[PATCH] staging: greybus: uart: fix racy TIOCMIWAIT implementation

[Johan Hovold <johan@kernel.org>]

The wakeup condition needs to be checked after adding the task to the
waitqueue and updating the task state to avoid missing a racing modem
status update or disconnect.

Fixes: e68453ed28c5 ("greybus: uart-gb: now builds, more framework added")
Cc: stable@vger.kernel.org	# 4.9
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
---
 drivers/staging/greybus/uart.c | 23 ++++++++++++++---------
 1 file changed, 14 insertions(+), 9 deletions(-)

diff --git a/drivers/staging/greybus/uart.c b/drivers/staging/greybus/uart.c
index 24b4dab069c3..bcfea0a02155 100644
--- a/drivers/staging/greybus/uart.c
+++ b/drivers/staging/greybus/uart.c
@@ -643,7 +643,10 @@ static int wait_serial_change(struct gb_tty *gb_tty, unsigned long arg)
 	if (!(arg & (TIOCM_DSR | TIOCM_RI | TIOCM_CD)))
 		return -EINVAL;
 
-	do {
+	add_wait_queue(&gb_tty->wioctl, &wait);
+	for (;;) {
+		set_current_state(TASK_INTERRUPTIBLE);
+
 		spin_lock_irq(&gb_tty->read_lock);
 		old = gb_tty->oldcount;
 		new = gb_tty->iocount;
@@ -657,18 +660,20 @@ static int wait_serial_change(struct gb_tty *gb_tty, unsigned long arg)
 		if ((arg & TIOCM_RI) && (old.rng != new.rng))
 			break;
 
-		add_wait_queue(&gb_tty->wioctl, &wait);
-		set_current_state(TASK_INTERRUPTIBLE);
-		schedule();
-		remove_wait_queue(&gb_tty->wioctl, &wait);
 		if (gb_tty->disconnected) {
-			if (arg & TIOCM_CD)
-				break;
 			retval = -ENODEV;
-		} else if (signal_pending(current)) {
+			break;
+		}
+
+		schedule();
+
+		if (signal_pending(current)) {
 			retval = -ERESTARTSYS;
+			break;
 		}
-	} while (!retval);
+	}
+	__set_current_state(TASK_RUNNING);
+	remove_wait_queue(&gb_tty->wioctl, &wait);
 
 	return retval;
 }
-- 
2.55.0