* David Miller <[EMAIL PROTECTED]> wrote:
> From: Ingo Molnar <[EMAIL PROTECTED]>
> 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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