netdev - Re: [PATCH net-next v6 1/8] vxlan: Restructure vxlan socket apis.

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Date:	Mon, 5 Aug 2013 11:24:23 -0700
From:	Pravin Shelar <pshelar@...ira.com>
To:	Stephen Hemminger <stephen@...workplumber.org>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH net-next v6 1/8] vxlan: Restructure vxlan socket apis.

On Mon, Aug 5, 2013 at 11:10 AM, Stephen Hemminger
<stephen@...workplumber.org> wrote:
> On Mon, 5 Aug 2013 08:49:36 -0700
> Pravin Shelar <pshelar@...ira.com> wrote:
>
>> On Sun, Aug 4, 2013 at 2:42 PM, Stephen Hemminger
>> <stephen@...workplumber.org> wrote:
>> > On Thu,  1 Aug 2013 11:44:40 -0700
>> > Pravin B Shelar <pshelar@...ira.com> wrote:
>> >
>> >> @@ -1642,59 +1651,56 @@ static struct vxlan_sock *vxlan_socket_create(struct net *net, __be16 port)
>> >>                        &vxlan_addr.sin_addr, ntohs(vxlan_addr.sin_port), rc);
>> >>               sk_release_kernel(sk);
>> >>               kfree(vs);
>> >> -             return ERR_PTR(rc);
>> >> +             return;
>> >>       }
>> >> +     atomic_set(&vs->refcnt, 0);
>> >>
>> >>       /* Disable multicast loopback */
>> >>       inet_sk(sk)->mc_loop = 0;
>> >> +     spin_lock(&vn->sock_lock);
>> >> +     hlist_add_head_rcu(&vs->hlist, vs_head(net, port));
>> >> +     spin_unlock(&vn->sock_lock);
>> >
>> > Overall I am fine with this set, except for this.
>> > The change causes a socket to be put into the hash list with a ref count
>> > of 0 and then you increment the ref count. If some other thread
>> > finds the socket and then decrements the ref count, it would go
>> > below zero and might leak, crash or delete it prematurely.
>> >
>> > This concerns me.
>>
>> There is no code path which decrements ref-cnt after lookup. Therefore
>> above case is not possible.
>> But I agree this does not look good, so I will change it so not to
>> have refcnt zero vs on hash-table.
>
> I was concerned about:
>
> CPU 0:
>    vxlan_socket_create
>      Make a vs structure with refcnt == 0
>      Add it to hlist
>
>      <preempt or other long IRQ>
>                                          CPU 1: Another vxlan being setup (batch script?)
>                                                 vxlan_init
>                                                      vxlan_find_port finds that vs
>                                                      atomic_inc(refcnt) now 1
>                                           <close or error>
>                                                 vxlan_uninit
>                                                   vxlan_sock_release
>                                                      atomic_dec_and_test (now 0)
>                                                        delete from hlist
>                                                        queue work for deletion
>
>      now decides to continue with vs
>      Either vs is already freed, or it exists
>      in limbo
>
ok, I see problem. v7 version fixes this issue.
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