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

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Message-ID: <4A0A9AC1.2090603@volkswagen.de>
Date:	Wed, 13 May 2009 12:02:41 +0200
From:	Oliver Hartkopp <oliver.hartkopp@...kswagen.de>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Wolfgang Grandegger <wg@...ndegger.com>
CC:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/7] [PATCH 3/8] can: CAN Network device driver and
 Netlink interface

Andrew Morton wrote:
> On Tue, 12 May 2009 11:28:00 +0200 Wolfgang Grandegger <wg@...ndegger.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?

Best regards,
Oliver

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