netdev - Re: Question about the barrier() in hlist_nulls_for_each_entry

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Date: Fri, 21 Jul 2023 16:47:54 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Alan Huang <mmpgouride@...il.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()

On Fri, Jul 21, 2023 at 4:31 PM Alan Huang <mmpgouride@...il.com> wrote:
>
>
> > 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);
>

Nope, the patch you want to revert (while it did fix (by pure luck
???) a real bug back in the days) was replacing

ACCESS_ONCE()

by

barrier();
ACCESS_ONCE();

(There is one ACCESS_ONCE(), not two of them)

BTW,
  barrier();
  followed by an arbitrary number of barrier(); back to back,
translates to one barrier()

Frankly, I would not change the code, unless someone can explain what
was the issue.
(Perhaps there was a missing barrier elsewhere)