Re: [PATCH V4 1/3] USB: serial: f81232: clear overrun flag

Oliver Neukum
2019-04-02T12:38:11Z

On Di, 2019-04-02 at 13:26 +0800, Ji-Ze Hong (Peter Hong) wrote:
> The F81232 will report data and LSR with bulk like following format:
> bulk-in data: [LSR(1Byte)+DATA(1Byte)][LSR(1Byte)+DATA(1Byte)]...
>
> LSR will auto clear frame/parity/break error flag when reading by H/W,
> but overrrun will only cleared when reading LSR. So this patch add a
> worker to read LSR when overrun and flush the worker on close() &
> suspend().

Hi,

I really hate doing this to you, but you are exchanging one race
condition for another race. Exact explanation below.

> @@ -315,6 +319,7 @@ static void f81232_process_read_urb(struct urb *urb)
>
> if (lsr & UART_LSR_OE) {
> port->icount.overrun++;
> + schedule_work(&priv->lsr_work);

Again you schedule the work. It may run anytime.
The check you put into the work item needs to go here.

> +static void f81232_lsr_worker(struct work_struct *work)
> +{
> + struct f81232_private *priv;
> + struct usb_serial_port *port;
> + struct usb_serial *serial;
> + int status;
> + u8 tmp;
> +
> + priv = container_of(work, struct f81232_private, lsr_work);
> + port = priv->port;
> + serial = port->serial;
> +
> + if (serial->suspending) {

There is no locking. f81232_resume() can run here.
This test
if (port_priv->lsr_work_resched) {
will evaluate to false

> + priv->lsr_work_resched = true;
> + return;
> + }
> +
> + status = f81232_get_register(port, LINE_STATUS_REGISTER, &tmp);
> + if (status)
> + dev_warn(&port->dev, "read LSR failed: %d\n", status);
> +}
> +
> static int f81232_port_probe(struct usb_serial_port *port)
> {
> struct f81232_private *priv;
> @@ -613,6 +643,7 @@ static int f81232_port_probe(struct usb_serial_port *port)
>
> mutex_init(&priv->lock);
> INIT_WORK(&priv->interrupt_work, f81232_interrupt_work);
> + INIT_WORK(&priv->lsr_work, f81232_lsr_worker);
>
> usb_set_serial_port_data(port, priv);
>
> @@ -632,6 +663,30 @@ static int f81232_port_remove(struct usb_serial_port *port)
> return 0;
> }
>
> +static int f81232_suspend(struct usb_serial *serial, pm_message_t message)
> +{
> + struct f81232_private *port_priv;
> +
> + port_priv = usb_get_serial_port_data(serial->port[0]);
> + flush_work(&port_priv->lsr_work);

Strictly speaking useless.

> +
> + return 0;
> +}
> +
> +static int f81232_resume(struct usb_serial *serial)
> +{
> + struct f81232_private *port_priv;
> +
> + port_priv = usb_get_serial_port_data(serial->port[0]);
> +
> + if (port_priv->lsr_work_resched) {
> + port_priv->lsr_work_resched = false;

Strictly speaking even that is a race, as you have no guarantee that
your work queue is run before the system is suspended again. You
are in a task context. There is no reason to defer action.

> + schedule_work(&port_priv->lsr_work);
> + }
> +
> + return usb_serial_generic_resume(serial);
> +}

Regards
Oliver