[NETNS]: The generic per-net pointers.

Add the elastic array of void * pointer to the struct net.
The access rules are simple:

 1. register the ops with register_pernet_gen_device to get
    the id of your private pointer
 2. call net_assign_generic() to put the private data on the
    struct net (most preferably this should be done in the
    ->init callback of the ops registered)
 3. do not store any private reference on the net_generic array;
 4. do not change this pointer while the net is alive;
 5. use the net_generic() to get the pointer.

When adding a new pointer, I copy the old array, replace it
with a new one and schedule the old for kfree after an RCU
grace period.

Since the net_generic explores the net->gen array inside rcu
read section and once set the net->gen->ptr[x] pointer never 
changes, this grants us a safe access to generic pointers.

Quoting Paul: "... RCU is protecting -only- the net_generic 
structure that net_generic() is traversing, and the [pointer]
returned by net_generic() is protected by a reference counter 
in the upper-level struct net."

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/netns/generic.h

@@ -0,0 +1,49 @@
+/*
+ * generic net pointers
+ */
+
+#ifndef __NET_GENERIC_H__
+#define __NET_GENERIC_H__
+
+#include <linux/rcupdate.h>
+
+/*
+ * Generic net pointers are to be used by modules to put some private
+ * stuff on the struct net without explicit struct net modification
+ *
+ * The rules are simple:
+ * 1. register the ops with register_pernet_gen_device to get the id
+ *    of your private pointer;
+ * 2. call net_assign_generic() to put the private data on the struct
+ *    net (most preferably this should be done in the ->init callback
+ *    of the ops registered);
+ * 3. do not change this pointer while the net is alive;
+ * 4. do not try to have any private reference on the net_generic object.
+ *
+ * After accomplishing all of the above, the private pointer can be
+ * accessed with the net_generic() call.
+ */
+
+struct net_generic {
+	unsigned int len;
+	struct rcu_head rcu;
+
+	void *ptr[0];
+};
+
+static inline void *net_generic(struct net *net, int id)
+{
+	struct net_generic *ng;
+	void *ptr;
+
+	rcu_read_lock();
+	ng = rcu_dereference(net->gen);
+	BUG_ON(id == 0 || id > ng->len);
+	ptr = ng->ptr[id - 1];
+	rcu_read_unlock();
+
+	return ptr;
+}
+
+extern int net_assign_generic(struct net *net, int id, void *data);
+#endif