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

From: Oliver Hartkopp <[email-masked]>
Date: Wed, 13 May 2009 14:08:04 +0200

Wolfgang Grandegger wrote:
> Oliver Hartkopp wrote:
>   
>> Andrew Morton wrote:
>>     
>>> On Tue, 12 May 2009 11:28:00 +0200 Wolfgang Grandegger
>>> <[email-masked]> 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