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SubjectRe: [PATCH RFC tip/core/rcu 04/23] net: Make accesses to ->br_port safe for sparse RCU
On Wed, 12 May 2010 14:33:23 -0700
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:

> diff --git a/net/bridge/br_fdb.c b/net/bridge/br_fdb.c
> index 9101a4e..3f66cd1 100644
> --- a/net/bridge/br_fdb.c
> +++ b/net/bridge/br_fdb.c
> @@ -246,7 +246,7 @@ int br_fdb_test_addr(struct net_device *dev, unsigned char *addr)
> return 0;
>
> rcu_read_lock();
> - fdb = __br_fdb_get(dev->br_port->br, addr);
> + fdb = __br_fdb_get(br_port(dev)->br, addr);
> ret = fdb && fdb->dst->dev != dev &&
> fdb->dst->state == BR_STATE_FORWARDING;
> rcu_read_unlock();
> diff --git a/net/bridge/br_private.h b/net/bridge/br_private.h
> index 846d7d1..4fedb60 100644
> --- a/net/bridge/br_private.h
> +++ b/net/bridge/br_private.h
> @@ -229,6 +229,14 @@ static inline int br_is_root_bridge(const struct net_bridge *br)
> return !memcmp(&br->bridge_id, &br->designated_root, 8);
> }
>
> +static inline struct net_bridge_port *br_port(const struct net_device *dev)
> +{
> + if (!dev)
> + return NULL;
> +
> + return rcu_dereference(dev->br_port);
> +}

Looks like this is wrapping existing problems, and hurting not helping.

Why introduce a wrapper that could return NULL and not check the
result?

I would rather that:
1. dev should never be null in this cases so the first if() is
unnecessary, and confuses the semantics.
2. don't use wrapper br_port()
3. have callers check that rcu_dereference(dev->br_port) did not
return NULL.
If they derefernce does return NULL, then it means other CPU
has started tear down and this CPU should just go home quietly.

--


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