linux-kernel - Re: unregister_netdevice: waiting for eth0 to become free. Usage count = 1

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Message-ID: <61d95fe2-edfd-9fc4-5e21-5b96e4e03c9f@gmail.com>
Date:   Sun, 13 Aug 2017 10:24:53 -0600
From:   David Ahern <dsahern@...il.com>
To:     Wei Wang <weiwan@...gle.com>, Ido Schimmel <idosch@...sch.org>
Cc:     Cong Wang <xiyou.wangcong@...il.com>,
        John Stultz <john.stultz@...aro.org>,
        Martin KaFai Lau <kafai@...com>,
        lkml <linux-kernel@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Linux USB List <linux-usb@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Felipe Balbi <felipe.balbi@...ux.intel.com>
Subject: Re: unregister_netdevice: waiting for eth0 to become free. Usage
 count = 1

On 8/12/17 1:42 PM, Wei Wang wrote:
> Hi Ido,
> 
>>> -     if ((rt->dst.dev == dev || !dev) &&
>>> +     if ((rt->dst.dev == dev || !dev ||
>>> +          rt->rt6i_idev->dev == dev) &&
>>
>> Can you please explain why this line is needed? While host routes aren't
>> removed from the FIB by rt6_ifdown() (when dst.dev goes down), they are
>> removed later on in addrconf_ifdown().
>>
> 
> Yes.. Agree. But one difference is that if the route is removed from
> addrconf_ifdown(), dst_dev_put() won't be called to release the
> devices before doing dst_release(). It is OK if dst_release() sees the
> refcnt on dst already drops to 0 and directly destroys the dst. But I
> think it will cause problem if at the time, the dst is still held by
> some other users because then the refcnt on the device going down will
> not get released.
> That's why I think we should remove the dst with either dst->dev ==
> going down dev or rt6->rt6i_idev->dev == going down dev from the fib6
> tree always because there, we always call dst_dev_put() to release the
> device.
> 
>> With your patch, if I check the return value of ip6_del_rt() in
>> __ipv6_ifa_notify() I see that -ENONET is returned. Because the host
>> route was already removed by rt6_ifdown(). When the line in question is
>> removed from the patch I don't get the error anymore.
>>
> 
> Right. That is expected as the route is already removed from the tree.
> 
>> Is it possible that in John's case the host route was correctly removed
>> from the FIB and that the unreleased reference was due to a wrong check
>> in ip6_dst_ifdown() (which you patched correctly AFAICT)?
>>
> 
> Yes. possible. But as I explained earlier, I still think we should
> also remove routes with rt6->rt6i_idev->dev == going down dev from the
> tree.

Looking at my patch to move host routes from loopback to device with the
address, I have this:

@@ -2789,7 +2808,8 @@ static int fib6_ifdown(struct rt6_info *rt, void *arg)
        const struct arg_dev_net *adn = arg;
        const struct net_device *dev = adn->dev;

-       if ((rt->dst.dev == dev || !dev) &&
+       if ((rt->dst.dev == dev || !dev ||
+            (netdev_unregistering(dev) && rt->rt6i_idev->dev == dev)) &&
            rt != adn->net->ipv6.ip6_null_entry &&
            (rt->rt6i_nsiblings == 0 ||
             (dev && netdev_unregistering(dev)) ||