[PATCH -next v2 22/26] pty, n_tty: Simplify input processing on final close

[Peter Hurley]

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@xxxxxxxxxxxxxxx>
Signed-off-by: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
---
 drivers/tty/n_tty.c | 46 ++++++++++++++++++++--------------------------
 drivers/tty/pty.c   |  1 +
 2 files changed, 21 insertions(+), 26 deletions(-)

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 0cb0e12..112eda7 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -2197,34 +2197,28 @@ 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)
-					break;
-				if (file->f_flags & O_NONBLOCK) {
-					retval = -EAGAIN;
-					break;
-				}
-				if (signal_pending(current)) {
-					retval = -ERESTARTSYS;
-					break;
-				}
-				n_tty_set_room(tty);
-				up_read(&tty->termios_rwsem);
+				retval = -EIO;
+				break;
+			}
+			if (tty_hung_up_p(file))
+				break;
+			if (!timeout)
+				break;
+			if (file->f_flags & O_NONBLOCK) {
+				retval = -EAGAIN;
+				break;
+			}
+			if (signal_pending(current)) {
+				retval = -ERESTARTSYS;
+				break;
+			}
+			n_tty_set_room(tty);
+			up_read(&tty->termios_rwsem);
 
-				timeout = schedule_timeout(timeout);
+			timeout = schedule_timeout(timeout);
 
-				down_read(&tty->termios_rwsem);
-				continue;
-			}
+			down_read(&tty->termios_rwsem);
+			continue;
 		}
 		__set_current_state(TASK_RUNNING);
 
diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
index bee9776..a9d256d 100644
--- a/drivers/tty/pty.c
+++ b/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);
-- 
2.1.3

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