Re: [PATCH] md: do not use ++ in rcu_dereference() argument

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Sep 09, 2010 at 05:14:43PM +0200, Arnd Bergmann wrote:
> On Tuesday 07 September 2010, Paul E. McKenney wrote:
> > On Tue, Sep 07, 2010 at 10:00:58PM +0200, Arnd Bergmann wrote:
> > > On Tuesday 07 September 2010 21:21:55 Kulikov Vasiliy wrote:
> > > > #define __rcu_dereference_check(p, c, space) \
> > > >         ({ \
> > > >                 typeof(*p) *_________p1 = (typeof(*p)*__force )ACCESS_ONCE(p); \
> > > >                                                                    ^
> > > >                 rcu_lockdep_assert(c); \
> > > >                 (void) (((typeof (*p) space *)p) = p); \
> > > >                                       ^     ^
> > > >                 smp_read_barrier_depends(); \
> > > >                 ((typeof(*p) __force __kernel *)(_________p1)); \
> > > >         })
> > > > 
> > > > If I understand this, it is evaluated three times, right?
> > > 
> > > Yes, that looks like my own fault, I added that :(
> > > 
> > > This patch seems to fix it, but I need to think about it some more
> > > to make sure it still does everything we need.
> > 
> > Let me know when you are satisfied with it, and then I will pick it up.
> 
> I guess it would be good to put it in now. I haven't had the time
> to try out all cases, but the current code in -next is definitely
> broken, so please put the fix in now.

Hmmm...  One approach would be have a secondary macro that was:

	#define __rcu_dereference_check_sparse(p, space) \
		(void) (((typeof (*p) space *)p) = p);

when running sparse and:

	#define __rcu_dereference_check_sparse(p, space)

otherwise.

Would that do the trick?

							Thanx, Paul