Re: [PATCH net-2.6.26 2/2][NETNS]: The generic per-net pointers.

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Sat, Apr 12, 2008 at 01:44:45PM +0400, Pavel Emelyanov wrote:
> 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."

;-)  One of the nice things about both reference counting and RCU
is that you can safely "sandwich" the protection regions in this
way.  If you try it with pure locking, you are prone to deadlock.

Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

> Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
> 
> ---
>  include/net/net_namespace.h |    2 +
>  include/net/netns/generic.h |   49 ++++++++++++++++++++++++++++++++++
>  net/core/net_namespace.c    |   62 +++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 113 insertions(+), 0 deletions(-)
>  create mode 100644 include/net/netns/generic.h
> 
> diff --git a/include/net/net_namespace.h b/include/net/net_namespace.h
> index 6971fdb..e3d4eb4 100644
> --- a/include/net/net_namespace.h
> +++ b/include/net/net_namespace.h
> @@ -19,6 +19,7 @@ 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
> @@ -57,6 +58,7 @@ 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 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
> --- 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);
> -- 
> 1.5.3.4
>