On Tue, 28 Oct 2014 11:37:31 +0800 "billbonaparte" <programme110@xxxxxxxxx> wrote:
> Hi, all:
> sorry for sending this mail again, the last mail doesn't show text
> clearly.
This one also mangles the text, so I cannot follow the race you are
describing. I'll try to reconstruct...
> In function __nf_conntrack_confirm, we check the conntrack if it was
> already dead, before insert it into hash-table.
> We do this because if we insert an already 'dead' hash, it will
> block further use of that particular connection.
Have you run into this problem in practice, or is this based on a
theory?
> but we don't do that right.
> let's consider the following case:
>
[tried to reconstruct]
> cpu1 cpu2
> __nf_conntrack_confirm get_next_corpse
> lock corresponding hash-list ....
> check nf_ct_is_dying(ct) ....
> ..... for_each_possible_cpu(cpu) {
> ..... (processing &pcpu->unconfirmed)
> ..... spin_lock_bh(&pcpu->lock);
> ..... set_bit(IPS_DYING_BIT, &ct->status);
> ..... spin_unlock_bh(&pcpu_lock);
> spin_lock_bh(&pcpu->lock);
> nf_ct_del_from_dying_or_unconfirmed_list(ct);
> spin_unlock_bh(&pcpu_lock);
>
> add_timer(&ct->timeout);
> ct->status |= IPS_CONFIRMED;
> __nf_conntrack_hash_insert(ct);
> /* the conntrack has been seted as dying*/
Yes, I think you are correct. There is a race. As we are modifying
the ct->status, without holding the hash bucket lock.
> The above case reveal two problems:
> 1. we may insert a dead conntrack to hash-table, it will block
> further use of that particular connection.
> 2. operation on ct->status should be atomic, because it race aginst
> get_next_corpse.
> due to this reason, the operation on ct->status in
> nf_nat_setup_info should be atomic as well.
>
> if we want to resolve the first problem, we must delete the
> unconfirmed conntrack from unconfirmed-list first, then check if it is
> already dead.
Guess that would be one approach.
> Am I right to do this ?
> Appreciate any comments and reply.
Perhaps we could get rid of unconfirmed list handling in get_next_corpse?
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Sr. Network Kernel Developer at Red Hat
Author of http://www.iptv-analyzer.org
LinkedIn: http://www.linkedin.com/in/brouer
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