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+	e = t->entries[curcpu];
 	i = 0;
-	IPT_ENTRY_ITERATE(t->entries[curcpu],
+	IPT_ENTRY_ITERATE(e,
 			  t->size,
 			  set_entry_to_counter,

Hum, current cpu might be interrupted by NIC, since you only disabled preemption.
set_entry_to_counter() might get garbage.
I suppose I already mentioned it :)

Need to change preempt_disable to local_bh_disable.

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 			  counters,
 			  &i);
 
 	for_each_possible_cpu(cpu) {
+		void *p;
+
 		if (cpu == curcpu)
 			continue;
+
+		/* Swizzle the values and wait */
+		e->counters = ((struct xt_counters) { 0, 0 });

I dont see what you want to do here...

e->counters is the counter associated with rule #0

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+		p = t->entries[cpu];
+		rcu_assign_pointer(t->entries[cpu], e);
+		synchronize_net();

Oh well, not this synchronize_net() :)

This wont provide atomic sampling of counters for whole CPUS, and introduce large delays
on big machines (NR_CPUS >= 64)

For large number of CPU's it  would be possible to allocate
a full set of ncpu-1 new counters, then swap them in and then do one synchronize net.
Or instead of synchronize_net, maybe a "wait for CPU #n" to go through
grace period.

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Why would this not provide the moral equivalent of atomic sampling?
The code above switches to another counter set, and waits for a grace
period.  Shouldn't this mean that all CPUs that were incrementing the
old set of counters have finished doing so, so that the aggregate count
covers all CPUs that started their increments before the pointer switch?
Same as acquiring a write lock, which would wait for all CPUs that
started their increments before starting the write-lock acquisition.
CPUs that started their increments after starting the write acquisition
would not be accounted for in the total, same as the RCU approach.

Steve's approach does delay reading out the counters, but it avoids
delaying any CPU trying to increment the counters.

I see your point, but this is not what Stephen implemented.

So.. CPU will increments which counters, if not delayed ?

The concept is that only the sum of all the counters matters, not
which CPU has the count.

How counters will be synced again after our 'iptables -L' finished ?

"iptable -L" is not supposed to miss some counters updates (only some packets
 might be droped at NIC level because we spend time in the collection).
If packets matches some rules, we really want up2date counters.

Maybe we need for this collection an extra "cpu", to collect 
all increments that were done when CPUs where directed to a 
"secondary table/counters"

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So what am I missing here?

Well, I saw one synchronize_net() inside the for_each_possible_cpu(cpu) loop.
Say we have NR_CPUS=4096, how long will it takes to perform "iptables -L" ?



General/intuitive idea would be :

switch pointers to a newly allocated table (and zeroed counters)
wait one RCU grace period
collect/sum all counters of "old" table + (all cpus) into user provided table
restore previous table
wait one RCU grace period
disable_bh()
collect/sum all counters of "new and temporary" table (all cpus) and
reinject them into local cpu table (this cpu should not be interrupted)
enable_bh()

This way, "iptables -L" is not too expensive and doesnt block packet processing at all.

Pretty much what I said.