linux-kernel - Re: [PATCH tip/core/rcu 28/48] net: Make accesses to ->br

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Message-ID: <20100506231211.GK2325@linux.vnet.ibm.com>
Date:	Thu, 6 May 2010 16:12:11 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Stephen Hemminger <shemminger@...ux-foundation.org>,
	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,
	David Miller <davem@...emloft.net>
Subject: Re: [PATCH tip/core/rcu 28/48] net: Make accesses to ->br_port
 safe for sparse RCU

On Thu, May 06, 2010 at 04:09:25PM +0200, Arnd Bergmann wrote:
> On Thursday 06 May 2010, Paul E. McKenney wrote:
> > On Tue, May 04, 2010 at 11:41:49PM +0200, Arnd Bergmann wrote:
> > > On Tuesday 04 May 2010 23:26:31 Stephen Hemminger wrote:
> > > > > The new versions of the rcu_dereference() APIs requires that any pointers
> > > > > passed to one of these APIs be fully defined.  The ->br_port field
> > > > > in struct net_device points to a struct net_bridge_port, which is an
> > > > > incomplete type.  This commit therefore changes ->br_port to be a void*,
> > > > > and introduces a br_port() helper function to convert the type to struct
> > > > > net_bridge_port, and applies this new helper function where required.
> > > > 
> > > > I would rather make the bridge hook generic and not take a type argument.
> > > 
> > > Not sure if you were confused by the comment in the same way that I was.
> > > 
> > > The bridge hook is not impacted by this at all, since we can either pass
> > > a void* or a struct net_bridge_port* to it. The br_port() helper
> > > is used for all the places where we actually want to dereference 
> > > dev->br_port and access its contents.
> > 
> > What should I change in the commit message to clear this up?
> > 
> > Of course, if the code needs to change, please let me know what should
> > change there as well.
> 
> I think it's both ok, I was mostly confused by the discussion we had earlier.
> Maybe add a sentence like:
> 
>  The br_handle_frame_hook now needs a forward declaration of struct net_bridge_port.

Done!

> Or you just change br_handle_frame_hook to take a void* to avoid the forward
> declaration. Not sure what Stephen was referring to really.

This sounds like a way to make things quite a bit more intrusive, so
holding off on this.

							Thanx, Paul
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