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

Paul E. McKenney
2010-09-10T03:46:03Z

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