From dec827d174d7f76c457238800183ca864a639365 Mon Sep 17 00:00:00 2001
From: Pavel Emelyanov <xemul@openvz.org>
Date: Tue, 15 Apr 2008 00:36:08 -0700
Subject: [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/net_namespace.h |  2 ++
 include/net/netns/generic.h | 49 +++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 51 insertions(+)
 create mode 100644 include/net/netns/generic.h

(limited to 'include')

diff --git a/include/net/net_namespace.h b/include/net/net_namespace.h
index 72fad1a0956..f880b0f9f10 100644
--- a/include/net/net_namespace.h
+++ b/include/net/net_namespace.h
@@ -20,6 +20,7 @@ struct proc_dir_entry;
 struct net_device;
 struct sock;
 struct ctl_table_header;
+struct net_generic;
 
 struct net {
 	atomic_t		count;		/* To decided when the network
@@ -61,6 +62,7 @@ struct net {
 #ifdef CONFIG_NETFILTER
 	struct netns_xt		xt;
 #endif
+	struct net_generic	*gen;
 };
 
 
diff --git a/include/net/netns/generic.h b/include/net/netns/generic.h
new file mode 100644
index 00000000000..0c04fd2a700
--- /dev/null
+++ b/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
-- 
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