netdev - Re: [PATCH net-next v7 1/3] rculist: Add hlist_nulls_replace_rcu() and hlist_nulls_replace_init

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Message-ID: <CANn89iJ15RFYq65t57sW=F1jZigbr5xTbPNLVY53cKtpMKLotA@mail.gmail.com>
Date: Mon, 13 Oct 2025 00:31:50 -0700
From: Eric Dumazet <edumazet@...gle.com>
To: xuanqiang.luo@...ux.dev
Cc: kuniyu@...gle.com, "Paul E. McKenney" <paulmck@...nel.org>, kerneljasonxing@...il.com, 
	davem@...emloft.net, kuba@...nel.org, netdev@...r.kernel.org, 
	Xuanqiang Luo <luoxuanqiang@...inos.cn>, Frederic Weisbecker <frederic@...nel.org>, 
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>
Subject: Re: [PATCH net-next v7 1/3] rculist: Add hlist_nulls_replace_rcu()
 and hlist_nulls_replace_init_rcu()

On Fri, Sep 26, 2025 at 12:41 AM <xuanqiang.luo@...ux.dev> wrote:
>
> From: Xuanqiang Luo <luoxuanqiang@...inos.cn>
>
> Add two functions to atomically replace RCU-protected hlist_nulls entries.
>
> Keep using WRITE_ONCE() to assign values to ->next and ->pprev, as
> mentioned in the patch below:
> commit efd04f8a8b45 ("rcu: Use WRITE_ONCE() for assignments to ->next for
> rculist_nulls")
> commit 860c8802ace1 ("rcu: Use WRITE_ONCE() for assignments to ->pprev for
> hlist_nulls")
>
> Signed-off-by: Xuanqiang Luo <luoxuanqiang@...inos.cn>
> ---
>  include/linux/rculist_nulls.h | 59 +++++++++++++++++++++++++++++++++++
>  1 file changed, 59 insertions(+)
>
> diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h
> index 89186c499dd4..c26cb83ca071 100644
> --- a/include/linux/rculist_nulls.h
> +++ b/include/linux/rculist_nulls.h
> @@ -52,6 +52,13 @@ static inline void hlist_nulls_del_init_rcu(struct hlist_nulls_node *n)
>  #define hlist_nulls_next_rcu(node) \
>         (*((struct hlist_nulls_node __rcu __force **)&(node)->next))
>
> +/**
> + * hlist_nulls_pprev_rcu - returns the dereferenced pprev of @node.
> + * @node: element of the list.
> + */
> +#define hlist_nulls_pprev_rcu(node) \
> +       (*((struct hlist_nulls_node __rcu __force **)(node)->pprev))
> +
>  /**
>   * hlist_nulls_del_rcu - deletes entry from hash list without re-initialization
>   * @n: the element to delete from the hash list.
> @@ -152,6 +159,58 @@ static inline void hlist_nulls_add_fake(struct hlist_nulls_node *n)
>         n->next = (struct hlist_nulls_node *)NULLS_MARKER(NULL);
>  }
>
> +/**
> + * hlist_nulls_replace_rcu - replace an old entry by a new one
> + * @old: the element to be replaced
> + * @new: the new element to insert
> + *
> + * Description:
> + * Replace the old entry with the new one in a RCU-protected hlist_nulls, while
> + * permitting racing traversals.
> + *
> + * The caller must take whatever precautions are necessary (such as holding
> + * appropriate locks) to avoid racing with another list-mutation primitive, such
> + * as hlist_nulls_add_head_rcu() or hlist_nulls_del_rcu(), running on this same
> + * list.  However, it is perfectly legal to run concurrently with the _rcu
> + * list-traversal primitives, such as hlist_nulls_for_each_entry_rcu().
> + */
> +static inline void hlist_nulls_replace_rcu(struct hlist_nulls_node *old,
> +                                          struct hlist_nulls_node *new)
> +{
> +       struct hlist_nulls_node *next = old->next;
> +
> +       WRITE_ONCE(new->next, next);
> +       WRITE_ONCE(new->pprev, old->pprev);
I do not think these two WRITE_ONCE() are needed.

At this point new is not yet visible.

The following  rcu_assign_pointer() is enough to make sure prior
writes are committed to memory.

> +       rcu_assign_pointer(hlist_nulls_pprev_rcu(new), new);
> +       if (!is_a_nulls(next))
> +               WRITE_ONCE(next->pprev, &new->next);
> +}
> +
> +/**
> + * hlist_nulls_replace_init_rcu - replace an old entry by a new one and
> + * initialize the old
> + * @old: the element to be replaced
> + * @new: the new element to insert
> + *
> + * Description:
> + * Replace the old entry with the new one in a RCU-protected hlist_nulls, while
> + * permitting racing traversals, and reinitialize the old entry.
> + *
> + * Note: @old must be hashed.
> + *
> + * The caller must take whatever precautions are necessary (such as holding
> + * appropriate locks) to avoid racing with another list-mutation primitive, such
> + * as hlist_nulls_add_head_rcu() or hlist_nulls_del_rcu(), running on this same
> + * list. However, it is perfectly legal to run concurrently with the _rcu
> + * list-traversal primitives, such as hlist_nulls_for_each_entry_rcu().
> + */
> +static inline void hlist_nulls_replace_init_rcu(struct hlist_nulls_node *old,
> +                                               struct hlist_nulls_node *new)
> +{
> +       hlist_nulls_replace_rcu(old, new);
> +       WRITE_ONCE(old->pprev, NULL);
> +}
> +
>  /**
>   * hlist_nulls_for_each_entry_rcu - iterate over rcu list of given type
>   * @tpos:      the type * to use as a loop cursor.
> --
> 2.25.1
>