Re: [PATCH 01/34] net: add skb_dst_set_noref_force

[Pablo Neira Ayuso <pablo@netfilter.org>]

Hi Simon,

On Fri, Mar 29, 2013 at 01:11:18PM +0900, Simon Horman wrote:
> From: Julian Anastasov <ja@ssi.bg>
> 
> Rename skb_dst_set_noref to __skb_dst_set_noref and
> add force flag as suggested by David Miller. The new wrapper
> skb_dst_set_noref_force will force dst entries that are not
> cached to be attached as skb dst without taking reference
> as long as provided dst is reclaimed after RCU grace period.
> 
> Signed-off-by: Julian Anastasov <ja@ssi.bg>
> Signed-off by: Hans Schillstrom <hans@schillstrom.com>
> Signed-off-by: Simon Horman <horms@verge.net.au>

I think was acked by David, right?

> ---
>  include/linux/skbuff.h |   35 ++++++++++++++++++++++++++++++++++-
>  net/core/dst.c         |    9 +++++----
>  2 files changed, 39 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 878e0ee..364e244 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -575,7 +575,40 @@ static inline void skb_dst_set(struct sk_buff *skb, struct dst_entry *dst)
>  	skb->_skb_refdst = (unsigned long)dst;
>  }
>  
> -extern void skb_dst_set_noref(struct sk_buff *skb, struct dst_entry *dst);
> +extern void __skb_dst_set_noref(struct sk_buff *skb, struct dst_entry *dst,
> +				bool force);
> +
> +/**
> + * skb_dst_set_noref - sets skb dst, hopefully, without taking reference
> + * @skb: buffer
> + * @dst: dst entry
> + *
> + * Sets skb dst, assuming a reference was not taken on dst.
> + * If dst entry is cached, we do not take reference and dst_release
> + * will be avoided by refdst_drop. If dst entry is not cached, we take
> + * reference, so that last dst_release can destroy the dst immediately.
> + */
> +static inline void skb_dst_set_noref(struct sk_buff *skb, struct dst_entry *dst)
> +{
> +	__skb_dst_set_noref(skb, dst, false);
> +}
> +
> +/**
> + * skb_dst_set_noref_force - sets skb dst, without taking reference
> + * @skb: buffer
> + * @dst: dst entry
> + *
> + * Sets skb dst, assuming a reference was not taken on dst.
> + * No reference is taken and no dst_release will be called. While for
> + * cached dsts deferred reclaim is a basic feature, for entries that are
> + * not cached it is caller's job to guarantee that last dst_release for
> + * provided dst happens when nobody uses it, eg. after a RCU grace period.
> + */
> +static inline void skb_dst_set_noref_force(struct sk_buff *skb,
> +					   struct dst_entry *dst)
> +{
> +	__skb_dst_set_noref(skb, dst, true);
> +}
>  
>  /**
>   * skb_dst_is_noref - Test if skb dst isn't refcounted
> diff --git a/net/core/dst.c b/net/core/dst.c
> index 35fd12f..df9cc81 100644
> --- a/net/core/dst.c
> +++ b/net/core/dst.c
> @@ -320,27 +320,28 @@ void __dst_destroy_metrics_generic(struct dst_entry *dst, unsigned long old)
>  EXPORT_SYMBOL(__dst_destroy_metrics_generic);
>  
>  /**
> - * skb_dst_set_noref - sets skb dst, without a reference
> + * __skb_dst_set_noref - sets skb dst, without a reference
>   * @skb: buffer
>   * @dst: dst entry
> + * @force: if force is set, use noref version even for DST_NOCACHE entries
>   *
>   * Sets skb dst, assuming a reference was not taken on dst
>   * skb_dst_drop() should not dst_release() this dst
>   */
> -void skb_dst_set_noref(struct sk_buff *skb, struct dst_entry *dst)
> +void __skb_dst_set_noref(struct sk_buff *skb, struct dst_entry *dst, bool force)
>  {
>  	WARN_ON(!rcu_read_lock_held() && !rcu_read_lock_bh_held());
>  	/* If dst not in cache, we must take a reference, because
>  	 * dst_release() will destroy dst as soon as its refcount becomes zero
>  	 */
> -	if (unlikely(dst->flags & DST_NOCACHE)) {
> +	if (unlikely((dst->flags & DST_NOCACHE) && !force)) {
>  		dst_hold(dst);
>  		skb_dst_set(skb, dst);
>  	} else {
>  		skb->_skb_refdst = (unsigned long)dst | SKB_DST_NOREF;
>  	}
>  }
> -EXPORT_SYMBOL(skb_dst_set_noref);
> +EXPORT_SYMBOL(__skb_dst_set_noref);
>  
>  /* Dirty hack. We did it in 2.2 (in __dst_free),
>   * we have _very_ good reasons not to repeat
> -- 
> 1.7.10.4
>