Re: PROBLEM: reproducible crash KVM+nf_conntrack all recent 2.6kernels

From: Jon Masters
Date: Fri Jan 29 2010 - 03:43:14 EST


So I did some poking (still trying to figure out netfilter a little
internally) and looked over the handling of connection tracking. The
oops reports I have been getting generally lie in __nf_conntrack_find,
specifically within a hlist iterator that looks up the information for
the current connection in a per-net namespace hashtable (under RCU, it's
been locked already by the time we get in here). Here's the piece:

hlist_nulls_for_each_entry_rcu(h, n, &net->ct.hash[hash],
hnnode) {
if (nf_ct_tuple_equal(tuple, &h->tuple)) {
NF_CT_STAT_INC(net, found);
return h;
NF_CT_STAT_INC(net, searched);

Instrumenting the kernel at the moment and then setting up more of a
debugging environment to poke at what goes wrong here. Perhaps there's
some broken RCU assumption - I just spent the last few hours reading
over netfilter source and Paul's RCU docs again to brush up.

Perhaps you netdev folks can let me know if there's a handy netfilter
debugging guide somewhere.


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