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

Wolfgang Grandegger
2009-05-13T11:37:34Z

Andrew Morton wrote:
> 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.

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.