[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>

diff --git a/include/net/net_namespace.h b/include/net/net_namespace.h
index 72fad1a..f880b0f 100644 (file)
--- 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 (file)
index 0000000..0c04fd2
--- /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

diff --git a/net/core/net_namespace.c b/net/core/net_namespace.c
index 2197d51..763674e 100644 (file)
--- 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);