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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: <CANn89iKJWw7zUP-E_d=Yhaz=Qw0R3Ae7ULaGgrtsi1yf2pfpGg@mail.gmail.com>
Date:   Fri, 21 Jul 2023 03:59:33 +0800
Cc:     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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 <CANn89iKJWw7zUP-E_d=Yhaz=Qw0R3Ae7ULaGgrtsi1yf2pfpGg@mail.gmail.com>
To:     Eric Dumazet <edumazet@google.com>
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> 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.

It caused a problem without barrier(), and the deprecated ACCESS_ONCE() =
didn=E2=80=99t help:

	https://lore.kernel.org/all/519D19DA.50400@yandex-team.ru/

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
> Do you have a patch for review ?

Possibly next month. :)

>=20
>=20
>>=20
>>=20
>> [1] =
https://lore.kernel.org/all/1369699930.3301.494.camel@edumazet-glaptop/
>>=20
>> Thanks,
>> Alan