pty, n_tty: Simplify input processing on final close

When releasing one end of a pty pair, that end may just have written
to the other, which the input processing worker, flush_to_ldisc(), is
still working on but has not completed the copy to the other end's
read buffer. So input may not appear to be available to a waiting
reader but yet TTY_OTHER_CLOSED is now observed. The n_tty line
discipline has worked around this by waiting for input processing
to complete and then re-checking if input is available before
exiting with -EIO.

Since the tty/ldisc lock reordering, the wait for input processing
to complete can now occur during final close before setting
TTY_OTHER_CLOSED. In this way, a waiting reader is guaranteed to
see input available (if any) before observing TTY_OTHER_CLOSED.
Reviewed-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/n_tty.c

@@ -2197,14 +2197,9 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
 
 		if (!input_available_p(tty, 0)) {
 			if (test_bit(TTY_OTHER_CLOSED, &tty->flags)) {
-				up_read(&tty->termios_rwsem);
-				tty_flush_to_ldisc(tty);
-				down_read(&tty->termios_rwsem);
-				if (!input_available_p(tty, 0)) {
 				retval = -EIO;
 				break;
 			}
-			} else {
 			if (tty_hung_up_p(file))
 				break;
 			if (!timeout)
@@ -2225,7 +2220,6 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
 			down_read(&tty->termios_rwsem);
 			continue;
 		}
-		}
 		__set_current_state(TASK_RUNNING);
 
 		if (ldata->icanon && !L_EXTPROC(tty)) {

drivers/tty/pty.c

@@ -53,6 +53,7 @@ static void pty_close(struct tty_struct *tty, struct file *filp)
 	/* Review - krefs on tty_link ?? */
 	if (!tty->link)
 		return;
+	tty_flush_to_ldisc(tty->link);
 	set_bit(TTY_OTHER_CLOSED, &tty->link->flags);
 	wake_up_interruptible(&tty->link->read_wait);
 	wake_up_interruptible(&tty->link->write_wait);