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SubjectRe: [PATCH 2/2] usbnet: Fix a race between usbnet_stop() and the BH
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On Fri, 2015-08-28 at 11:09 +0300, Eugene Shatokhin wrote:
> > 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?
>
> Given enough time, what prevents defer_bh() from calling
> tasklet_schedule(&dev->bh) *after* usbnet_stop() calls tasklet_kill()?
>
> Consider the following situation (assuming '&&' are changed to '||'
> in
> that while loop in usbnet_terminate_urbs() as they should be):
>
> CPU0 CPU1
> usbnet_stop() defer_bh() with list == dev->rxq
> usbnet_terminate_urbs()
> __skb_unlink() removes the last
> skb from dev->rxq.
> dev->rxq, dev->txq and dev->done
> are now empty.
> while (!skb_queue_empty()...)
> The loop ends because all 3
> queues are now empty.
>
> usbnet_terminate_urbs() ends.
>
> usbnet_stop() continues:
> usbnet_status_stop(dev);
> ...
> del_timer_sync (&dev->delay);
> tasklet_kill (&dev->bh);
> __skb_queue_tail(&dev->done, skb);
> if (dev->done.qlen == 1)
> tasklet_schedule(&dev->bh);
>
> The BH is scheduled at this point, which is not what was intended.
> The
> race window is small, but still.

This looks possible. I cannot come up with a better fix, although
it isn't nice. Thanks for finding this.

Regards
Oliver




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