netdev - Re: [PATCH net-next-2.6] bridge: add _

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Date:	Sun, 14 Nov 2010 10:47:56 -0800
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next-2.6] bridge: add __rcu annotations

On Sat, 13 Nov 2010 23:04:21 +0100
Eric Dumazet <eric.dumazet@...il.com> wrote:

> Le samedi 13 novembre 2010 à 10:13 -0800, Stephen Hemminger a écrit :
> > On Sat, 13 Nov 2010 18:58:50 +0100
> > Eric Dumazet <eric.dumazet@...il.com> wrote:
> > 
> > > Le samedi 13 novembre 2010 à 09:35 -0800, Stephen Hemminger a écrit :
> > > > On Sat, 13 Nov 2010 09:15:28 +0100
> > > > Eric Dumazet <eric.dumazet@...il.com> wrote:
> > > > 
> > > > > diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> > > > > index 578debb..ffbd177 100644
> > > > > --- a/include/linux/netdevice.h
> > > > > +++ b/include/linux/netdevice.h
> > > > > @@ -996,7 +996,10 @@ struct net_device {
> > > > >  #endif
> > > > >  
> > > > >  	rx_handler_func_t	*rx_handler;
> > > > > -	void			*rx_handler_data;
> > > > > +	union {
> > > > > +		void				*rx_handler_data;
> > > > > +		struct net_bridge_port __rcu	*br_port_rcu;
> > > > > +	};
> > > > >  
> > > > >  	struct netdev_queue __rcu *ingress_queue;
> > > > 
> > > > I don't like making the generic hook typed again.
> > > > We don't do this for other callbacks, timers, workqueues, ...
> > > > Why is it necessary for RCU notation.
> > > > 
> > > 
> > > because rcu_dereference() needs the type for __CHECKER__/sparse checks
> > > 
> > > #define __rcu_dereference_check(p, c, space) \
> > >         ({ \
> > >                 typeof(*p) *_________p1 = (typeof(*p)*__force )ACCESS_ONCE(p); \
> > >                 rcu_lockdep_assert(c); \
> > >                 rcu_dereference_sparse(p, space); \
> > >                 smp_read_barrier_depends(); \
> > >                 ((typeof(*p) __force __kernel *)(_________p1)); \
> > >         })
> > > 
> > > So using a "void *ptr" is not an option
> > > 
> > > Its also cleaner to use
> > > 
> > > rcu_dereference(dev->br_port_rcu)
> > > 
> > > instead of 
> > > 
> > > (struct net_bridge_port *)rcu_dereference(dev->rx_handler_data)
> > 
> > There must be a better way. What about use of that hook by macvlan and openvswitch?
> 
> macvlan and openvswitch (is it part of linux yet ???)
> 
> I honestly dont understand your point Stephen, maybe you could explain a
> bit more what is the problem ?
> 
> I use a union, like many other ones in the kernel. This is the first
> time I ear this is not good to add type safety.
> 
> You can use either one or other field at your convenience.
> 
> If you are talking about stacking hooks, that has nothing to do with
> this (cleanup) rcu patch, but previous introduction of
> rx_handler_data/rx_handler ?
> 
> Please run sparse on x86_64 machine and watch all the warnings in bridge
> code. (with CONFIG_SPARSE_RCU_POINTER=y)
> 
> Me confused.
> 
> 

You combined three different sets of changes and introduced a needless
union. I will split them up and show you what I want.


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