[PATCH 2/3] tty: n_hdlc: fix read() hangup race

[Johan Hovold <johan@kernel.org>]

Make sure to set the task state before checking the break conditions in
the read() wait loop to avoid missing a racing wakeup.

Note that before commit c96cf923a98d ("tty: Don't block on IO when ldisc
change is pending") a racing task hanging up the tty could get stuck in
uninterruptible sleep when there are blocked readers, but since then
read() incorrectly returns -EAGAIN instead.

The false-positive stuck hangup issue was flagged by an LLM.

Fixes: 1035b63d3c6f ("n_hdlc: fix read and write locking")
Cc: stable@vger.kernel.org	# 2.6.38
Signed-off-by: Johan Hovold <johan@kernel.org>
---
 drivers/tty/n_hdlc.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/n_hdlc.c b/drivers/tty/n_hdlc.c
index 0fbc44968a44..02676ae9b112 100644
--- a/drivers/tty/n_hdlc.c
+++ b/drivers/tty/n_hdlc.c
@@ -431,6 +431,8 @@ static ssize_t n_hdlc_tty_read(struct tty_struct *tty, struct file *file,
 	add_wait_queue(&tty->read_wait, &wait);
 
 	for (;;) {
+		set_current_state(TASK_INTERRUPTIBLE);
+
 		if (test_bit(TTY_OTHER_CLOSED, &tty->flags)) {
 			ret = -EIO;
 			break;
@@ -438,8 +440,6 @@ static ssize_t n_hdlc_tty_read(struct tty_struct *tty, struct file *file,
 		if (tty_hung_up_p(file))
 			break;
 
-		set_current_state(TASK_INTERRUPTIBLE);
-
 		rbuf = n_hdlc_buf_get(&n_hdlc->rx_buf_list);
 		if (rbuf)
 			break;
-- 
2.55.0