25.08.2015 00:01, Bjørn Mork пишет:
Eugene Shatokhin <eugene.shatok...@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?
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.
Regards,
Eugene
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