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: <bdc27331-e1a3-4e49-ba58-d5b41171be3e@linux.dev>
Date: Wed, 17 Sep 2025 11:26:55 +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 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.

>> +       WRITE_ONCE(new->pprev, old->pprev);
> As you don't use WRITE_ONCE() for ->next, the new node must
> not be published yet, so WRITE_ONCE() is unnecessary for ->pprev
> too.

I noticed that point. My understanding is that using WRITE_ONCE()
for new->pprev follows the approach in hlist_replace_rcu() to
match the READ_ONCE() in hlist_nulls_unhashed_lockless() and
hlist_unhashed_lockless().

>
>> +       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)) {
> As mentioned in v1, this check is redundant.

Apologies for bringing this up again. My understanding is that
replacing a node requires checking if the old node is unhashed.

If so, we need a return value to inform the caller that the
replace operation would fail.

>
>> +               __hlist_nulls_replace_rcu(old, new);
>> +               WRITE_ONCE(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.
>> --
>> 2.25.1
>>