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

From: Patrick McHardy <[email-masked]>
Date: Wed, 17 Jun 2009 17:34:12 +0200

Eric Dumazet wrote:
> Patrick McHardy a écrit :
>> I'm having some trouble figuring out the exact events that would
>> lead to the timer base corruption. Ingo, could you please test
>> this patch to make sure it also fixes the problem?
> 
> ;)
> 
> Event can be described as following :
> 
> CPU1					CPU2
> 
> /* __nf_conntrack_confirm() */
> __nf_conntrack_hash_insert(ct, hash, repl_hash);
> // now 'ct' is visible by other cpus
> 					// search conntrack and find ct
> // timeout.expires becomes absolute here
> ct->timeout.expires += jiffies;
> add_timer(&ct->timeout);
> 
> 					/* __nf_ct_refresh_acct() */
> 					if (!nf_ct_is_confirmed(ct)) {
> 					// we *believe* timeout.expires 
> 					// is not yet in use by timer code
> 					// and is still a relative quantity.
> 					// We want to 'update' it but we should not !
> 						ct->timeout.expires = extra_jiffies;  << CORRUPTION >>
> 					} else {
> // too late :(
> set_bit(IPS_CONFIRMED_BIT, &ct->status);
> 
> This is how I understood the problem, but I may be wrong ?

Thats one case that can happen, but that wouldn't corrupt the
timer base AFAICS. Also the callpath shows that it actually went
into the mod_timer_pending() path *and* timer_pending() was true.