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

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

Steffen Trumtrar <s.trumtrar@pengutronix.de> writes:

> 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.
>

Scratch that. This patch is bogus, I'm stupid, port-lock is already held
by uart_write apart from the hrtimer-callback.

Sorry for the noise.


Best regards,
Steffen Trumtrar

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