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

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Date:	Wed, 12 May 2010 19:00:08 -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 06:33:59PM -0700, Stephen Hemminger wrote:
> On Wed, 12 May 2010 15:35:25 -0700
> "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > 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.
> 
> Then ok. leave the wrapper, but get rid of the !dev part.
> 
> I can do it if you want.

Done!

I would normally accept your offer in a heartbeat, but there are
dependencies among the patches.  :-(

> Still don't like changing working code to conform to code checking tools.
> Especially when code checking tool is missing bad RCU usage that already
> exists (like this case). It is a big problem if code assumes rcu_deref
> always returns non NULL.

Indeed, I would have liked some way of making this work without having
to make rcu_dereference() know about the pointed-too type.  I have not
yet been able to find one, though.  :-(

							Thanx, Paul
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