Subject: Re: [netfilter bug] BUG: using smp_processor_id() in preemptible [00000000] code: ssh/9115, caller is ipt_do_table+0xc8/0x559

From: Ingo Molnar <[email-masked]>
Date: Thu, 2 Apr 2009 23:16:06 +0200

* Ingo Molnar <[email-masked]> wrote:

> * Eric Dumazet <[email-masked]> wrote:
> 
> > David put into its tree fix for that a few hours ago
> > 
> > commit fa9a86ddc8ecd2830a5e773facc250f110300ae7
> > 
> > (netfilter: iptables: lock free counters) forgot to disable BH
> > in arpt_do_table(), ipt_do_table() and  ip6t_do_table()
> > 
> > Use rcu_read_lock_bh() instead of rcu_read_lock() cures the problem.
> 
> ok, got your fix (attached below), thanks Eric for the pointer.
> 
> But i think my fix might be slightly better, because it does not 
> manipulate the preempt counter and leaves preemption enabled.
> 
> There's no BH context worries since this code did not seem to have 
> BH protection before either. (it used a plain read_lock(), not 
> read_lock_bh(), AFAICS)
> 
> I dont see any preemption worries either. I must be missing 
> something :)

as per the other mail - what i missed was that the old code _did_ 
use read_lock_bh(), which did not get carried over into the 
rcu_read_lock().

So this fix affects basically all things netfilter, not just 
rcu-preempt - a plain rcu_read_lock() doesnt protect against BH 
context interaction.

	Ingo