle functions to re-evaluate conditions
if interrupted by the complement flow control function.
Reported-by: Vincent Pillet <vincentx.pillet@intel.com>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/n_tty.c

@@ -1468,14 +1468,14 @@ static void n_tty_receive_buf(struct tty_struct *tty, const unsigned char *cp,
 	 * mode.  We don't want to throttle the driver if we're in
 	 * canonical mode and don't have a newline yet!
 	 */
-	if (tty->receive_room < TTY_THRESHOLD_THROTTLE)
-		tty_throttle(tty);
-
-        /* FIXME: there is a tiny race here if the receive room check runs
-           before the other work executes and empties the buffer (upping
-           the receiving room and unthrottling. We then throttle and get
-           stuck. This has been observed and traced down by Vincent Pillet/
-           We need to address this when we sort out out the rx path locking */
+	while (1) {
+		tty_set_flow_change(tty, TTY_THROTTLE_SAFE);
+		if (tty->receive_room >= TTY_THRESHOLD_THROTTLE)
+			break;
+		if (!tty_throttle_safe(tty))
+			break;
+	}
+	__tty_set_flow_change(tty, 0);
 }
 
 int is_ignored(int sig)
@@ -1944,11 +1944,17 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
 		 * longer than TTY_THRESHOLD_UNTHROTTLE in canonical mode,
 		 * we won't get any more characters.
 		 */
-		if (n_tty_chars_in_buffer(tty) <= TTY_THRESHOLD_UNTHROTTLE) {
+		while (1) {
+			tty_set_flow_change(tty, TTY_UNTHROTTLE_SAFE);
+			if (n_tty_chars_in_buffer(tty) > TTY_THRESHOLD_UNTHROTTLE)
+				break;
+			if (!tty->count)
+				break;
 			n_tty_set_room(tty);
-			if (tty->count)
-				tty_unthrottle(tty);
+			if (!tty_unthrottle_safe(tty))
+				break;
 		}
+		__tty_set_flow_change(tty, 0);
 
 		if (b - buf >= minimum)
 			break;