netdev - Re: [PATCH net-next v4 1/3] rculist: Add __hlist_nulls_replace_rcu() and hlist_nulls_replace_init

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Message-ID: <6715c8c6-f252-42d3-b9b2-3032cd38c65f@linux.dev>
Date: Tue, 23 Sep 2025 09:59:56 +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 v4 1/3] rculist: Add __hlist_nulls_replace_rcu()
 and hlist_nulls_replace_init_rcu()


在 2025/9/23 08:19, Kuniyuki Iwashima 写道:
> On Sat, Sep 20, 2025 at 4:00 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:
>> efd04f8a8b45 ("rcu: Use WRITE_ONCE() for assignments to ->next for rculist_nulls")
>> 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 | 52 +++++++++++++++++++++++++++++++++++
>>   1 file changed, 52 insertions(+)
>>
>> diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h
>> index 89186c499dd4..d86331ce22c4 100644
>> --- a/include/linux/rculist_nulls.h
>> +++ b/include/linux/rculist_nulls.h
>> @@ -152,6 +152,58 @@ 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
> nit: '__' is not needed as there is not no-'__' version.
>
>
>> + * @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);
>> +       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.
>> + *
>> + * Note: @old should be hashed.
> nit: s/should/must/

Will fix them in the next version!

Thanks, Kuniyuki!

>> + *
>> + * 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
>>