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    SubjectRe: [PATCH v2 3/7] [PATCH 3/8] can: CAN Network device driver and Netlink interface
    Oliver Hartkopp wrote:
    > Wolfgang Grandegger wrote:
    >> Oliver Hartkopp wrote:
    >>
    >>> Andrew Morton 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.
    >>>>
    >>>>
    >>>>
    >>> Hi Wolfgang,
    >>>
    >>> would it be an appropriate solution, just to invoke
    >>>
    >>> netif_stop_queue() in can_bus_off()
    >>>
    >>> and invoke
    >>>
    >>> netif_wake_queue() in can_restart_now()
    >>>
    >>> ???
    >>>
    >>> In a BUSOFF condition we're not able to send CAN frames anyway, so we
    >>> can disable the device queue and the we won't need any netif_tx_lock()
    >>> right?
    >>>
    >>> AFAIK this was the original implementation before some of the latest
    >>> improvement with the netif_carrier stuff.
    >>>
    >>> What do you think?
    >>>
    >>
    >> The problem is the "manual" restart triggered via the netlink interface,
    >> which can occur in the middle of ndo_start_xmit().
    >>
    >>
    >
    > Ah, i see.
    >
    > What if the manual restart via netlink would also stop the queue and
    > start the timer?

    It will not help if the restart is triggered in the middle of
    ndo_start_xmit().

    Wolfgang.


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