linux-kernel - Re: [PATCH rcu 6/6] rcu: Use WRITE_ONCE() for assignments to ->next for rculist

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Message-ID: <9651c3b4-0791-5cd7-567f-f65d28bc8fae@joelfernandes.org>
Date:   Tue, 18 Jul 2023 09:49:51 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>, rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...a.com,
        rostedt@...dmis.org, Alan Huang <mmpgouride@...il.com>
Subject: Re: [PATCH rcu 6/6] rcu: Use WRITE_ONCE() for assignments to ->next
 for rculist_nulls

On 7/17/23 14:03, Paul E. McKenney wrote:
> From: Alan Huang <mmpgouride@...il.com>
> 
> When the objects managed by rculist_nulls are allocated with
> SLAB_TYPESAFE_BY_RCU, old readers may still hold references to an object
> even though it is just now being added, which means the modification of
> ->next is visible to readers.  This patch therefore uses WRITE_ONCE()
> for assignments to ->next.
> 
> Signed-off-by: Alan Huang <mmpgouride@...il.com>
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>

Did we ever conclude that the READ_ONCE() counterparts were not needed? ;-)

But incremental progress and all, so this LGTM:
Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>

thanks,

  - Joel


> ---
>   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 {