On 6/13/24 18:26, Keith Busch wrote:
> On Thu, Jun 13, 2024 at 10:26:11AM +0530, Nilay Shroff wrote:
>> On 6/12/24 21:21, Keith Busch wrote:
>>> +static inline void list_cut(struct list_head *list,
>>> + struct list_head *head, struct list_head *entry)
>>> +{
>>> + list->next = entry;
>>> + list->prev = head->prev;
>>> + head->prev = entry->prev;
>>> + entry->prev->next = head;
>>> + entry->prev = list;
>>> + list->prev->next = list;
>>> +}
>> I am wondering whether we really need the _rcu version of list_cut here?
>> I think that @head could point to an _rcu protected list and that's true
>> for this patch. So there might be concurrent readers accessing @head using
>> _rcu list-traversal primitives, such as list_for_each_entry_rcu().
>>
>> An _rcu version of list_cut():
>>
>> static inline void list_cut_rcu(struct list_head *list,
>> struct list_head *head, struct list_head *entry)
>> {
>> list->next = entry;
>> list->prev = head->prev;
>> head->prev = entry->prev;
>> rcu_assign_pointer(list_next_rcu(entry->prev), head);
>> entry->prev = list;
>> list->prev->next = list;
>> }
>
> I was initially thinking similiar, but this is really just doing a
> "list_del", and the rcu version calls the same generic __list_del()
> helper. To make this more clear, we could change
>
> head->prev = entry->prev;
> entry->prev->next = head;
>
> To just this:
>
> __list_del(entry->prev, head);
>
> And that also gets the "WRITE_ONCE" usage right.
Yeah this sounds reasonable.
>
> But that's not the problem for the rcu case. It's the last line that's
> the problem:
>
> list->prev->next = list;
>
> We can't change forward pointers for any element being detached from
> @head because a reader iterating the list may see that new pointer value
> and end up in the wrong list, breaking iteration. A synchronize rcu
> needs to happen before forward pointers can be mucked with, so it still
> needs to be done in two steps. Oh bother...
Agree and probably we may break it down using this API:
static inline void list_cut_rcu(struct list_head *list,
struct list_head *head, struct list_head *entry,
void (*sync)(void))
{
list->next = entry;
list->prev = head->prev;
__list_del(entry->prev, head);
sync();
entry->prev = list;
list->prev->next = list;
}
Thanks,
--Nilay
