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SubjectRe: unregister_netdevice: waiting for DEV to become free (2)
Hi David,

I looked at patchwork. This patch hasn't been accepted. Is there a plan
to resubmit? It is very useful. I had to debug refcnt issues multiple
times for my employer.

Thanks,

Stephen.

On Mon, Apr 15, 2019 at 09:35:01AM -0600, David Ahern wrote:
> On 4/15/19 7:36 AM, Tetsuo Handa wrote:
> > I traced using debug printk() patch shown below.
> >
>
> I find tracepoints (see attached patch) and perf are easier to use to
> debug device refcnt problems.
>
> For example, limit the stack you have to deal with via sysctl -w
> kernel.perf_event_max_stack=16, and add a filter (e.g., --filter 'name
> == "lo"') to limit collection to a specific device.

> From 068b1b8362ec5fd1b9dffdbd6e84474ada2eb829 Mon Sep 17 00:00:00 2001
> From: David Ahern <dsa@cumulusnetworks.com>
> Date: Thu, 11 Feb 2016 02:40:12 -0800
> Subject: [PATCH] Add tracepoints to dev_hold and dev_put
>
> Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
> ---
> include/linux/netdevice.h | 6 ++++++
> include/trace/events/net.h | 38 ++++++++++++++++++++++++++++++++++++++
> net/core/dev.c | 21 +++++++++++++++++++++
> 3 files changed, 65 insertions(+)
>
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index 219f53c30cb3..7ef6fc672dfb 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -3193,6 +3193,7 @@ extern int netdev_budget;
> /* Called by rtnetlink.c:rtnl_unlock() */
> void netdev_run_todo(void);
>
> +#if 0
> /**
> * dev_put - release reference to device
> * @dev: network device
> @@ -3214,6 +3215,11 @@ static inline void dev_hold(struct net_device *dev)
> {
> this_cpu_inc(*dev->pcpu_refcnt);
> }
> +#else
> +void dev_put(struct net_device *dev);
> +void dev_hold(struct net_device *dev);
> +
> +#endif
>
> /* Carrier loss detection, dial on demand. The functions netif_carrier_on
> * and _off may be called from IRQ context, but it is caller
> diff --git a/include/trace/events/net.h b/include/trace/events/net.h
> index 49cc7c3de252..9ed73dfe9d09 100644
> --- a/include/trace/events/net.h
> +++ b/include/trace/events/net.h
> @@ -236,6 +236,44 @@ DEFINE_EVENT(net_dev_rx_verbose_template, netif_rx_ni_entry,
> TP_ARGS(skb)
> );
>
> +TRACE_EVENT(dev_put,
> +
> + TP_PROTO(struct net_device *dev),
> +
> + TP_ARGS(dev),
> +
> + TP_STRUCT__entry(
> + __string( name, dev->name )
> + __field( int, refcnt )
> + ),
> +
> + TP_fast_assign(
> + __assign_str(name, dev->name);
> + __entry->refcnt = netdev_refcnt_read(dev);
> + ),
> +
> + TP_printk("dev=%s refcnt %d", __get_str(name), __entry->refcnt)
> +);
> +
> +TRACE_EVENT(dev_hold,
> +
> + TP_PROTO(struct net_device *dev),
> +
> + TP_ARGS(dev),
> +
> + TP_STRUCT__entry(
> + __string( name, dev->name )
> + __field( int, refcnt )
> + ),
> +
> + TP_fast_assign(
> + __assign_str(name, dev->name);
> + __entry->refcnt = netdev_refcnt_read(dev);
> + ),
> +
> + TP_printk("dev=%s refcnt %d", __get_str(name), __entry->refcnt)
> +);
> +
> #endif /* _TRACE_NET_H */
>
> /* This part must be outside protection */
> diff --git a/net/core/dev.c b/net/core/dev.c
> index f1284835b8c9..99ac067afd18 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -8117,3 +8117,24 @@ static int __init net_dev_init(void)
> }
>
> subsys_initcall(net_dev_init);
> +
> +
> +void dev_put(struct net_device *dev)
> +{
> + this_cpu_dec(*dev->pcpu_refcnt);
> + trace_dev_put(dev);
> +}
> +EXPORT_SYMBOL(dev_put);
> +
> +/**
> + * dev_hold - get reference to device
> + * @dev: network device
> + *
> + * Hold reference to device to keep it from being freed.
> + */
> +void dev_hold(struct net_device *dev)
> +{
> + this_cpu_inc(*dev->pcpu_refcnt);
> + trace_dev_hold(dev);
> +}
> +EXPORT_SYMBOL(dev_hold);
> --
> 2.1.4
>

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