netfilter: Remove now-redundant smp_read_barrier_depends()

READ_ONCE() now implies smp_read_barrier_depends(), which means that
the instances in arpt_do_table(), ipt_do_table(), and ip6t_do_table()
are now redundant.  This commit removes them and adjusts the comments.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Pablo Neira Ayuso <pablo@netfilter.org>
Cc: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Cc: Florian Westphal <fw@strlen.de>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: <netfilter-devel@vger.kernel.org>
Cc: <coreteam@netfilter.org>
Cc: <netdev@vger.kernel.org>

net/ipv4/netfilter/arp_tables.c

@@ -202,13 +202,8 @@ unsigned int arpt_do_table(struct sk_buff *skb,
 
 	local_bh_disable();
 	addend = xt_write_recseq_begin();
-	private = table->private;
+	private = READ_ONCE(table->private); /* Address dependency. */
 	cpu     = smp_processor_id();
-	/*
-	 * Ensure we load private-> members after we've fetched the base
-	 * pointer.
-	 */
-	smp_read_barrier_depends();
 	table_base = private->entries;
 	jumpstack  = (struct arpt_entry **)private->jumpstack[cpu];
 

net/ipv4/netfilter/ip_tables.c

@@ -260,13 +260,8 @@ ipt_do_table(struct sk_buff *skb,
 	WARN_ON(!(table->valid_hooks & (1 << hook)));
 	local_bh_disable();
 	addend = xt_write_recseq_begin();
-	private = table->private;
+	private = READ_ONCE(table->private); /* Address dependency. */
 	cpu        = smp_processor_id();
-	/*
-	 * Ensure we load private-> members after we've fetched the base
-	 * pointer.
-	 */
-	smp_read_barrier_depends();
 	table_base = private->entries;
 	jumpstack  = (struct ipt_entry **)private->jumpstack[cpu];
 

net/ipv6/netfilter/ip6_tables.c

@@ -282,12 +282,7 @@ ip6t_do_table(struct sk_buff *skb,
 
 	local_bh_disable();
 	addend = xt_write_recseq_begin();
-	private = table->private;
-	/*
-	 * Ensure we load private-> members after we've fetched the base
-	 * pointer.
-	 */
-	smp_read_barrier_depends();
+	private = READ_ONCE(table->private); /* Address dependency. */
 	cpu        = smp_processor_id();
 	table_base = private->entries;
 	jumpstack  = (struct ip6t_entry **)private->jumpstack[cpu];