netdev - 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 23:53:23 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Wolfgang Grandegger <wg@...ndegger.com>, 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 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.

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?

<boggles at the size of the inlined netif_tx_lock()>
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