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Subject: Re: [PATCH rcu 6/6] rcu: Use WRITE_ONCE() for assignments to ->next
 for rculist_nulls
From:   Alan Huang <mmpgouride@gmail.com>
In-Reply-To: <9651c3b4-0791-5cd7-567f-f65d28bc8fae@joelfernandes.org>
Date:   Tue, 18 Jul 2023 22:48:07 +0800
Cc:     "Paul E. McKenney" <paulmck@kernel.org>, rcu@vger.kernel.org,
        linux-kernel@vger.kernel.org, kernel-team@meta.com,
        rostedt@goodmis.org
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Message-Id: <BE64BF3F-69AD-4522-965E-0B7E4054D737@gmail.com>
References: <6127192c-da9b-4599-9738-6e8f92e6c75c@paulmck-laptop>
 <20230717180317.1097590-6-paulmck@kernel.org>
 <9651c3b4-0791-5cd7-567f-f65d28bc8fae@joelfernandes.org>
To:     Joel Fernandes <joel@joelfernandes.org>
Precedence: bulk


> 2023=E5=B9=B47=E6=9C=8818=E6=97=A5 21:49=EF=BC=8CJoel Fernandes =
<joel@joelfernandes.org> =E5=86=99=E9=81=93=EF=BC=9A
>=20
> On 7/17/23 14:03, Paul E. McKenney wrote:
>> From: Alan Huang <mmpgouride@gmail.com>
>> When the objects managed by rculist_nulls are allocated with
>> SLAB_TYPESAFE_BY_RCU, old readers may still hold references to an =
object
>> even though it is just now being added, which means the modification =
of
>> ->next is visible to readers.  This patch therefore uses WRITE_ONCE()
>> for assignments to ->next.
>> Signed-off-by: Alan Huang <mmpgouride@gmail.com>
>> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
>=20
> Did we ever conclude that the READ_ONCE() counterparts were not =
needed? ;-)

Read-side is already protected by rcu_dereference_raw() in =
hlist_nulls_for_each_entry_{rcu, safe}.

>=20
> But incremental progress and all, so this LGTM:
> Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
>=20
> thanks,
>=20
> - Joel
>=20
>=20
>> ---
>>  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 =3D h->first;
>>  - n->next =3D 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 =3D i;
>>     if (last) {
>> - n->next =3D last->next;
>> + WRITE_ONCE(n->next, last->next);
>>   n->pprev =3D &last->next;
>>   rcu_assign_pointer(hlist_nulls_next_rcu(last), n);
>>   } else {
>=20