Re: [bug] __nf_ct_refresh_acct(): WARNING: at lib/list_debug.c:30__list_add+0x7d/0xad()

From: Patrick McHardy
Date: Wed Jun 17 2009 - 07:51:58 EST

Patrick McHardy wrote:
Eric Dumazet wrote:
IPS_CONFIRMED_BIT is set under nf_conntrack_lock (in __nf_conntrack_confirm()),
we probably want to add a synchronisation under ct->lock as well,
or __nf_ct_refresh_acct() could set ct->timeout.expires to extra_jiffies,
while a different cpu could confirm the conntrack.

Before the conntrack is confirmed, it is exclusively handled by a
single CPU. I agree that we need to make sure the IPS_CONFIRMED_BIT
is visible before we add the conntrack to the hash table since the
lookup is lockless, but simply moving the set_bit before the hash
insertion should be fine I think.

A slightly changed version which moves hash insertion to the end
and adds a comment about ordering. This make sure the timer is
actually running before the conntrack can be found be other CPUs.

diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
index 5f72b94..e2cc707 100644
--- a/net/netfilter/nf_conntrack_core.c
+++ b/net/netfilter/nf_conntrack_core.c
@@ -425,7 +425,6 @@ __nf_conntrack_confirm(struct sk_buff *skb)
/* Remove from unconfirmed list */

- __nf_conntrack_hash_insert(ct, hash, repl_hash);
/* Timer relative to confirmation time, not original
setting time, otherwise we'd get timer wrap in
weird delay cases. */
@@ -433,8 +432,15 @@ __nf_conntrack_confirm(struct sk_buff *skb)
set_bit(IPS_CONFIRMED_BIT, &ct->status);
+ /* Since the lookup is lockless, hash insertion must be after starting the
+ * timer and setting the CONFIRMED bit. The RCU barriers guarantee that no
+ * other CPU can find the conntrack before the above stores are visible.
+ */
+ __nf_conntrack_hash_insert(ct, hash, repl_hash);
NF_CT_STAT_INC(net, insert);
help = nfct_help(ct);
if (help && help->helper)
nf_conntrack_event_cache(IPCT_HELPER, ct);