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

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Date:	Wed, 12 May 2010 15:35:25 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Stephen Hemminger <shemminger@...ux-foundation.org>
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, May 12, 2010 at 02:44:53PM -0700, Stephen Hemminger wrote:
> 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?

Fair point!

> 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.

OK.

The reason for br_port() is to allow ->br_port to be a void*.  If we
eliminate br_port(), then it is necessary to make the definition of the
struct net_bridge_port available everywhere that ->br_port is given to
rcu_dereference().  The reason for this is that Arnd's sparse-based RCU
checking code uses __rcu to tag the data pointed to by an RCU-protected
pointer.  This in turn means that rcu_dereference() and friends must
now have access to the pointed-to type, as is done in patch 6 in this
series.

One way to make struct net_bridge_port available is to put:

	#include "../../net/bridge/br_private.h"

in include/linux/netdevice.h.

However, when I try this, I get lots of build errors, which was what led
us to the path of making ->br_port be a void*, thus requiring the br_port()
helper function in cases where the caller needs the underlying type.

What should we be doing instead?

							Thanx, Paul
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