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

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Date:   Tue, 09 May 2017 16:09:08 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Cong Wang <xiyou.wangcong@...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, 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;

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.