n_tty: Split n_tty_chars_in_buffer() for reader-only interface

N_TTY .chars_in_buffer() method requires serialized access if
the current thread is not the single-consumer, n_tty_read().

Separate the internal interface; prepare for lockless read-side.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/n_tty.c

@@ -277,7 +277,7 @@ static void n_tty_flush_buffer(struct tty_struct *tty)
  *	Locking: read_lock
  */
 
-static ssize_t n_tty_chars_in_buffer(struct tty_struct *tty)
+static ssize_t chars_in_buffer(struct tty_struct *tty)
 {
 	struct n_tty_data *ldata = tty->disc_data;
 	unsigned long flags;
@@ -295,6 +295,11 @@ static ssize_t n_tty_chars_in_buffer(struct tty_struct *tty)
 	return n;
 }
 
+static ssize_t n_tty_chars_in_buffer(struct tty_struct *tty)
+{
+	return chars_in_buffer(tty);
+}
+
 /**
  *	is_utf8_continuation	-	utf8 multibyte check
  *	@c: byte to check
@@ -2032,7 +2037,7 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
 		}
 
 		/* If there is enough space in the read buffer now, let the
-		 * low-level driver know. We use n_tty_chars_in_buffer() to
+		 * low-level driver know. We use chars_in_buffer() to
 		 * check the buffer, as it now knows about canonical mode.
 		 * Otherwise, if the driver is throttled and the line is
 		 * longer than TTY_THRESHOLD_UNTHROTTLE in canonical mode,
@@ -2040,7 +2045,7 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
 		 */
 		while (1) {
 			tty_set_flow_change(tty, TTY_UNTHROTTLE_SAFE);
-			if (n_tty_chars_in_buffer(tty) > TTY_THRESHOLD_UNTHROTTLE)
+			if (chars_in_buffer(tty) > TTY_THRESHOLD_UNTHROTTLE)
 				break;
 			if (!tty->count)
 				break;