[PATCH] tty: pty: don't flush line discipline of another side in pty_flush_buffer

[Wang YanQing]

Don't flush line discipline of another side, because user could already see
the data in line discipline through poll, TIOCINQ or FIONREAD. Current behavior
break the expection of userspace application in below situations.

Example1:
Thread A                                 Thread B
--------                                 --------
n_tty_poll return POLLIN
                                         CTRL-C trigger pty_flush_buffer
                                           tty_buffer_flush
                                             n_tty_flush_buffer
TIOCINQ or FIONREAD
  return zero number of bytes

Example2:
Thread A                                 Thread B
--------                                 --------
TIOCINQ or FIONREAD
  return valid number of bytes

                                         CTRL-C trigger pty_flush_buffer
                                           tty_buffer_flush
                                             n_tty_flush_buffer
n_tty_read still block due to no data

I meet this problem(Example1) in konsole, current behavior cause konsole to hangup
forever.

It has high possibility to trigger the trouble I meet to execute below commands a
few times in konsole with kernel has commit 1d1d14da12e79a6c05fbe1a975401f0f56c93316
("pty: Fix buffer flush deadlock") which bring this behavior:
1:cat BigFile
2:CTRL-C

Signed-off-by: Wang YanQing <udknight@xxxxxxxxx>
---
 drivers/tty/pty.c | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
index a23fa5e..2b90738 100644
--- a/drivers/tty/pty.c
+++ b/drivers/tty/pty.c
@@ -216,16 +216,11 @@ static int pty_signal(struct tty_struct *tty, int sig)
 static void pty_flush_buffer(struct tty_struct *tty)
 {
 	struct tty_struct *to = tty->link;
-	struct tty_ldisc *ld;
 
 	if (!to)
 		return;
 
-	ld = tty_ldisc_ref(to);
-	tty_buffer_flush(to, ld);
-	if (ld)
-		tty_ldisc_deref(ld);
-
+	tty_buffer_flush(to, NULL);
 	if (to->packet) {
 		spin_lock_irq(&tty->ctrl_lock);
 		tty->ctrl_status |= TIOCPKT_FLUSHWRITE;
-- 
1.8.5.6.2.g3d8a54e.dirty