Re: [PATCH net] ipv6: add complete rcu protection around np->opt

[Eric Dumazet]

On Tue, 2015-12-01 at 12:11 +0100, Hannes Frederic Sowa wrote:
> Hi Eric,
> 
> On Mon, Nov 30, 2015, at 04:37, Eric Dumazet wrote:
> > -                       opt = xchg(&np->opt, NULL);
> > -                       if (opt)
> > -                               sock_kfree_s(sk, opt, opt->tot_len);
> > +                       opt = xchg((__force struct ipv6_txoptions
> > **)&np->opt,
> > +                                  NULL);
> > +                       if (opt) {
> > +                               atomic_sub(opt->tot_len,
> > &sk->sk_omem_alloc);
> > +                               txopt_put(opt);
> > +                       }
> >  			pktopt = xchg(&np->pktoptions, NULL);
> >  			kfree_skb(pktopt);
> 
> Is here something special going on (because of the xchg). I don't see
> why you cannot simply use a RCU_INIT_POINTER?
> 
> Thanks,
> Hannes

Yes, I mentioned this earlier, and will be addressed in net-next tree
later.

(Same for np->pktoptions)

The xchg() here does not bring additional protection as we are the last
user of np.

Thanks.




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