Re: [PATCH 4/8] serial: core: Take port mutex for send_xchar() tty method

[Peter Hurley <peter@hurleysoftware.com>]

On 12/12/2015 04:36 PM, Peter Hurley wrote:
> Prepare for separate methods of preventing unsafe uart port access
> when sending xchar; claim port mutex for the tty ops method (which
> might sleep) and rcu for the throttle/unthrottle uses.
> 
> The implied limitation is that uart drivers which support
> AUTOXOFF flow control cannot define a send_xchar() method that sleeps.

This patch doesn't do what I want, so I need to rework this solution.

Regards,
Peter Hurley


> Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
> ---
>  drivers/tty/serial/serial_core.c | 30 +++++++++++++++++++-----------
>  1 file changed, 19 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
> index 4430518..806eba81 100644
> --- a/drivers/tty/serial/serial_core.c
> +++ b/drivers/tty/serial/serial_core.c
> @@ -610,23 +610,31 @@ static void uart_flush_buffer(struct tty_struct *tty)
>   * This function is used to send a high-priority XON/XOFF character to
>   * the device
>   */
> -static void uart_send_xchar(struct tty_struct *tty, char ch)
> +static void __uart_send_xchar(struct uart_port *uport, char ch)
>  {
> -	struct uart_state *state = tty->driver_data;
> -	struct uart_port *port = state->uart_port;
>  	unsigned long flags;
>  
> -	if (port->ops->send_xchar)
> -		port->ops->send_xchar(port, ch);
> +	if (uport->ops->send_xchar)
> +		uport->ops->send_xchar(uport, ch);
>  	else {
> -		spin_lock_irqsave(&port->lock, flags);
> -		port->x_char = ch;
> +		spin_lock_irqsave(&uport->lock, flags);
> +		uport->x_char = ch;
>  		if (ch)
> -			port->ops->start_tx(port);
> -		spin_unlock_irqrestore(&port->lock, flags);
> +			uport->ops->start_tx(port);
> +		spin_unlock_irqrestore(&uport->lock, flags);
>  	}
>  }
>  
> +static void uart_send_xchar(struct tty_struct *tty, char ch)
> +{
> +	struct uart_state *state = tty->driver_data;
> +	struct uart_port *uport = state->uart_port;
> +
> +	mutex_lock(&state->port.mutex);
> +	__uart_send_xchar(uport, ch);
> +	mutex_unlock(&state->port.mutex);
> +}
> +
>  static void uart_throttle(struct tty_struct *tty)
>  {
>  	struct uart_state *state = tty->driver_data;
> @@ -644,7 +652,7 @@ static void uart_throttle(struct tty_struct *tty)
>  	}
>  
>  	if (mask & UPSTAT_AUTOXOFF)
> -		uart_send_xchar(tty, STOP_CHAR(tty));
> +		__uart_send_xchar(port, STOP_CHAR(tty));
>  
>  	if (mask & UPSTAT_AUTORTS)
>  		uart_clear_mctrl(port, TIOCM_RTS);
> @@ -667,7 +675,7 @@ static void uart_unthrottle(struct tty_struct *tty)
>  	}
>  
>  	if (mask & UPSTAT_AUTOXOFF)
> -		uart_send_xchar(tty, START_CHAR(tty));
> +		__uart_send_xchar(port, START_CHAR(tty));
>  
>  	if (mask & UPSTAT_AUTORTS)
>  		uart_set_mctrl(port, TIOCM_RTS);
>