netdev - Re: [PATCH net-next v3 1/3] rculist: Add __hlist_nulls_replace_rcu() and hlist_nulls_replace_init

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Message-ID: <ce3d224d-0d17-4510-9bad-494832dbce8b@linux.dev>
Date: Thu, 18 Sep 2025 14:09:08 +0800
From: luoxuanqiang <xuanqiang.luo@...ux.dev>
To: Kuniyuki Iwashima <kuniyu@...gle.com>
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 v3 1/3] rculist: Add __hlist_nulls_replace_rcu()
 and hlist_nulls_replace_init_rcu()


在 2025/9/17 12:43, Kuniyuki Iwashima 写道:
> On Tue, Sep 16, 2025 at 9:27 PM Kuniyuki Iwashima <kuniyu@...gle.com> wrote:
>> On Tue, Sep 16, 2025 at 8:27 PM luoxuanqiang <xuanqiang.luo@...ux.dev> wrote:
>>>
>>> 在 2025/9/17 02:58, Kuniyuki Iwashima 写道:
>>>> On Tue, Sep 16, 2025 at 3:31 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 | 61 +++++++++++++++++++++++++++++++++++
>>>>>    1 file changed, 61 insertions(+)
>>>>>
>>>>> diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h
>>>>> index 89186c499dd4..8ed604f65a3e 100644
>>>>> --- a/include/linux/rculist_nulls.h
>>>>> +++ b/include/linux/rculist_nulls.h
>>>>> @@ -152,6 +152,67 @@ 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;
>>> Do we need to use WRITE_ONCE() here, as mentioned in efd04f8a8b45
>>> ("rcu: Use WRITE_ONCE() for assignments to ->next for rculist_nulls")?
>>> I am more inclined to think that it is necessary.
>> Good point, then WRITE_ONCE() makes sense.
> and it would be nice to have such reasoning in the commit
> message as we were able to learn from efd04f8a8b45.

Okay, I'll add an explanation in the next version. Really
appreciate you confirming this!