Eugene Shatokhin <eugene.shatokhin@xxxxxxxxxx> 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
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