diff options
| author | Uwe Kleine-König <u.kleine-koenig@pengutronix.de> | 2022-02-15 17:02:36 +0100 |
|---|---|---|
| committer | Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2022-04-15 14:18:18 +0200 |
| commit | 1da082f728c72737c08ee38a4ae70f408dca67cc (patch) | |
| tree | ef6d5908ac11c89f690b7b44ace2411a2b4341a7 | |
| parent | 617d9c0b98063e6be8d81bada85969874cf43404 (diff) | |
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@pengutronix.de>
Link: https://lore.kernel.org/r/20220215160236.344236-1-u.kleine-koenig@pengutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
| -rw-r--r-- | drivers/tty/serial/8250/8250_port.c | 12 |
1 files changed, 12 insertions, 0 deletions
diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c index 777ef1a9591c..87567515591e 100644 --- a/drivers/tty/serial/8250/8250_port.c +++ b/drivers/tty/serial/8250/8250_port.c @@ -1545,6 +1545,18 @@ static inline void start_tx_rs485(struct uart_port *port) if (!(up->port.rs485.flags & SER_RS485_RX_DURING_TX)) serial8250_stop_rx(&up->port); + /* + * 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; mcr = serial8250_in_MCR(up); |