[PATCH v2] staging: greybus: uart: fix racy TIOCMIWAIT implementation

[Johan Hovold <johan@kernel.org>]

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

Store the old icount on entry and do not update it in the completion
handler to avoid missing events or reporting spurious ones (due to modem
status changes before TIOCMIWAIT was called).

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>
---

Changes in v2
 - store old icount on entry only


 drivers/staging/greybus/uart.c | 30 ++++++++++++++++++------------
 1 file changed, 18 insertions(+), 12 deletions(-)

diff --git a/drivers/staging/greybus/uart.c b/drivers/staging/greybus/uart.c
index 24b4dab069c3..d47f00751e7a 100644
--- a/drivers/staging/greybus/uart.c
+++ b/drivers/staging/greybus/uart.c
@@ -53,7 +53,6 @@ struct gb_tty {
 	spinlock_t read_lock;
 	spinlock_t write_lock;
 	struct async_icount iocount;
-	struct async_icount oldcount;
 	wait_queue_head_t wioctl;
 	struct mutex mutex;
 	u8 ctrlin;	/* input control lines */
@@ -643,11 +642,16 @@ static int wait_serial_change(struct gb_tty *gb_tty, unsigned long arg)
 	if (!(arg & (TIOCM_DSR | TIOCM_RI | TIOCM_CD)))
 		return -EINVAL;
 
-	do {
+	spin_lock_irq(&gb_tty->read_lock);
+	old = gb_tty->iocount;
+	spin_unlock_irq(&gb_tty->read_lock);
+
+	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;
-		gb_tty->oldcount = new;
 		spin_unlock_irq(&gb_tty->read_lock);
 
 		if ((arg & TIOCM_DSR) && (old.dsr != new.dsr))
@@ -657,18 +661,20 @@ 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