Re: [PATCH v2] rcu: fix a race in hlist_nulls_for_each_entry_rcu macro

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

On Mon, 2013-05-27 at 21:55 +0400, Roman Gushchin wrote:
> Hi, Paul!
> 
> > On 25.05.2013 15:37, Paul E. McKenney wrote:
> >> Again, I believe that your retry logic needs to extend back into the
> >> calling function for your some_func() example above.
> 
> And what do you think about the following approach (diff below)?
> 
> It seems to me, it's enough clear (especially with good accompanying comments)
> and produces a good binary code (without significant overhead).
> Also, we will remove a hidden reef in using rcu-protected (h)list traverses with restarts.
> 

> diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h
> index 2ae1371..4af5ee5 100644
> --- a/include/linux/rculist_nulls.h
> +++ b/include/linux/rculist_nulls.h
> @@ -107,7 +107,8 @@ static inline void hlist_nulls_add_head_rcu(struct hlist_nulls_node *n,
>    *
>    */
>   #define hlist_nulls_for_each_entry_rcu(tpos, pos, head, member)                        \
> -       for (pos = rcu_dereference_raw(hlist_nulls_first_rcu(head));            \
> +       for (ACCESS_ONCE(*(head)),                                              \
> +               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)))

It looks like this still relies on gcc being friendly here.

I repeat again : @head here is a constant.

Macro already uses ACCESS_ONCE(), we only have to instruct gcc that
caching the value is forbidden if we restart the loop 
(aka "goto begin;" see Documentation/RCU/rculist_nulls.txt line 146)

Adding a barrier() is probably what we want.

I cooked followed patch and it fixes the problem.

diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h
index 2ae1371..4dc51b2 100644
--- a/include/linux/rculist_nulls.h
+++ b/include/linux/rculist_nulls.h
@@ -105,8 +105,12 @@ static inline void hlist_nulls_add_head_rcu(struct hlist_nulls_node *n,
  * @head:	the head for your list.
  * @member:	the name of the hlist_nulls_node within the struct.
  *
+ * The barrier() is needed to make sure compiler doesn't cache first element,
+ * as this loop can be restarted.
+ * (cf Documentation/RCU/rculist_nulls.txt around line 146)
  */
 #define hlist_nulls_for_each_entry_rcu(tpos, pos, head, member)			\
+	barrier();								\
 	for (pos = rcu_dereference_raw(hlist_nulls_first_rcu(head));		\
 		(!is_a_nulls(pos)) &&						\
 		({ tpos = hlist_nulls_entry(pos, typeof(*tpos), member); 1; }); \