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

On Mon, Nov 11, 2019 at 08:56:32AM -0800, Stephen Hemminger wrote:

On Mon, 11 Nov 2019 22:05:03 +0800
Tony Lu [off-list ref] wrote:

quoted

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?

From code analysis, there should be a little performance backwards.
I don't have the benchmark data for now. I will make a kernel module to
take a series of benchmarks for dev_put/dev_hold. Actually there is a plan
to take a whole solution for this issue. The benchmarks will be done
after this.

Cheers
Tony Lu