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

Peter Hurley
2014-11-05T17:50:11Z

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>
---
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