[PATCH v4 3/4] serial: fix TIOCMIWAIT race

[Johan Hovold <johan@kernel.org>]

The task state must be updated before checking the wakeup condition to
avoid missing a racing modem status update.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: sashiko-bot@kernel.org
Link: https://lore.kernel.org/r/20260904115542.E20D11F00A3D@smtp.kernel.org
Cc: stable@vger.kernel.org
Signed-off-by: Johan Hovold <johan@kernel.org>
---
 drivers/tty/serial/serial_core.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 139b93876229..f7b30f77b231 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -1230,12 +1230,12 @@ static int uart_wait_modem_status(struct tty_struct *tty, struct uart_state *sta
 
 	add_wait_queue(&port->delta_msr_wait, &wait);
 	for (;;) {
+		set_current_state(TASK_INTERRUPTIBLE);
+
 		uart_port_lock_irq(uport);
 		memcpy(&cnow, &uport->icount, sizeof(struct uart_icount));
 		uart_port_unlock_irq(uport);
 
-		set_current_state(TASK_INTERRUPTIBLE);
-
 		if (((arg & TIOCM_RNG) && (cnow.rng != cprev.rng)) ||
 		    ((arg & TIOCM_DSR) && (cnow.dsr != cprev.dsr)) ||
 		    ((arg & TIOCM_CD)  && (cnow.dcd != cprev.dcd)) ||
-- 
2.55.0