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; });
\
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