Re: [PATCHv2 net] usbnet: fix cyclical race on disconnect with work queue

[Paolo Abeni <pabeni@redhat.com>]

On 9/5/24 15:46, Oliver Neukum wrote:
> The work can submit URBs and the URBs can schedule the work.
> This cycle needs to be broken, when a device is to be stopped.
> Use a flag to do so.
> This is a design issue as old as the driver.
> 
> Signed-off-by: Oliver Neukum <oneukum@suse.com>
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> CC: stable@vger.kernel.org
> ---
> 
> v2: fix PM reference issue
> 
>   drivers/net/usb/usbnet.c   | 37 ++++++++++++++++++++++++++++---------
>   include/linux/usb/usbnet.h | 17 +++++++++++++++++
>   2 files changed, 45 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/net/usb/usbnet.c b/drivers/net/usb/usbnet.c
> index 18eb5ba436df..2506aa8c603e 100644
> --- a/drivers/net/usb/usbnet.c
> +++ b/drivers/net/usb/usbnet.c
> @@ -464,10 +464,15 @@ static enum skb_state defer_bh(struct usbnet *dev, struct sk_buff *skb,
>   void usbnet_defer_kevent (struct usbnet *dev, int work)
>   {
>   	set_bit (work, &dev->flags);
> -	if (!schedule_work (&dev->kevent))
> -		netdev_dbg(dev->net, "kevent %s may have been dropped\n", usbnet_event_names[work]);
> -	else
> -		netdev_dbg(dev->net, "kevent %s scheduled\n", usbnet_event_names[work]);
> +	if (!usbnet_going_away(dev)) {
> +		if (!schedule_work(&dev->kevent))
> +			netdev_dbg(dev->net,
> +				   "kevent %s may have been dropped\n",
> +				   usbnet_event_names[work]);
> +		else
> +			netdev_dbg(dev->net,
> +				   "kevent %s scheduled\n", usbnet_event_names[work]);
> +	}
>   }
>   EXPORT_SYMBOL_GPL(usbnet_defer_kevent);
>   
> @@ -535,7 +540,8 @@ static int rx_submit (struct usbnet *dev, struct urb *urb, gfp_t flags)
>   			tasklet_schedule (&dev->bh);
>   			break;
>   		case 0:
> -			__usbnet_queue_skb(&dev->rxq, skb, rx_start);
> +			if (!usbnet_going_away(dev))
> +				__usbnet_queue_skb(&dev->rxq, skb, rx_start);
>   		}
>   	} else {
>   		netif_dbg(dev, ifdown, dev->net, "rx: stopped\n");
> @@ -843,9 +849,18 @@ int usbnet_stop (struct net_device *net)
>   
>   	/* deferred work (timer, softirq, task) must also stop */
>   	dev->flags = 0;
> -	del_timer_sync (&dev->delay);
> -	tasklet_kill (&dev->bh);
> +	del_timer_sync(&dev->delay);
> +	tasklet_kill(&dev->bh);
>   	cancel_work_sync(&dev->kevent);
> +
> +	/* We have cyclic dependencies. Those calls are needed
> +	 * to break a cycle. We cannot fall into the gaps because
> +	 * we have a flag
> +	 */
> +	tasklet_kill(&dev->bh);
> +	del_timer_sync(&dev->delay);
> +	cancel_work_sync(&dev->kevent);

I guess you do the shutdown twice because a running tasklet or timer 
could re-schedule the others? If so, what prevent the rescheduling to 
happen in the 2nd iteration? why can't you add usbnet_going_away() 
checks on tasklet and timer reschedule point?

Thanks,

Paolo