This is a note to let you know that I've just added the patch titled serial: 8250: Fix race condition in RTS-after-send handling to the 5.15-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: serial-8250-fix-race-condition-in-rts-after-send-han.patch and it can be found in the queue-5.15 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. commit 23b37f776f2ee455475bfecf59c95efddc5a20cc Author: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx> Date: Tue Feb 15 17:02:36 2022 +0100 serial: 8250: Fix race condition in RTS-after-send handling [ Upstream commit dedab69fd650ea74710b2e626e63fd35584ef773 ] Set em485->active_timer = NULL isn't always enough to take out the stop timer. While there is a check that it acts in the right state (i.e. waiting for RTS-after-send to pass after sending some chars) but the following might happen: - CPU1: some chars send, shifter becomes empty, stop tx timer armed - CPU0: more chars send before RTS-after-send expired - CPU0: shifter empty irq, port lock taken - CPU1: tx timer triggers, waits for port lock - CPU0: em485->active_timer = &em485->stop_tx_timer, hrtimer_start(), releases lock() - CPU1: get lock, see em485->active_timer == &em485->stop_tx_timer, tear down RTS too early This fix bases on research done by Steffen Trumtrar. Fixes: b86f86e8e7c5 ("serial: 8250: fix potential deadlock in rs485-mode") Signed-off-by: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx> Link: https://lore.kernel.org/r/20220215160236.344236-1-u.kleine-koenig@xxxxxxxxxxxxxx Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c index b470bc747b99..868ccb3e16cf 100644 --- a/drivers/tty/serial/8250/8250_port.c +++ b/drivers/tty/serial/8250/8250_port.c @@ -1623,6 +1623,18 @@ static inline void start_tx_rs485(struct uart_port *port) struct uart_8250_port *up = up_to_u8250p(port); struct uart_8250_em485 *em485 = up->em485; + /* + * While serial8250_em485_handle_stop_tx() is a noop if + * em485->active_timer != &em485->stop_tx_timer, it might happen that + * the timer is still armed and triggers only after the current bunch of + * chars is send and em485->active_timer == &em485->stop_tx_timer again. + * So cancel the timer. There is still a theoretical race condition if + * the timer is already running and only comes around to check for + * em485->active_timer when &em485->stop_tx_timer is armed again. + */ + if (em485->active_timer == &em485->stop_tx_timer) + hrtimer_try_to_cancel(&em485->stop_tx_timer); + em485->active_timer = NULL; if (em485->tx_stopped) {