Date:   Sat, 24 Jun 2023 02:31:12 +0800
From:   Alan Huang <mmpgouride@...il.com>
To:     paulmck@...nel.org, will@...nel.org
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] rcu: Add necessary WRITE_ONCE()


> 2023年6月23日 下午1:17，Paul E. McKenney <paulmck@...nel.org> 写道：
> 
> On Wed, Jun 21, 2023 at 10:08:28AM +0800, Alan Huang wrote:
>> 
>>> 2023年6月21日 06:26，Paul E. McKenney <paulmck@...nel.org> 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@...il.com>
>>> 
>>> 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()?
> 
> You might well have located a larger issue.  We want to be able to use
> KCSAN to find unintended data races, but as you noted, there might
> be different requirements for RCU-protected linked lists and for
> lock-protected linked lists.  If there are, then there is probably
> existing linked-list code that is using the wrong primitive, for
> example, using (or failing to use) the one that Eric Dumazet provided.
> For example, mismatched API usage might be causing the differences in
> uses of _ONCE() primitives that you are calling out.

I noticed a thread:

https://lore.kernel.org/lkml/20200324153643.15527-2-will@kernel.org/

It seems like Will wanted to remove that hlist_unhashed_lockless()?
But I can’t find any further updates.

Will: Can you tell me what happened later?

> 
> Would you be interested in digging into this?

I’d like to.

> 
> You will of course need to be able to build and run kernels with KCSAN
> enabled, which is not hard to do given a laptop that can build a kernel
> and run a guest OS.

I’ll do that, :)

> 
> 							Thanx, Paul
> 
>> 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

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