Subject: Re: [PATCH 2/14][NETNS]: Generic per-net pointers.

From: Daniel Lezcano <[email-masked]>
Date: Fri, 11 Apr 2008 15:43:55 +0200

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 change this pointer while the net is alive;
>  4. 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.
> 
> Signed-off-by: Pavel Emelyanov <[email-masked]>
> 
> ---
>  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..e8a6d27
> --- /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.
> + *
> + * 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 7ef3bac..b384840 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);

Why do you need to allocate more than sizeof(struct net_generic) ?

> +	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;
>  }
> 
> @@ -384,3 +399,50 @@ 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;

shouldn't it be rcu_dereferenced ?

> +	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);