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Subject: Re: [PATCH v2] rcu: Add necessary WRITE_ONCE()
From:   Alan Huang <mmpgouride@gmail.com>
In-Reply-To: <50c4aa37-388b-449c-8184-00a9d69471fc@paulmck-laptop>
Date:   Wed, 21 Jun 2023 10:08:28 +0800
Cc:     rcu@vger.kernel.org, linux-kernel@vger.kernel.org
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Message-Id: <B9A94CB4-DB9B-43D2-9D34-ADB4F7EA737D@gmail.com>
References: <20230620171346.207076-1-mmpgouride@gmail.com>
 <50c4aa37-388b-449c-8184-00a9d69471fc@paulmck-laptop>
To:     paulmck@kernel.org
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> 2023=E5=B9=B46=E6=9C=8821=E6=97=A5 06:26=EF=BC=8CPaul E. McKenney =
<paulmck@kernel.org> =E5=86=99=E9=81=93=EF=BC=9A
>=20
> 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.
>>=20
>> Also add WRITE_ONCE() to pair with the READ_ONCE() in hlist_empty().
>>=20
>> Signed-off-by: Alan Huang <mmpgouride@gmail.com>
>=20
> On hlist_nulls_add_tail_rcu(), good catch, thank you!
>=20
> 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

>=20
> Or is there some use case that I am missing?
>=20
> 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.
>=20
> Thanx, Paul
>=20
>> ---
>> Changelog:
>> V1 -> V2:=20
>>  Add WRITE_ONCE in hlist_del_init to pair with READ_ONCE in
>>  hlist_unhashed_lockless.
>>=20
>> include/linux/list.h          | 9 +++++----
>> include/linux/list_nulls.h    | 2 +-
>> include/linux/rculist_nulls.h | 2 +-
>> 3 files changed, 7 insertions(+), 6 deletions(-)
>>=20
>> 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 =3D LIST_POISON1;
>> - n->pprev =3D LIST_POISON2;
>> + WRITE_ONCE(n->pprev, LIST_POISON2);
>> }
>>=20
>> /**
>> @@ -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 =3D NULL;
>> + WRITE_ONCE(n->pprev, NULL);
>> }
>> }
>>=20
>> @@ -1026,8 +1027,8 @@ static inline void hlist_move_list(struct =
hlist_head *old,
>> {
>> new->first =3D old->first;
>> if (new->first)
>> - new->first->pprev =3D &new->first;
>> - old->first =3D NULL;
>> + WRITE_ONCE(new->first->pprev, &new->first);
>> + WRITE_ONCE(old->first, NULL);
>> }
>>=20
>> #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,
>>=20
>> n->next =3D first;
>> WRITE_ONCE(n->pprev, &h->first);
>> - h->first =3D 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,
>>=20
>> if (last) {
>> n->next =3D last->next;
>> - n->pprev =3D &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);
>> --=20
>> 2.34.1
>>=20