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.