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@xxxxxxxxx>
Acked-by: Oliver Neukum <oneukum@xxxxxxxx>
> ---
> 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);
> - }
> }
>
> /*
