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

From: Eric Dumazet
Date: Wed Jun 17 2009 - 08:34:25 EST

Patrick McHardy a écrit :
> Eric Dumazet wrote:
>> Patrick McHardy a écrit :
>>> 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.
>> Problem is timeout.expires is either a relative or absolute timeout,
>> and changes happen
>> in __nf_conntrack_confirm() or __nf_ct_refresh_acct().
>> We must have a synchronization (an barriers), a single bit wont be
>> enough.
> Please have a look at the second patch I just sent. It relies
> on the RCU barriers to make sure all stores are visible before
> other CPUs can find the conntrack.

Sorry, I dont understand how your second patch corrects the problem.

This (unconfirmed) conntrack is visible by another cpu. This other
cpu can call __nf_ct_refresh_acct() while this cpu runs
in __nf_conntrack_confirm()

@@ -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)

<<<< another cpu could here change timeout.expires (thinking its still relative) >>>>

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);
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