[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>

1 files changed, 62 insertions, 0 deletions

diff --git a/net/core/net_namespace.c b/net/core/net_namespace.c
index 2197d51aef3b..763674e1e593 100644
--- a/net/core/net_namespace.c
+++ b/net/core/net_namespace.c
@@ -7,6 +7,7 @@
 #include <linux/sched.h>
 #include <linux/idr.h>
 #include <net/net_namespace.h>
+#include <net/netns/generic.h>
 
 /*
  *      Our network namespace constructor/destructor lists
@@ -21,6 +22,8 @@ LIST_HEAD(net_namespace_list);
 struct net init_net;
 EXPORT_SYMBOL(init_net);
 
+#define INITIAL_NET_GEN_PTRS    13 /* +1 for len +2 for rcu_head */
+
 /*
  * setup_net runs the initializers for the network namespace object.
  */
@@ -29,10 +32,21 @@ static __net_init int setup_net(struct net *net)
         /* Must be called with net_mutex held */
         struct pernet_operations *ops;
         int error;
+        struct net_generic *ng;
 
         atomic_set(&net->count, 1);
         atomic_set(&net->use_count, 0);
 
+        error = -ENOMEM;
+        ng = kzalloc(sizeof(struct net_generic) +
+                        INITIAL_NET_GEN_PTRS * sizeof(void *), GFP_KERNEL);
+        if (ng == NULL)
+                goto out;
+
+        ng->len = INITIAL_NET_GEN_PTRS;
+        INIT_RCU_HEAD(&ng->rcu);
+        rcu_assign_pointer(net->gen, ng);
+
         error = 0;
         list_for_each_entry(ops, &pernet_list, list) {
                 if (ops->init) {
@@ -54,6 +68,7 @@ out_undo:
         }
 
         rcu_barrier();
+        kfree(ng);
         goto out;
 }
 
@@ -386,3 +401,50 @@ void unregister_pernet_gen_device(int id, struct pernet_operations *ops)
         mutex_unlock(&net_mutex);
 }
 EXPORT_SYMBOL_GPL(unregister_pernet_gen_device);
+
+static void net_generic_release(struct rcu_head *rcu)
+{
+        struct net_generic *ng;
+
+        ng = container_of(rcu, struct net_generic, rcu);
+        kfree(ng);
+}
+
+int net_assign_generic(struct net *net, int id, void *data)
+{
+        struct net_generic *ng, *old_ng;
+
+        BUG_ON(!mutex_is_locked(&net_mutex));
+        BUG_ON(id == 0);
+
+        ng = old_ng = net->gen;
+        if (old_ng->len >= id)
+                goto assign;
+
+        ng = kzalloc(sizeof(struct net_generic) +
+                        id * sizeof(void *), GFP_KERNEL);
+        if (ng == NULL)
+                return -ENOMEM;
+
+        /*
+         * Some synchronisation notes:
+         *
+         * The net_generic explores the net->gen array inside rcu
+         * read section. Besides once set the net->gen->ptr[x]
+         * pointer never changes (see rules in netns/generic.h).
+         *
+         * That said, we simply duplicate this array and schedule
+         * the old copy for kfree after a grace period.
+         */
+
+        ng->len = id;
+        INIT_RCU_HEAD(&ng->rcu);
+        memcpy(&ng->ptr, &old_ng->ptr, old_ng->len);
+
+        rcu_assign_pointer(net->gen, ng);
+        call_rcu(&old_ng->rcu, net_generic_release);
+assign:
+        ng->ptr[id - 1] = data;
+        return 0;
+}
+EXPORT_SYMBOL_GPL(net_assign_generic);