netdev - Re: Passive OS fingerprint xtables match.

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Date:	Wed, 11 Mar 2009 00:54:44 +0300
From:	Evgeniy Polyakov <zbr@...emap.net>
To:	Jesper Dangaard Brouer <hawk@...u.dk>
Cc:	Patrick McHardy <kaber@...sh.net>, netdev <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Netfilter Development Mailinglist 
	<netfilter-devel@...r.kernel.org>,
	Jan Engelhardt <jengelh@...ozas.de>, hawk@...x.dk
Subject: Re: Passive OS fingerprint xtables match.

Hi.

On Tue, Mar 10, 2009 at 10:01:30PM +0100, Jesper Dangaard Brouer (hawk@...u.dk) wrote:
> >+static void __exit ipt_osf_fini(void)
> >+{
> >+	struct ipt_osf_finger *f;
> >+	int i;
> >+
> >+	cn_del_callback(&cn_osf_id);
> >+	xt_unregister_match(&ipt_osf_match);
> >+
> >+	rcu_read_lock();
> >+	for (i=0; i<ARRAY_SIZE(ipt_osf_fingers); ++i) {
> >+		struct ipt_osf_finger_storage *st = &ipt_osf_fingers[i];
> >+
> >+		list_for_each_entry_rcu(f, &st->finger_list, finger_entry) {
> 
> spin_lock(&st->finger_lock); //???
> >+			list_del_rcu(&f->finger_entry);
> spin_unlock(&st->finger_lock);
> 
> >+			call_rcu(&f->rcu_head, ipt_osf_finger_free_rcu);
> >+		}
> >+	}
> >+	rcu_read_unlock();
> 
> Should the list_del_rcu() not be protected by a spinlock?

Not required at this place - all users are already unregistered and
no code can access this list except module exit path.

> >+	rcu_barrier();
> 
> In some of my code I call synchronize_net(), is it enough to call 
> rcu_barrier()?

It is enough here, rcu_barrier() will wait until all scheduled
call_rcu() are completed, that's what we need. But in some cases we
should only wait for the whole grace period to elapse, then one has to use
synchronize_rcu() and friends. rcu_barrier() will wait for the callbacks
to be executed, while they are executed after grace period has elapsed,
so it implicitly includes synchronize_rcu(), but effectively they are
the same: both functions register rcu callback and wait for the
completion, rcu_barrier() is a bit more enhanced, since it has several
types.

> What is the difference between:
> 
>  synchronize_rcu()
>  synchronize_net()

Those are essentially the same - synchronize_net() has additional
might_sleep()  call. Both will wait until grace period elapced - i.e.
all currently RCU protected sections completed.

>  rcu_barrier()

It will wait until all scheduled rcu callbacks are executed.

So from the description they look different, but implementation
suggestes that effectively they are the same, except that there are a
bit different invocation types for the barrier.

-- 
	Evgeniy Polyakov
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