netdev - Re: [PATCH net-next v4 2/2] vxlan: allow specifying multiple default destinations

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Date:	Sun, 23 Jun 2013 17:14:40 -0700
From:	Stephen Hemminger <stephen@...workplumber.org>
To:	Mike Rapoport <mike.rapoport@...ellosystems.com>
Cc:	netdev@...r.kernel.org, David Stevens <dlstevens@...ibm.com>,
	Thomas Graf <tgraf@...g.ch>
Subject: Re: [PATCH net-next v4 2/2] vxlan: allow specifying multiple
 default destinations

On Sun, 23 Jun 2013 19:22:23 +0300
Mike Rapoport <mike.rapoport@...ellosystems.com> wrote:

> A list of multiple default destinations can be used in environments that
> disable multicast on the infrastructure level, e.g. public clouds.
> 
> Signed-off-by: Mike Rapoport <mike.rapoport@...ellosystems.com>
> ---
>  drivers/net/vxlan.c          | 268 +++++++++++++++++++++++++++++++++++++++++--
>  include/uapi/linux/if_link.h |  17 +++
>  2 files changed, 276 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/net/vxlan.c b/drivers/net/vxlan.c
> index e5fb6568..f57a0d94 100644
> --- a/drivers/net/vxlan.c
> +++ b/drivers/net/vxlan.c
> @@ -103,6 +103,7 @@ struct vxlan_rdst {
>  	u32			 remote_vni;
>  	u32			 remote_ifindex;
>  	struct list_head	 list;
> +	struct rcu_head		 rcu;
>  };

The use of remotes_cnt here is not SMP safe.
You are using remotes_cnt to size the buffer for dumping, but then the list
of remotes might change during the dump.

There a a couple of alternatives here:
1. Put a hard limit on the number of remotes per MAC.
2. When there are multiple destnations, just dump multiple entries, like
   multipath routing does.

I prefer #2 because it also allows for a cleaner API on creation.

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