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

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Date:	Wed, 13 May 2009 13:37:16 +0200
From:	Wolfgang Grandegger <wg@...ndegger.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	Oliver Hartkopp <oliver.hartkopp@...kswagen.de>
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 23:30:52 -0700 Andrew Morton <akpm@...ux-foundation.org> 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.

Oops, I have obviously overlook that.

> Also, I wonder if it's safe to take netif_tx_lock() from a timer
> handler when other parts of the code might be taking it from process
> context (I didn't check).
> 
> lockdep should be able to detect this, and I trust this code has been
> fully runtime tested with lockdep enabled?

Well, CONFIG_PROVE_LOCKING would be cool, but I'm unable to enabled it
for my MPC5200 test system. Only 64bit PowerPC's seem to support
TRACE_IRQFLAGS_SUPPORT. I'm going to test the code on a PC as well.

Thanks,

Wolfgang.
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