netdev - Re: [Patch net-next] net_sched: hold tcf

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Message-ID: <CAM_iQpU6=rtb+-T9NiiEJ+799PtJo=j1=EBuAL76Ac7=w=MKHA@mail.gmail.com>
Date:	Thu, 17 Jul 2014 09:45:36 -0700
From:	Cong Wang <xiyou.wangcong@...il.com>
To:	Jamal Hadi Salim <jhs@...atatu.com>
Cc:	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	Cong Wang <cwang@...pensource.com>
Subject: Re: [Patch net-next] net_sched: hold tcf_lock in netdevice notifier

On Thu, Jul 17, 2014 at 5:49 AM, Jamal Hadi Salim <jhs@...atatu.com> wrote:
> On 07/16/14 17:25, Cong Wang wrote:
>>
>> From: Cong Wang <cwang@...pensource.com>
>>
>> We modify mirred action (m->tcfm_dev) in netdev event, we need to
>> prevent on-going mirred actions from reading freed m->tcfm_dev.
>> So we need to acquire this spin lock.
>>
>
> Cong,
>
> What setup prompted this? We have no problems dealing with deleted
> devices in the data path. It seems harmless otherwise.
>


I found this during code review, don't see any real crash. This
is why I sent it for net-next not net. :)

The reason why it doesn't trigger a crash is I think dev_put()
doesn't immediately release dev when refcount hits zero,
instead it defers that to rtnl_unlock().
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