netdev - Re: [PATCH] net: remove static inline from dev_put/dev

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Date:   Mon, 11 Nov 2019 08:56:32 -0800
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Tony Lu <tonylu@...ux.alibaba.com>
Cc:     davem@...emloft.net, shemminger@...l.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: remove static inline from dev_put/dev_hold

On Mon, 11 Nov 2019 22:05:03 +0800
Tony Lu <tonylu@...ux.alibaba.com> 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
> 
> 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.
> 
> Signed-off-by: Tony Lu <tonylu@...ux.alibaba.com>

In the past dev_hold/dev_put was in the hot path for several
operations. What is the performance implication of doing this?