> 2023年7月11日 04:30，Paul E. McKenney <paulmck@xxxxxxxxxx> 写道：
>
> On Mon, Jul 10, 2023 at 04:01:07PM -0400, Joel Fernandes wrote:
>> On Thu, Jul 6, 2023 at 6:29 AM Alan Huang <mmpgouride@xxxxxxxxx> wrote:
>>>
>>> When the objects managed by rculist_nulls are allocated with
>>> SLAB_TYPESAFE_BY_RCU, readers may still hold references to this
>>> object that is being added, which means the modification of ->next
>>> is visible to readers. So, this patch uses WRITE_ONCE() for assignments
>>> to ->next.
>>>
>>> Signed-off-by: Alan Huang <mmpgouride@xxxxxxxxx>
>>> ---
>>> include/linux/rculist_nulls.h | 4 ++--
>>> 1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h
>>> index ba4c00dd8005..89186c499dd4 100644
>>> --- a/include/linux/rculist_nulls.h
>>> +++ b/include/linux/rculist_nulls.h
>>> @@ -101,7 +101,7 @@ static inline void hlist_nulls_add_head_rcu(struct hlist_nulls_node *n,
>>> {
>>> struct hlist_nulls_node *first = h->first;
>>>
>>> - n->next = first;
>>> + WRITE_ONCE(n->next, first);
>>> WRITE_ONCE(n->pprev, &h->first);
>>> rcu_assign_pointer(hlist_nulls_first_rcu(h), n);
>>> if (!is_a_nulls(first))
>>> @@ -137,7 +137,7 @@ static inline void hlist_nulls_add_tail_rcu(struct hlist_nulls_node *n,
>>> last = i;
>>>
>>> if (last) {
>>> - n->next = last->next;
>>> + WRITE_ONCE(n->next, last->next);
>>> n->pprev = &last->next;
>>> rcu_assign_pointer(hlist_nulls_next_rcu(last), n);
>>> } else {
>>
>> Don't you need READ_ONCE() for the read-side accesses as well?
>
> Both hlist_nulls_for_each_entry_rcu() and hlist_nulls_for_each_entry_safe()
> use rcu_dereference_raw(). To your point, both hlist_nulls_first_rcu()
> and hlist_nulls_next_rcu() look rather strange. I would have expected
> them to also use rcu_dereference_raw().
hlist_nulls_first_rcu() and hlist_nulls_next_rcu() were added in commit:
67bdbffd696f(“rculist: avoid __rcu annotations”)
According to the commit message, this avoids warnings from missing __rcu annotations.
>
> Thanx, Paul
