Re: [PATCH v2 3/7] [PATCH 3/8] can: CAN Network device driver and Netlink interface

Oliver Hartkopp
2009-05-13T12:09:44Z

Wolfgang Grandegger wrote:
> Oliver Hartkopp wrote:
>
>> Andrew Morton wrote:
>>
>>> On Tue, 12 May 2009 11:28:00 +0200 Wolfgang Grandegger
>>> <wg@grandegger.com> wrote:
>>>
>>>
>>>
>>>> +int can_restart_now(struct net_device *dev)
>>>> +{
>>>> + struct can_priv *priv = netdev_priv(dev);
>>>> + struct net_device_stats *stats = &dev->stats;
>>>> + struct sk_buff *skb;
>>>> + struct can_frame *cf;
>>>> + int err;
>>>> +
>>>> + /* Synchronize with dev->hard_start_xmit() */
>>>> + netif_tx_lock(dev);
>>>> +
>>>> + /* Ensure that no more messages can go out */
>>>> + if (netif_carrier_ok(dev))
>>>> + netif_carrier_off(dev);
>>>> +
>>>> + /* Ensure that no more messages can come in */
>>>> + err = priv->do_set_mode(dev, CAN_MODE_STOP);
>>>> + if (err)
>>>> + return err;
>>>> +
>>>> + /* Now it's save to clean up */
>>>> + del_timer_sync(&priv->restart_timer);
>>>>
>>>>
>>> This is deadlockable.
>>>
>>> It calls del_timer_sync() while holding netif_tx_lock(). But the timer
>>> handler (can_restart_now()) also takes netif_tx_lock(). So if the
>>> timer handler is presently running, it's sitting there spinning in
>>> netif_tx_lock(). And del_timer_sync() is sitting there waiting for the
>>> timer handler to complete.
>>>
>>>
>>>
>>>
>> Hi Wolfgang,
>>
>> would it be an appropriate solution, just to invoke
>>
>> netif_stop_queue() in can_bus_off()
>>
>> and invoke
>>
>> netif_wake_queue() in can_restart_now()
>>
>> ???
>>
>> In a BUSOFF condition we're not able to send CAN frames anyway, so we
>> can disable the device queue and the we won't need any netif_tx_lock()
>> right?
>>
>> AFAIK this was the original implementation before some of the latest
>> improvement with the netif_carrier stuff.
>>
>> What do you think?
>>
>
> The problem is the "manual" restart triggered via the netlink interface,
> which can occur in the middle of ndo_start_xmit().
>
>

Ah, i see.

What if the manual restart via netlink would also stop the queue and
start the timer?

Regards,
Oliver