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Subject: Re: Question about the barrier() in hlist_nulls_for_each_entry_rcu()
From:   Alan Huang <mmpgouride@gmail.com>
In-Reply-To: <1E0741E0-2BD9-4FA3-BA41-4E83315A10A8@joelfernandes.org>
Date:   Fri, 21 Jul 2023 22:27:04 +0800
Cc:     Eric Dumazet <edumazet@google.com>, linux-kernel@vger.kernel.org,
        netdev@vger.kernel.org, rcu@vger.kernel.org,
        "Paul E. McKenney" <paulmck@kernel.org>, roman.gushchin@linux.dev
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Message-Id: <1AF98387-B78C-4556-BE2E-E8F88ADACF8A@gmail.com>
References: <E9CF24C7-3080-4720-B540-BAF03068336B@gmail.com>
 <1E0741E0-2BD9-4FA3-BA41-4E83315A10A8@joelfernandes.org>
To:     Joel Fernandes <joel@joelfernandes.org>
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> 2023=E5=B9=B47=E6=9C=8821=E6=97=A5 20:54=EF=BC=8CJoel Fernandes =
<joel@joelfernandes.org> =E5=86=99=E9=81=93=EF=BC=9A
>=20
>=20
>=20
>> On Jul 20, 2023, at 4:00 PM, Alan Huang <mmpgouride@gmail.com> wrote:
>>=20
>> =EF=BB=BF
>>> 2023=E5=B9=B47=E6=9C=8821=E6=97=A5 03:22=EF=BC=8CEric Dumazet =
<edumazet@google.com> =E5=86=99=E9=81=93=EF=BC=9A
>>>=20
>>>> On Thu, Jul 20, 2023 at 8:54=E2=80=AFPM Alan Huang =
<mmpgouride@gmail.com> wrote:
>>>>=20
>>>> Hi,
>>>>=20
>>>> I noticed a commit c87a124a5d5e(=E2=80=9Cnet: force a reload of =
first item in hlist_nulls_for_each_entry_rcu=E2=80=9D)
>>>> and a related discussion [1].
>>>>=20
>>>> After reading the whole discussion, it seems like that ptr->field =
was cached by gcc even with the deprecated
>>>> ACCESS_ONCE(), so my question is:
>>>>=20
>>>>      Is that a compiler bug? If so, has this bug been fixed today, =
ten years later?
>>>>=20
>>>>      What about READ_ONCE(ptr->field)?
>>>=20
>>> Make sure sparse is happy.
>>=20
>> It caused a problem without barrier(), and the deprecated =
ACCESS_ONCE() didn=E2=80=99t help:
>>=20
>>   https://lore.kernel.org/all/519D19DA.50400@yandex-team.ru/
>>=20
>> So, my real question is: With READ_ONCE(ptr->field), are there still =
some unusual cases where gcc=20
>> decides not to reload ptr->field?
>=20
> I am a bit doubtful there will be strong (any?) interest in replacing =
the barrier() with READ_ONCE() without any tangible reason, regardless =
of whether a gcc issue was fixed.
>=20
> But hey, if you want to float the idea=E2=80=A6

We already had the READ_ONCE() in rcu_deference_raw().

The barrier() here makes me think we need write code like below:
=09
	READ_ONCE(head->first);
	barrier();
	READ_ONCE(head->first);

With READ_ONCE (or the deprecated ACCESS_ONCE),
I don=E2=80=99t think a compiler should cache the value of head->first.

>=20
> Thanks,
>=20
> - Joel
>=20
>>=20
>>>=20
>>> Do you have a patch for review ?
>>=20
>> Possibly next month. :)
>>=20
>>>=20
>>>=20
>>>>=20
>>>>=20
>>>> [1] =
https://lore.kernel.org/all/1369699930.3301.494.camel@edumazet-glaptop/
>>>>=20
>>>> Thanks,
>>>> Alan
>>=20