Re: [PATCH] rcu: fix a race in hlist_nulls_for_each_entry_rcu macro

[Eric Dumazet <eric.dumazet@gmail.com>]

On Tue, 2013-05-21 at 05:09 -0700, Paul E. McKenney wrote:

> > 
> > -#define hlist_nulls_first_rcu(head) \
> > -	(*((struct hlist_nulls_node __rcu __force **)&(head)->first))
> > +#define hlist_nulls_first_rcu(head)			\
> > +	(*((struct hlist_nulls_node __rcu __force **)	\
> > +	   &((volatile typeof(*head) *)head)->first))
> 
> Why not use ACCESS_ONCE() or (better) rcu_dereference_raw() here?

More exactly we have :

#define list_entry_rcu(ptr, type, member) \
        ({typeof (*ptr) __rcu *__ptr = (typeof (*ptr) __rcu __force *)ptr; \
         container_of((typeof(ptr))rcu_dereference_raw(__ptr), type, member); \
        })

#define list_for_each_entry_rcu(pos, head, member) \
        for (pos = list_entry_rcu((head)->next, typeof(*pos), member); \
                &pos->member != (head); \
                pos = list_entry_rcu(pos->member.next, typeof(*pos), member))
<< and >>

#define hlist_nulls_for_each_entry_rcu(tpos, pos, head, member)                 \
        for (pos = rcu_dereference_raw(hlist_nulls_first_rcu(head));            \
                (!is_a_nulls(pos)) &&                                           \
                ({ tpos = hlist_nulls_entry(pos, typeof(*tpos), member); 1; }); \
                pos = rcu_dereference_raw(hlist_nulls_next_rcu(pos)))

We need to change hlist_nulls_for_each_entry_rcu() to use same construct,
so that the rcu_dereference_raw() is performed at the right place.