PPPOE lockdep report in dev_queue_xmit+0x8b8/0x900

From: Eric Dumazet <edumazet@google.com>

On Mon, 2013-02-18 at 19:43 +0200, Yanko Kaneti wrote:
> Hello,
> 
> I've had the following lockdep report for the last couple of years of
> kernels. I don't think I've had a lockup during that time related to
> pppoe.
> 
> The pppoe entry in the MAINTAINERS file lists Michal Ostrowski <mostrows@earthlink.net>
> which unfortunately bounces.
> 

Thanks for the report.

Could you please test following patch ?

[PATCH] ppp: set qdisc_tx_busylock to avoid LOCKDEP splat

If a qdisc is installed on a ppp device, its possible to get
a lockdep splat under stress, because nested dev_queue_xmit() can
lock busylock a second time (on a different device, so its a false
positive)

Avoid this problem using a distinct lock_class_key for team
devices.

Reported-by: Yanko Kaneti <yaneti@declera.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
---
 drivers/net/ppp/ppp_generic.c |    8 ++++++++
 1 file changed, 8 insertions(+)



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On Mon, 2013-02-18 at 11:50 -0800, Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@google.com>
> 
> On Mon, 2013-02-18 at 19:43 +0200, Yanko Kaneti wrote:
> > Hello,
> > 
> > I've had the following lockdep report for the last couple of years of
> > kernels. I don't think I've had a lockup during that time related to
> > pppoe.
> > 
> > The pppoe entry in the MAINTAINERS file lists Michal Ostrowski <mostrows@earthlink.net>
> > which unfortunately bounces.
> > 
> 
> Thanks for the report.
> 
> Could you please test following patch ?

It looks like it has done the job. I am running a kernel with the patch
and workload that otherwise inevitably triggers the splat and so far it
has been quiet.

Thanks

> 
> [PATCH] ppp: set qdisc_tx_busylock to avoid LOCKDEP splat
> 
> If a qdisc is installed on a ppp device, its possible to get
> a lockdep splat under stress, because nested dev_queue_xmit() can
> lock busylock a second time (on a different device, so its a false
> positive)
> 
> Avoid this problem using a distinct lock_class_key for team
> devices.
> 
> Reported-by: Yanko Kaneti <yaneti@declera.com>
> Signed-off-by: Eric Dumazet <edumazet@google.com>
> ---
>  drivers/net/ppp/ppp_generic.c |    8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
> index 4fd754e..3db9131 100644
> --- a/drivers/net/ppp/ppp_generic.c
> +++ b/drivers/net/ppp/ppp_generic.c
> @@ -1058,7 +1058,15 @@ ppp_get_stats64(struct net_device *dev, struct rtnl_link_stats64 *stats64)
>         return stats64;
>  }
>  
> +static struct lock_class_key ppp_tx_busylock;
> +static int ppp_dev_init(struct net_device *dev)
> +{
> +       dev->qdisc_tx_busylock = &ppp_tx_busylock;
> +       return 0;
> +}
> +
>  static const struct net_device_ops ppp_netdev_ops = {
> +       .ndo_init        = ppp_dev_init,
>         .ndo_start_xmit  = ppp_start_xmit,
>         .ndo_do_ioctl    = ppp_net_ioctl,
>         .ndo_get_stats64 = ppp_get_stats64,
> 
> 
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