netdev - [PATCH] netfilter: Use hlist_add_head_rcu() in nf_conntrack_set

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Date:	Tue, 24 Mar 2009 20:54:53 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Patrick McHardy <kaber@...sh.net>
CC:	mbizon@...ebox.fr, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Joakim Tjernlund <Joakim.Tjernlund@...nsmode.se>,
	avorontsov@...mvista.com, netdev@...r.kernel.org
Subject: [PATCH] netfilter: Use hlist_add_head_rcu() in nf_conntrack_set_hashsize()

Eric Dumazet a écrit :
> 
> We are working on a SLAB_DESTROY_BY_RCU implementation so that
> conntrack wont use call_rcu() anymore, give us a couple of days :)
> 

While working on this stuff, I found one suspect use of hlist_add_head()

Its not a hot path, I believe following patch would make sure nothing
wrong happens.

If a chain contains element A and B, then we might build a new table
with a new chain containing B and A (in this reverse order), and
a cpu could see A->next = B (new pointer),  B->next = A (old pointer)

Thanks

[PATCH] netfilter: Use hlist_add_head_rcu() in nf_conntrack_set_hashsize()

Using hlist_add_head() in nf_conntrack_set_hashsize() is quite dangerous.
Without any barrier, one CPU could see a loop while doing its lookup.
Its true new table cannot be seen by another cpu, but previous table is still
readable.

Signed-off-by: Eric Dumazet <dada1@...mosbay.com>

diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
index 55befe5..54e983f 100644
--- a/net/netfilter/nf_conntrack_core.c
+++ b/net/netfilter/nf_conntrack_core.c
@@ -1121,7 +1121,7 @@ int nf_conntrack_set_hashsize(const char *val, struct kernel_param *kp)
 					struct nf_conntrack_tuple_hash, hnode);
 			hlist_del_rcu(&h->hnode);
 			bucket = __hash_conntrack(&h->tuple, hashsize, rnd);
-			hlist_add_head(&h->hnode, &hash[bucket]);
+			hlist_add_head_rcu(&h->hnode, &hash[bucket]);
 		}
 	}
 	old_size = nf_conntrack_htable_size;


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