[PATCH 5.15.y] serial: fix TIOCMIWAIT race

[Sasha Levin <sashal@kernel.org>]

From: Johan Hovold <johan@kernel.org>

[ Upstream commit 40adffec8c0090533dfaa863ced7901b60bbf1dc ]

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@kernel.org
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://patch.msgid.link/20260910130816.642699-4-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[ sashal: Adjusted context for direct spin_lock_irq() and spin_unlock_irq() calls. ]
Signed-off-by: Sasha Levin <sashal@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 404354727d194..1cba822e068e9 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -1213,12 +1213,12 @@ static int uart_wait_modem_status(struct uart_state *state, unsigned long arg)
 
 	add_wait_queue(&port->delta_msr_wait, &wait);
 	for (;;) {
+		set_current_state(TASK_INTERRUPTIBLE);
+
 		spin_lock_irq(&uport->lock);
 		memcpy(&cnow, &uport->icount, sizeof(struct uart_icount));
 		spin_unlock_irq(&uport->lock);
 
-		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)) ||
-- 
2.53.0