linux-kernel - Re: [PATCH RFC tip/core/rcu 04/23] net: Make accesses to ->br

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Date:	Wed, 12 May 2010 14:44:53 -0700
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	dvhltc@...ibm.com, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com,
	Arnd Bergmann <arnd@...db.de>,
	David Miller <davem@...emloft.net>
Subject: Re: [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@...ux.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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