Re: [PATCH] net: remove static inline from dev_put/dev_hold

On Mon, Nov 11, 2019 at 6:12 AM Tony Lu [off-list ref] wrote:

This patch removes static inline from dev_put/dev_hold in order to help
trace the pcpu_refcnt leak of net_device.

We have sufferred this kind of issue for several times during
manipulating NIC between different net namespaces. It prints this
log in dmesg:

  unregister_netdevice: waiting for eth0 to become free. Usage count = 1

I debugged a nasty dst refcnt leak in TCP a long time ago, so I can
feel your pain.

However, it is hard to find out who called and leaked refcnt in time. It
only left the crime scene but few evidence. Once leaked, it is not
safe to fix it up on the running host. We can't trace dev_put/dev_hold
directly, for the functions are inlined and used wildly amoung modules.
And this issue is common, there are tens of patches fix net_device
refcnt leak for various causes.

To trace the refcnt manipulating, this patch removes static inline from
dev_put/dev_hold. We can use handy tools, such as eBPF with kprobe, to
find out who holds but forgets to put refcnt. This will not be called
frequently, so the overhead is limited.

I think tracepoint serves the purpose of tracking function call history,
you can add tracepoint for each of dev_put()/dev_hold(), which could
also inherit the trace filter and trigger too.

The netdev refcnt itself is not changed very frequently, but it is
refcnt'ed by other things like dst too which is changed frequently.
This is why usually when you see the netdev refcnt leak warning,
the problem is probably somewhere else, like dst refcnt leak.

Hope this helps.

Thanks.