linux-kernel - Re: Question about the barrier() in hlist_nulls_for_each_entry

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Message-Id: <EDE1C0AE-B479-49F9-995D-DA9CC1A6EA57@gmail.com>
Date:   Fri, 21 Jul 2023 22:31:13 +0800
From:   Alan Huang <mmpgouride@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        rcu@...r.kernel.org, "Paul E. McKenney" <paulmck@...nel.org>,
        roman.gushchin@...ux.dev
Subject: Re: Question about the barrier() in hlist_nulls_for_each_entry_rcu()


> 2023年7月21日 05:11，Eric Dumazet <edumazet@...gle.com> 写道：
> 
> On Thu, Jul 20, 2023 at 10:00 PM Alan Huang <mmpgouride@...il.com> wrote:
>> 
>> 
>>> 2023年7月21日 03:22，Eric Dumazet <edumazet@...gle.com> 写道：
>>> 
>>> On Thu, Jul 20, 2023 at 8:54 PM Alan Huang <mmpgouride@...il.com> wrote:
>>>> 
>>>> 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/
>> 
>> 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; });)


> 
> 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);

?

> 
>> 
>>> 
>>> Do you have a patch for review ?
>> 
>> Possibly next month. :)
>> 
>>> 
>>> 
>>>> 
>>>> 
>>>> [1] https://lore.kernel.org/all/1369699930.3301.494.camel@edumazet-glaptop/
>>>> 
>>>> Thanks,
>>>> Alan