Re: [PATCH v2] rcu: Add necessary WRITE_ONCE()

[Alan Huang]

> 2023年6月21日 06:26，Paul E. McKenney <paulmck@xxxxxxxxxx> 写道：
> 
> On Tue, Jun 20, 2023 at 05:13:46PM +0000, Alan Huang wrote:
>> Commit c54a2744497d("list: Add hlist_unhashed_lockless()") and
>> commit 860c8802ace1("rcu: Use WRITE_ONCE() for assignments to
>> ->pprev for hlist_nulls") added various WRITE_ONCE() to pair with
>> the READ_ONCE() in hlist_unhashed_lockless(), but there are still
>> some places where WRITE_ONCE() was not added, this commit adds that.
>> 
>> Also add WRITE_ONCE() to pair with the READ_ONCE() in hlist_empty().
>> 
>> Signed-off-by: Alan Huang <mmpgouride@xxxxxxxxx>
> 
> On hlist_nulls_add_tail_rcu(), good catch, thank you!
> 
> On the others, are there really cases where a lockless read races with
> the update?  At first glance, that sounds like a usage bug.  For example,
> as I understand it, when you use something like hlist_del(), you are
> supposed to ensure that there are no concurrent readers.  Which is the
> point of the assignment of the special value LIST_POISON2, right?

Do you mean there are cases where a lockless read races with hlist_add_head/hlist_add_before
hlist_add_behind/__hlist_del, but there is no real case where a lockless read races with the hlist_del_init/hlist_del
hlist_move_list?

There may be no real case where a lockless read races with the hlist_del_init/hlist_del
hlist_move_list. But for the sake of completeness, I added those WRITE_ONCE, after all, if there is WRITE_ONCE
in __hlist_del, why not add WRITE_ONCE in its caller, like hlist_del()?

Thanks,
Alan

> 
> Or is there some use case that I am missing?
> 
> If I am not missing something, then switching the non-RCU APIs to
> WRITE_ONCE() would be a step backwards, because it would make it harder
> for tools like KCSAN to find bugs.
> 
> Thanx, Paul
> 
>> ---
>> Changelog:
>> V1 -> V2: 
>>  Add WRITE_ONCE in hlist_del_init to pair with READ_ONCE in
>>  hlist_unhashed_lockless.
>> 
>> include/linux/list.h          | 9 +++++----
>> include/linux/list_nulls.h    | 2 +-
>> include/linux/rculist_nulls.h | 2 +-
>> 3 files changed, 7 insertions(+), 6 deletions(-)
>> 
>> diff --git a/include/linux/list.h b/include/linux/list.h
>> index ac366958ea..3a29b95bfe 100644
>> --- a/include/linux/list.h
>> +++ b/include/linux/list.h
>> @@ -912,7 +912,7 @@ static inline void hlist_del(struct hlist_node *n)
>> {
>> __hlist_del(n);
>> n->next = LIST_POISON1;
>> - n->pprev = LIST_POISON2;
>> + WRITE_ONCE(n->pprev, LIST_POISON2);
>> }
>> 
>> /**
>> @@ -925,7 +925,8 @@ static inline void hlist_del_init(struct hlist_node *n)
>> {
>> if (!hlist_unhashed(n)) {
>> __hlist_del(n);
>> - INIT_HLIST_NODE(n);
>> + n->next = NULL;
>> + WRITE_ONCE(n->pprev, NULL);
>> }
>> }
>> 
>> @@ -1026,8 +1027,8 @@ static inline void hlist_move_list(struct hlist_head *old,
>> {
>> new->first = old->first;
>> if (new->first)
>> - new->first->pprev = &new->first;
>> - old->first = NULL;
>> + WRITE_ONCE(new->first->pprev, &new->first);
>> + WRITE_ONCE(old->first, NULL);
>> }
>> 
>> #define hlist_entry(ptr, type, member) container_of(ptr,type,member)
>> diff --git a/include/linux/list_nulls.h b/include/linux/list_nulls.h
>> index fa6e8471bd..b63b0589fa 100644
>> --- a/include/linux/list_nulls.h
>> +++ b/include/linux/list_nulls.h
>> @@ -95,7 +95,7 @@ static inline void hlist_nulls_add_head(struct hlist_nulls_node *n,
>> 
>> n->next = first;
>> WRITE_ONCE(n->pprev, &h->first);
>> - h->first = n;
>> + WRITE_ONCE(h->first, n);
>> if (!is_a_nulls(first))
>> WRITE_ONCE(first->pprev, &n->next);
>> }
>> diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h
>> index ba4c00dd80..c65121655b 100644
>> --- a/include/linux/rculist_nulls.h
>> +++ b/include/linux/rculist_nulls.h
>> @@ -138,7 +138,7 @@ static inline void hlist_nulls_add_tail_rcu(struct hlist_nulls_node *n,
>> 
>> if (last) {
>> n->next = last->next;
>> - n->pprev = &last->next;
>> + WRITE_ONCE(n->pprev, &last->next);
>> rcu_assign_pointer(hlist_nulls_next_rcu(last), n);
>> } else {
>> hlist_nulls_add_head_rcu(n, h);
>> -- 
>> 2.34.1
>>