Re: [PATCH 1/2] tty: serial: 8250: always call tx_chars under spinlock

[Steffen Trumtrar <s.trumtrar@pengutronix.de>]

Hi!

Johan Hovold <johan@kernel.org> writes:

> On Wed, Dec 09, 2020 at 10:17:27AM +0100, Steffen Trumtrar wrote:
>> In most cases serial8250_tx_chars is called with spinlock held.
>> Fix the remaining location, too.
>
> Please explain where __start_tx() is called without holding the port
> lock and consider fixing that up.
>
>> Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
>> ---
>>  drivers/tty/serial/8250/8250_port.c | 6 +++++-
>>  1 file changed, 5 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
>> index b0af13074cd3..3310c2b70138 100644
>> --- a/drivers/tty/serial/8250/8250_port.c
>> +++ b/drivers/tty/serial/8250/8250_port.c
>> @@ -1559,6 +1559,7 @@ static void serial8250_stop_tx(struct uart_port *port)
>>  static inline void __start_tx(struct uart_port *port)
>>  {
>>  	struct uart_8250_port *up = up_to_u8250p(port);
>> +	unsigned long flags;
>>
>>  	if (up->dma && !up->dma->tx_dma(up))
>>  		return;
>> @@ -1569,8 +1570,11 @@ static inline void __start_tx(struct uart_port *port)
>>
>>  			lsr = serial_in(up, UART_LSR);
>>  			up->lsr_saved_flags |= lsr & LSR_SAVE_FLAGS;
>> -			if (lsr & UART_LSR_THRE)
>> +			if (lsr & UART_LSR_THRE) {
>> +				spin_lock_irqsave(&port->lock, flags);
>>  				serial8250_tx_chars(up);
>> +				spin_unlock_irqrestore(&port->lock, flags);
>> +			}
>
> Since several callers of __start_tx() do hold the lock, this change will
> introduce a deadlock.
>

Meh, yeah :(

Seem like the correct solution would be to fix start_tx_rs485 and
serial8250_start_tx instead.


Best regards,
Steffen Trumtrar

--
Pengutronix e.K.                | Dipl.-Inform. Steffen Trumtrar |
Steuerwalder Str. 21            | https://www.pengutronix.de/    |
31137 Hildesheim, Germany       | Phone: +49-5121-206917-0       |
Amtsgericht Hildesheim, HRA 2686| Fax:   +49-5121-206917-5555    |