Re: [PATCH 2/2] usbnet: Fix a race between usbnet_stop() and the BH

["Bjørn Mork" <bjorn@mork.no>]

Eugene Shatokhin <eugene.shatokhin@rosalab.ru> writes:

> The race may happen when a device (e.g. YOTA 4G LTE Modem) is
> unplugged while the system is downloading a large file from the Net.
>
> Hardware breakpoints and Kprobes with delays were used to confirm that
> the race does actually happen.
>
> The race is on skb_queue ('next' pointer) between usbnet_stop()
> and rx_complete(), which, in turn, calls usbnet_bh().
>
> Here is a part of the call stack with the code where the changes to the
> queue happen. The line numbers are for the kernel 4.1.0:
>
> *0 __skb_unlink (skbuff.h:1517)
>     prev->next = next;
> *1 defer_bh (usbnet.c:430)
>     spin_lock_irqsave(&list->lock, flags);
>     old_state = entry->state;
>     entry->state = state;
>     __skb_unlink(skb, list);
>     spin_unlock(&list->lock);
>     spin_lock(&dev->done.lock);
>     __skb_queue_tail(&dev->done, skb);
>     if (dev->done.qlen == 1)
>         tasklet_schedule(&dev->bh);
>     spin_unlock_irqrestore(&dev->done.lock, flags);
> *2 rx_complete (usbnet.c:640)
>     state = defer_bh(dev, skb, &dev->rxq, state);
>
> At the same time, the following code repeatedly checks if the queue is
> empty and reads these values concurrently with the above changes:
>
> *0  usbnet_terminate_urbs (usbnet.c:765)
>     /* maybe wait for deletions to finish. */
>     while (!skb_queue_empty(&dev->rxq)
>         && !skb_queue_empty(&dev->txq)
>         && !skb_queue_empty(&dev->done)) {
>             schedule_timeout(msecs_to_jiffies(UNLINK_TIMEOUT_MS));
>             set_current_state(TASK_UNINTERRUPTIBLE);
>             netif_dbg(dev, ifdown, dev->net,
>                   "waited for %d urb completions\n", temp);
>     }
> *1  usbnet_stop (usbnet.c:806)
>     if (!(info->flags & FLAG_AVOID_UNLINK_URBS))
>         usbnet_terminate_urbs(dev);
>
> As a result, it is possible, for example, that the skb is removed from
> dev->rxq by __skb_unlink() before the check
> "!skb_queue_empty(&dev->rxq)" in usbnet_terminate_urbs() is made. It is
> also possible in this case that the skb is added to dev->done queue
> after "!skb_queue_empty(&dev->done)" is checked. So
> usbnet_terminate_urbs() may stop waiting and return while dev->done
> queue still has an item.

Exactly what problem will that result in?  The tasklet_kill() will wait
for the processing of the single element done queue, and everything will
be fine.  Or?


Bjørn