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

Wei Wang
2017-08-12T19:42:46Z

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.

Thanks.
Wei

On Sat, Aug 12, 2017 at 11:01 AM, Ido Schimmel <idosch@idosch.org> wrote:
> Hi Wei,
>
> On Fri, Aug 11, 2017 at 05:10:02PM -0700, Wei Wang wrote:
>> I think we have a potential fix for this issue.
>> Martin and I found that when addrconf_dst_alloc() creates a rt6, it is
>> possible that rt6->dst.dev points to loopback device while
>> rt6->rt6i_idev->dev points to a real device.
>> When the real device goes down, the current fib6 clean up code only
>> checks for rt6->dst.dev and assumes rt6->rt6i_idev->dev is the same.
>> That leaves unreleased refcnt on the real device if rt6->dst.dev
>> points to loopback dev.
>
> [...]
>
>> From 2d8861808c2029013f6b6e86120ba6902329145b Mon Sep 17 00:00:00 2001
>> From: Wei Wang <weiwan@google.com>
>> Date: Fri, 11 Aug 2017 16:36:04 -0700
>> Subject: [PATCH 1/2] potential fix for unregister_netdevice()
>>
>> Change-Id: I5d5f6f7a7ad0f5dd769f33487db17ff2570d52ea
>> ---
>> net/ipv6/route.c | 17 ++++++++---------
>> 1 file changed, 8 insertions(+), 9 deletions(-)
>>
>> diff --git a/net/ipv6/route.c b/net/ipv6/route.c
>> index 4d30c96a819d..105922903932 100644
>> --- a/net/ipv6/route.c
>> +++ b/net/ipv6/route.c
>> @@ -417,14 +417,12 @@ static void ip6_dst_ifdown(struct dst_entry *dst, struct net_device *dev,
>> struct net_device *loopback_dev =
>> dev_net(dev)->loopback_dev;
>>
>> - if (dev != loopback_dev) {
>> - if (idev && idev->dev == dev) {
>> - struct inet6_dev *loopback_idev =
>> - in6_dev_get(loopback_dev);
>> - if (loopback_idev) {
>> - rt->rt6i_idev = loopback_idev;
>> - in6_dev_put(idev);
>> - }
>> + if (idev && idev->dev != loopback_dev) {
>> + struct inet6_dev *loopback_idev =
>> + in6_dev_get(loopback_dev);
>> + if (loopback_idev) {
>> + rt->rt6i_idev = loopback_idev;
>> + in6_dev_put(idev);
>> }
>> }
>> }
>> @@ -2789,7 +2787,8 @@ 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 ||
>> + 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().
>
> 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.
>
> 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)?
>
> Thanks
>
>> rt != adn->net->ipv6.ip6_null_entry &&
>> (rt->rt6i_nsiblings == 0 ||
>> (dev && netdev_unregistering(dev)) ||
>> --
>> 2.14.0.434.g98096fd7a8-goog
>>