netdev - Re: [Patch net] ipv4: restore rt->fi for reference counting

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Date:   Tue, 9 May 2017 16:35:57 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [Patch net] ipv4: restore rt->fi for reference counting

On Tue, May 9, 2017 at 4:09 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Tue, 2017-05-09 at 15:54 -0700, Eric Dumazet wrote:
>> On Tue, 2017-05-09 at 15:52 -0700, Eric Dumazet wrote:
>> > On Tue, 2017-05-09 at 15:07 -0700, Cong Wang wrote:
>> > > On Tue, May 9, 2017 at 1:56 PM, Cong Wang <xiyou.wangcong@...il.com> wrote:
>> > > > Wait... if we transfer dst->dev to loopback_dev because we don't
>> > > > want to block unregister path, then we might have a similar problem
>> > > > for rt->fi too, fib_info is still referenced by dst, so these nh_dev's still
>> > > > hold the dev references...
>> > > >
>> > >
>> > > I finally come up with the attach patch... Do you mind to give it a try?
>> >
>> > I will, but this might be delayed by a few hours.
>> >
>> > In the mean time, it looks like you could try adding the following to
>> > your .config ;)
>> >
>> > CONFIG_IP_ROUTE_MULTIPATH=y
>> >
>> >
>>
>> +                               /* This should be fine, we are on unregister
>> +                                * path so synchronize_net() already waits for
>> +                                * existing readers. We have to release the
>> +                                * dev here because dst could still hold this
>> +                                * fib_info via rt->fi, we can't wait for GC.
>> +                                */
>> +                               RCU_INIT_POINTER(nexthop_nh->nh_dev, NULL);
>> +                               dev_put(dev);
>>                                 dead = fi->fib_nhs;
>>
>> dead = fi->fib_mhs looks wrong if you remove the break; statement ?
>>
>> -                               break;

This statement is only used to ensure we pass the "dead == fi->fib_nhs"
check right below the inner loop, it is fine to keep it without break since
fi is not changed in the inner loop.


>
> Also setting nexthop_nh->nh_dev to NULL looks quite dangerous
>
> We have plenty of sites doing :
>
> if (fi->fib_dev)
>     x = fi->fib_dev->field
>
> fib_route_seq_show() is one example.
>

All of them take RCU read lock, so, as I explained in the code comment,
they all should be fine because of synchronize_net() on unregister path.
Do you see anything otherwise?