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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: <43d29007-3c59-4497-a1e5-26f182a7f4c5@paulmck-laptop>
Date:   Thu, 3 Aug 2023 21:40:11 +0800
Cc:     Joel Fernandes <joel@joelfernandes.org>,
        Eric Dumazet <edumazet@google.com>,
        linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
        rcu@vger.kernel.org, roman.gushchin@linux.dev
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Message-Id: <784ABF9D-303F-4FC8-8AFF-A3FF319B4E7A@gmail.com>
References: <E9CF24C7-3080-4720-B540-BAF03068336B@gmail.com>
 <1E0741E0-2BD9-4FA3-BA41-4E83315A10A8@joelfernandes.org>
 <1AF98387-B78C-4556-BE2E-E8F88ADACF8A@gmail.com>
 <43d29007-3c59-4497-a1e5-26f182a7f4c5@paulmck-laptop>
To:     "Paul E. McKenney" <paulmck@kernel.org>
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> 2023=E5=B9=B48=E6=9C=881=E6=97=A5 =E4=B8=8A=E5=8D=884:09=EF=BC=8CPaul =
E. McKenney <paulmck@kernel.org> =E5=86=99=E9=81=93=EF=BC=9A
>=20
> On Fri, Jul 21, 2023 at 10:27:04PM +0800, Alan Huang wrote:
>>=20
>>> 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
>>=20
>> We already had the READ_ONCE() in rcu_deference_raw().
>>=20
>> The barrier() here makes me think we need write code like below:
>> =09
>> 	READ_ONCE(head->first);
>> 	barrier();
>> 	READ_ONCE(head->first);
>>=20
>> With READ_ONCE (or the deprecated ACCESS_ONCE),
>> I don=E2=80=99t think a compiler should cache the value of =
head->first.
>=20
> Apologies for the late reply!
>=20
> If both are READ_ONCE(), you should not need the barrier().  Unless =
there
> is some other code not shown in your example that requires it, that =
is.

And unless the compiler has a bug. :)=20

So, the barrier() in hlist_nulls_for_each_entry_rcu() is a workaround =
for a compiler bug.

>=20
> 							Thanx, Paul
>=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