netdev - Re: [PATCH net-next v6 1/3] rculist: Add hlist_nulls_replace_rcu() and hlist_nulls_replace_init

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Message-ID: <CAAVpQUAi2TLyODdvK=EAh0OyL_ZzLQWA_XrrQaspXTNdEmapWA@mail.gmail.com>
Date: Thu, 25 Sep 2025 21:02:45 -0700
From: Kuniyuki Iwashima <kuniyu@...gle.com>
To: luoxuanqiang <xuanqiang.luo@...ux.dev>
Cc: edumazet@...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 v6 1/3] rculist: Add hlist_nulls_replace_rcu()
 and hlist_nulls_replace_init_rcu()

On Thu, Sep 25, 2025 at 8:23 PM luoxuanqiang <xuanqiang.luo@...ux.dev> wrote:
[...]
> >> +{
> >> +       struct hlist_nulls_node *next = old->next;
> >> +
> >> +       WRITE_ONCE(new->next, next);
> >> +       WRITE_ONCE(new->pprev, old->pprev);
> >> +       rcu_assign_pointer(*(struct hlist_nulls_node __rcu **)new->pprev, new);
> > nit: define hlist_nulls_prev_rcu() like hlist_nulls_next_rcu().
>
> I'm wondering if defining a macro called hlist_nulls_prev_rcu() might
> be controversial, since it should actually be getting the prev->next
> rather than the prev itself.

See hlist_add_before_rcu() for an example:

    rcu_assign_pointer(hlist_pprev_rcu(n), n);

You can define hlist_nulls_pprev_rcu() and use it alike.

    rcu_assign_pointer(hlist_nulls_pprev_rcu(new), new);


[...]
> However, I noticed that in the definition of hlist_pprev_rcu(), it directly
> uses pprev:
>
> #define hlist_pprev_rcu(node)    (*((struct hlist_node __rcu **)((node)->pprev)))

Note it dereferences *((node)->pprev).  The macro is not to iterate the
lengthy cast.