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From: Tony Lu <>
To: Stephen Hemminger <>
Subject: Re: [PATCH] net: remove static inline from dev_put/dev_hold
Date: Tue, 12 Nov 2019 15:18:19 +0800	[thread overview]
Message-ID: <20191112071819.GB67139@TonyMac-Alibaba> (raw)
In-Reply-To: <20191111085632.24d88706@hermes.lan>

On Mon, Nov 11, 2019 at 08:56:32AM -0800, Stephen Hemminger wrote:
> On Mon, 11 Nov 2019 22:05:03 +0800
> Tony Lu <> 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 <>
> 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.

Tony Lu

  reply	other threads:[~2019-11-12  7:18 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-11 14:05 [PATCH] net: remove static inline from dev_put/dev_hold Tony Lu
2019-11-11 16:56 ` Stephen Hemminger
2019-11-12  7:18   ` Tony Lu [this message]
2019-11-11 17:21 ` Eric Dumazet
2019-11-12  9:48   ` Tony Lu
2019-11-11 21:26 ` Cong Wang
2019-11-12  8:47   ` Tony Lu

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