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From:   Joel Fernandes <joel@joelfernandes.org>
Mime-Version: 1.0 (1.0)
Subject: Re: Question about the barrier() in hlist_nulls_for_each_entry_rcu()
Date:   Fri, 21 Jul 2023 08:54:17 -0400
Message-Id: <1E0741E0-2BD9-4FA3-BA41-4E83315A10A8@joelfernandes.org>
References: <E9CF24C7-3080-4720-B540-BAF03068336B@gmail.com>
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
In-Reply-To: <E9CF24C7-3080-4720-B540-BAF03068336B@gmail.com>
To:     Alan Huang <mmpgouride@gmail.com>
Precedence: bulk


> 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@g=
oogle.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 it=
em 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 ca=
ched 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 y=
ears 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() di=
dn=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 u=
nusual cases where gcc=20
> decides not to reload ptr->field?

I am a bit doubtful there will be strong (any?) interest in replacing the ba=
rrier() with READ_ONCE() without any tangible reason, regardless of whether a=
 gcc issue was fixed.

But hey, if you want to float the idea=E2=80=A6

Thanks,

 - Joel

>=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-glapt=
op/
>>>=20
>>> Thanks,
>>> Alan
>=20