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

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Date:	Tue, 12 May 2009 23:30:52 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Wolfgang Grandegger <wg@...ndegger.com>
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

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.


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