netdev - Re: [patch 11/11] netfilter warning fix

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Date:	Tue, 6 Feb 2007 22:58:06 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	David Miller <davem@...emloft.net>
Cc:	akpm@...ux-foundation.org, netdev@...r.kernel.org, kaber@...sh.net,
	dipankar@...ibm.com, paulmck@...ibm.com
Subject: Re: [patch 11/11] netfilter warning fix

* David Miller <davem@...emloft.net> wrote:

> From: Ingo Molnar <mingo@...e.hu>
> Date: Tue, 6 Feb 2007 22:02:51 +0100
> 
> > So i'm wondering what other assumptions there are (or can be) about 
> > rcu_read_lock() also being a preempt-off point. Thanks!
> 
> I showed the examples in my detailed analysis yesterday.
> 
> Beause I love hearing myself say the same thing over and over so much 
> I'll restate it for you.
> 
> net/netfilter/nf_conntrack_core.c, calls:
> 
> 	l4proto = __nf_ct_l4proto_find((u_int16_t)pf, protonum);
> 
> whichs assumes that preemption is disabled.

you are right - i mistakenly read that mail only up to the point where 
you point out the (slightly) buggy NF_CT_STATIC_INC use and missed your 
final point about other coding having implicit preempt_disable() 
assumptions.

I've looked at __nf_ct_l4proto_find() and it's not obvious to me what 
the hidden preempt_disable() assumption is. Its main use seems to be of 
nf_ct_protos[] array, which is protected by nf_conntrack_lock. I'm 
wondering whether what you say suggests that it's safe to call 
__nf_ct_l4proto_find() without the nf_conntrack_lock locked (as read or 
as write), and if it's safe, how it protects against simultaneous 
modifications to the nf_ct_protos[] array. 

Ahh ... unregister does a synchronize_net(), right? That means that 
removal of the pointer only happens if all CPUs have gone through a 
quiescent state.

this means that this particular use could be fixed by converting the 
preempt_disable()/enable() pair in nf_ct_l4proto_find_get() to 
rcu_read_lock()/unlock(), correct? Furthermore, every user of 
synchronize_net() [and synchronize_rcu() in general] needs to be 
reviewed.

	Ingo
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