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serial: 8250: Fix race condition in RTS-after-send handling

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[ 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: b86f86e8e7 ("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>
This commit is contained in:
Uwe Kleine-König 2022-02-15 17:02:36 +01:00 committed by Greg Kroah-Hartman
parent 617d9c0b98
commit 1da082f728
1 changed files with 12 additions and 0 deletions
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12
drivers/tty/serial/8250/8250_port.c
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@ -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);