Subject: Re: [PATCH net-next v7 1/3] rculist: Add hlist_nulls_replace_rcu() and hlist_nulls_replace_init_rcu()

From: luoxuanqiang <[email-masked]>
Date: Mon, 13 Oct 2025 16:25:37 +0800

在 2025/10/13 15:31, Eric Dumazet 写道:
> On Fri, Sep 26, 2025 at 12:41 AM <[email-masked]> wrote:
>> From: Xuanqiang Luo <[email-masked]>
>>
>> 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:
>> commit efd04f8a8b45 ("rcu: Use WRITE_ONCE() for assignments to ->next for
>> rculist_nulls")
>> commit 860c8802ace1 ("rcu: Use WRITE_ONCE() for assignments to ->pprev for
>> hlist_nulls")
>>
>> Signed-off-by: Xuanqiang Luo <[email-masked]>
>> ---
>>   include/linux/rculist_nulls.h | 59 +++++++++++++++++++++++++++++++++++
>>   1 file changed, 59 insertions(+)
>>
>> diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h
>> index 89186c499dd4..c26cb83ca071 100644
>> --- a/include/linux/rculist_nulls.h
>> +++ b/include/linux/rculist_nulls.h
>> @@ -52,6 +52,13 @@ static inline void hlist_nulls_del_init_rcu(struct hlist_nulls_node *n)
>>   #define hlist_nulls_next_rcu(node) \
>>          (*((struct hlist_nulls_node __rcu __force **)&(node)->next))
>>
>> +/**
>> + * hlist_nulls_pprev_rcu - returns the dereferenced pprev of @node.
>> + * @node: element of the list.
>> + */
>> +#define hlist_nulls_pprev_rcu(node) \
>> +       (*((struct hlist_nulls_node __rcu __force **)(node)->pprev))
>> +
>>   /**
>>    * hlist_nulls_del_rcu - deletes entry from hash list without re-initialization
>>    * @n: the element to delete from the hash list.
>> @@ -152,6 +159,58 @@ 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
>> + * @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);
> I do not think these two WRITE_ONCE() are needed.
>
> At this point new is not yet visible.
>
> The following  rcu_assign_pointer() is enough to make sure prior
> writes are committed to memory.

Dear Eric,

I’m quoting your more detailed explanation from the other patch [0], thank
you for that!

However, regarding new->next, if the new object is allocated with
SLAB_TYPESAFE_BY_RCU, would we still encounter the same issue as in commit
efd04f8a8b45 (“rcu: Use WRITE_ONCE() for assignments to ->next for
rculist_nulls”)?

Also, for the WRITE_ONCE() assignments to ->pprev introduced in commit
860c8802ace1 (“rcu: Use WRITE_ONCE() for assignments to ->pprev for
hlist_nulls”) within hlist_nulls_add_head_rcu(), is that also unnecessary?

[0]: https://lore.kernel.org/all/CANn89iKQM=4wjCLxpg-m3jYoUm=[email-masked]/

Thanks!

>> +       rcu_assign_pointer(hlist_nulls_pprev_rcu(new), new);
>> +       if (!is_a_nulls(next))
>> +               WRITE_ONCE(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 must be hashed.
>> + *
>> + * 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
>>