Messages in this thread | | | Subject | Re: [PATCH 2/2] usbnet: Fix a race between usbnet_stop() and the BH | From | Eugene Shatokhin <> | Date | Fri, 28 Aug 2015 11:09:52 +0300 |
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25.08.2015 00:01, Bjørn Mork пишет: > 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?
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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