Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Mon, 11 Nov 2019 08:56:32 -0800
From:   Stephen Hemminger <stephen@networkplumber.org>
To:     Tony Lu <tonylu@linux.alibaba.com>
Cc:     davem@davemloft.net, shemminger@osdl.org, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] net: remove static inline from dev_put/dev_hold
Message-ID: <20191111085632.24d88706@hermes.lan>
In-Reply-To: <20191111140502.17541-1-tonylu@linux.alibaba.com>
References: <20191111140502.17541-1-tonylu@linux.alibaba.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Mon, 11 Nov 2019 22:05:03 +0800
Tony Lu <tonylu@linux.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@linux.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?