Re: [PATCH 1/2] rcu: Use WRITE_ONCE() for assignments to ->next for rculist_nulls

[Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>]

On Thu, Jul 6, 2023 at 6:29 AM Alan Huang <mmpgouride@xxxxxxxxx> wrote:
>
> When the objects managed by rculist_nulls are allocated with
> SLAB_TYPESAFE_BY_RCU, readers may still hold references to this
> object that is being added, which means the modification of ->next
> is visible to readers. So, this patch uses WRITE_ONCE() for assignments
> to ->next.
>
> Signed-off-by: Alan Huang <mmpgouride@xxxxxxxxx>
> ---
>  include/linux/rculist_nulls.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h
> index ba4c00dd8005..89186c499dd4 100644
> --- a/include/linux/rculist_nulls.h
> +++ b/include/linux/rculist_nulls.h
> @@ -101,7 +101,7 @@ static inline void hlist_nulls_add_head_rcu(struct hlist_nulls_node *n,
>  {
>         struct hlist_nulls_node *first = h->first;
>
> -       n->next = first;
> +       WRITE_ONCE(n->next, first);
>         WRITE_ONCE(n->pprev, &h->first);
>         rcu_assign_pointer(hlist_nulls_first_rcu(h), n);
>         if (!is_a_nulls(first))
> @@ -137,7 +137,7 @@ static inline void hlist_nulls_add_tail_rcu(struct hlist_nulls_node *n,
>                 last = i;
>
>         if (last) {
> -               n->next = last->next;
> +               WRITE_ONCE(n->next, last->next);
>                 n->pprev = &last->next;
>                 rcu_assign_pointer(hlist_nulls_next_rcu(last), n);
>         } else {

Don't you need READ_ONCE() for the read-side accesses as well?

thanks,

 - Joel