netdev - Re: [PATCH net-next v1 1/3] rculist: Add __hlist_nulls_replace_rcu() and hlist_nulls_replace_init

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Message-ID: <CAAVpQUCc1ajczhau-iOE8AWHySOowdg0XvSk_WUUmDy4ka_V4w@mail.gmail.com>
Date: Mon, 15 Sep 2025 19:10:37 -0700
From: Kuniyuki Iwashima <kuniyu@...gle.com>
To: luoxuanqiang <xuanqiang.luo@...ux.dev>
Cc: edumazet@...gle.com, kerneljasonxing@...il.com, davem@...emloft.net, 
	kuba@...nel.org, netdev@...r.kernel.org, 
	Xuanqiang Luo <luoxuanqiang@...inos.cn>
Subject: Re: [PATCH net-next v1 1/3] rculist: Add __hlist_nulls_replace_rcu()
 and hlist_nulls_replace_init_rcu()

On Mon, Sep 15, 2025 at 6:34 PM luoxuanqiang <xuanqiang.luo@...ux.dev> wrote:
>
>
> 在 2025/9/16 06:50, Kuniyuki Iwashima 写道:
> > On Mon, Sep 15, 2025 at 12:04 AM <xuanqiang.luo@...ux.dev> wrote:
> >> From: Xuanqiang Luo <luoxuanqiang@...inos.cn>
> >>
> >> Add two functions to atomically replace RCU-protected hlist_nulls entries.
> >>
> >> Signed-off-by: Xuanqiang Luo <luoxuanqiang@...inos.cn>
> >> ---
> >>   include/linux/rculist_nulls.h | 62 +++++++++++++++++++++++++++++++++++
> >>   1 file changed, 62 insertions(+)
> >>
> >> diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h
> >> index 89186c499dd4..eaa3a0d2f206 100644
> >> --- a/include/linux/rculist_nulls.h
> >> +++ b/include/linux/rculist_nulls.h
> >> @@ -152,6 +152,68 @@ 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;
> >> +
> >> +       new->next = next;
> >> +       WRITE_ONCE(new->pprev, old->pprev);
> >> +       rcu_assign_pointer(*(struct hlist_nulls_node __rcu **)new->pprev, new);
> >> +       if (!is_a_nulls(next))
> >> +               WRITE_ONCE(new->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.
> >> + *
> >> + * Return: true if the old entry was hashed and was replaced successfully, false
> >> + * otherwise.
> >> + *
> >> + * Note: hlist_nulls_unhashed() on the old node returns true after this.
> >> + * It is useful for RCU based read lockfree traversal if the writer side must
> >> + * know if the list entry is still hashed or already unhashed.
> >> + *
> >> + * 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 bool hlist_nulls_replace_init_rcu(struct hlist_nulls_node *old,
> >> +                                               struct hlist_nulls_node *new)
> >> +{
> >> +       if (!hlist_nulls_unhashed(old)) {
> > This is already checked by __sk_nulls_replace_node_init_rcu().
> >
> It seems to me that hlist_nulls_replace_init_rcu() checks whether the
> sk_nulls_node is unhashed, while __sk_nulls_replace_node_init_rcu() checks the
> sk_node's unhashed status.

sk_nulls_node and sk_node have the same size and
are defined in union so that we can reuse sk_hashed()
for both types.


> Perhaps these serve different purposes?
> This would maintain parity with how hlist_nulls_del_init_rcu verifies
> sk_nulls_node and __sk_nulls_del_node_init_rcu() checks sk_node.
>
>
> Thanks
> Xuanqiang
>
> >> +               __hlist_nulls_replace_rcu(old, new);
> >> +               old->pprev = NULL;
> >> +               return true;
> >> +       }
> >> +
> >> +       return false;
> >> +}
> >> +
> >>   /**
> >>    * hlist_nulls_for_each_entry_rcu - iterate over rcu list of given type
> >>    * @tpos:      the type * to use as a loop cursor.
> >> --
> >> 2.27.0
> >>