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

Andrew Morton
2009-05-13T06:55:59Z

On Tue, 12 May 2009 23:30:52 -0700 Andrew Morton <akpm@linux-foundation.org> wrote:

> On Tue, 12 May 2009 11:28:00 +0200 Wolfgang Grandegger <wg@grandegger.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()>