quoted

2023年7月21日 05:11，Eric Dumazet [off-list ref] 写道：

On Thu, Jul 20, 2023 at 10:00 PM Alan Huang [off-list ref] wrote:

quoted

quoted

2023年7月21日 03:22，Eric Dumazet [off-list ref] 写道：

On Thu, Jul 20, 2023 at 8:54 PM Alan Huang [off-list ref] wrote:

quoted

Hi,

I noticed a commit c87a124a5d5e(“net: force a reload of first item in hlist_nulls_for_each_entry_rcu”)
and a related discussion [1].

After reading the whole discussion, it seems like that ptr->field was cached by gcc even with the deprecated
ACCESS_ONCE(), so my question is:

      Is that a compiler bug? If so, has this bug been fixed today, ten years later?

      What about READ_ONCE(ptr->field)?

Make sure sparse is happy.

It caused a problem without barrier(), and the deprecated ACCESS_ONCE() didn’t help:

       https://lore.kernel.org/all/519D19DA.50400@yandex-team.ru/ (local)

So, my real question is: With READ_ONCE(ptr->field), are there still some unusual cases where gcc
decides not to reload ptr->field?

I can not really answer without seeing an actual patch...

The content of the potential patch:

diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h
index 89186c499dd4..bcd39670f359 100644
--- a/include/linux/rculist_nulls.h
+++ b/include/linux/rculist_nulls.h

@@ -158,15 +158,9 @@ static inline void hlist_nulls_add_fake(struct hlist_nulls_node *n)
  * @pos:       the &struct hlist_nulls_node to use as a loop cursor.
  * @head:      the head of the 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 [1],
- * as this loop can be restarted [2]
- * [1] Documentation/memory-barriers.txt around line 1533
- * [2] Documentation/RCU/rculist_nulls.rst around line 146
  */
 #define hlist_nulls_for_each_entry_rcu(tpos, pos, head, member)                        \
-       for (({barrier();}),                                                    \
-            pos = rcu_dereference_raw(hlist_nulls_first_rcu(head));            \
+       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)))

@@ -180,8 +174,7 @@ static inline void hlist_nulls_add_fake(struct hlist_nulls_node *n)
  * @member:    the name of the hlist_nulls_node within the struct.
  */
 #define hlist_nulls_for_each_entry_safe(tpos, pos, head, member)               \
-       for (({barrier();}),                                                    \
-            pos = rcu_dereference_raw(hlist_nulls_first_rcu(head));            \
+       for (pos = rcu_dereference_raw(hlist_nulls_first_rcu(head));            \
                (!is_a_nulls(pos)) &&                                           \
                ({ tpos = hlist_nulls_entry(pos, typeof(*tpos), member);        \
                   pos = rcu_dereference_raw(hlist_nulls_next_rcu(pos)); 1; });)

quoted

Why are you asking ? Are you tracking compiler bug fixes ?

The barrier() here makes me confused.

If we really need that, do we need:

        READ_ONCE(head->first);
        barrier();
        READ_ONCE(head->first);