netdev - Re: [PATCH net] net: ipv6: Do not duplicate DAD on link up

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Date:   Wed, 3 May 2017 15:17:30 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     David Ahern <dsahern@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>
Subject: Re: [PATCH net] net: ipv6: Do not duplicate DAD on link up

On Wed, May 3, 2017 at 2:29 PM, David Ahern <dsahern@...il.com> wrote:
> On 5/3/17 3:22 PM, Cong Wang wrote:
>>> Andrey's reproducer program runs in a very tight loop, calling
>>> 'unshare -n' and then spawning 2 sets of 14 threads running random ioctl
>>> calls. The relevant networking sequence:
>>>
>>> 1. New network namespace created via unshare -n
>>> - ip6tnl0 device is created in down state
>>>
>>> 2. address added to ip6tnl0
>>> - equivalent to ip -6 addr add dev ip6tnl0 fd00::bb/1
>>> - DAD is started on the address and when it completes the host
>>>   route is inserted into the FIB
>>>
>>> 3. ip6tnl0 is brought up
>>> - the new fixup_permanent_addr function restarts DAD on the address
>>>
>>> 4. exit namespace
>>> - teardown / cleanup sequence starts
>>> - once in a blue moon, lo teardown appears to happen BEFORE teardown
>>>   of ip6tunl0
>>
>> Why is this possible? Loopback device should be always the first one to
>> register therefore the last one to unregister.
>
> Andrey's config builds in all of the devices. Once in a blue moon I
> would see the host route get deleted b/c 'lo' was taken down.


I am not asking for why ip6tunl0 is there, I am asking about the
order you claimed above, why the teardown of lo is BEFORE ip6tunl0?
Again, lo is the first to register and last to unregister, so any other device
should be teared down before lo.

If you don't trust me, read net/core/dev.c:

        /* The loopback device is special if any other network devices
         * is present in a network namespace the loopback device must
         * be present. Since we now dynamically allocate and free the
         * loopback device ensure this invariant is maintained by
         * keeping the loopback device as the first device on the
         * list of network devices.  Ensuring the loopback devices
         * is the first device that appears and the last network device
         * that disappears.
         */

>>
>> Hmm? But addrconf_ifdown() is called when the device is going from UP
>> to down, how does your patch prevent a duplicated DAD for a device which
>> is going from DOWN to UP in step 3? This is very confusing.
>>
>
> read the sequence:
> 1. ip6tnl0 is in the down state
>
> 2. address is added to ip6tnl0
>    DAD starts, completes, inserts host route into FIB
>
> 3. ip6tnl0 is brought up
>    DAD starts, completes, inserts host route into FIB
>
> Between 2. and 3. lo is taken down.

By taken down, you mean ifdown, not unregister (aka tear down), right?
But you didn't mention this at all in your original changelog.

If you mean unregister, that means we have a much more serious
problem as I mentioned above.

>
> Since DAD is done for an address in 2, it does not need to be repeated in 3.
>
> The DAD in 3. was meant for addresses kept on an admin down. This patch
> does that.

Again, if you mean unregister above, ip6tunl0 should be unregistered before
lo, therefore 3 is impossible to happen.