usb: cdc-acm: fix race during wakeup blocking TX traffic

[Oliver Neukum <oneukum@suse.com>]

On Fr, 2019-03-22 at 16:53 +0100, Romain Izard wrote:
> When the kernel is compiled with preemption enabled, the URB completion
> handler can run in parallel with the work responsible for waking up the
> tty layer. If the URB handler sets the EVENT_TTY_WAKEUP bit during the
> call to tty_port_tty_wakeup() to signal that there is room for additional
> input, it will be cleared at the end of this call. As a result, TX traffic
> on the upper layer will be blocked.

Good catch.

> This can be seen with a kernel configured with CONFIG_PREEMPT, and a fast
> modem connected with PPP running over a USB CDC-ACM port.
> 
> Use test_and_clear_bit() instead, which ensures that each wakeup requested
> by the URB completion code will trigger a call to tty_port_tty_wakeup().
> 
> Fixes: 1aba579f3cf5 cdc-acm: handle read pipe errors
> Signed-off-by: Romain Izard <romain.izard.pro@gmail.com>
Acked-by: Oliver Neukum <oneukum@suse.com>
> ---
>  drivers/usb/class/cdc-acm.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c
> index 739f8960811a..ec666eb4b7b4 100644
> --- a/drivers/usb/class/cdc-acm.c
> +++ b/drivers/usb/class/cdc-acm.c
> @@ -558,10 +558,8 @@ static void acm_softint(struct work_struct *work)
>  		clear_bit(EVENT_RX_STALL, &acm->flags);
>  	}
>  
> -	if (test_bit(EVENT_TTY_WAKEUP, &acm->flags)) {
> +	if (test_and_clear_bit(EVENT_TTY_WAKEUP, &acm->flags))
>  		tty_port_tty_wakeup(&acm->port);
> -		clear_bit(EVENT_TTY_WAKEUP, &acm->flags);
> -	}
>  }
>  
>  /*