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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: <cc9b292c-99b1-bec9-ba8e-9c202b5835cd@joelfernandes.org>
Date:   Fri, 21 Jul 2023 23:54:34 +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: <7D269BDF-102C-46B8-B31C-A559D2E410E0@gmail.com>
References: <E9CF24C7-3080-4720-B540-BAF03068336B@gmail.com>
 <1E0741E0-2BD9-4FA3-BA41-4E83315A10A8@joelfernandes.org>
 <1AF98387-B78C-4556-BE2E-E8F88ADACF8A@gmail.com>
 <cc9b292c-99b1-bec9-ba8e-9c202b5835cd@joelfernandes.org>
To:     Joel Fernandes <joel@joelfernandes.org>
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> 2023=E5=B9=B47=E6=9C=8821=E6=97=A5 23:21=EF=BC=8CJoel Fernandes =
<joel@joelfernandes.org> =E5=86=99=E9=81=93=EF=BC=9A
>=20
> On 7/21/23 10:27, Alan Huang wrote:
>>> 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
>>>> 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:
>>=20
>> 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
>=20
> Right, it shouldn't need to cache. To Eric's point it might be risky =
to remove the barrier() and someone needs to explain that issue first =
(or IMO there needs to be another tangible reason like performance etc). =
Anyway, FWIW I wrote a simple program and I am not seeing the =
head->first cached with the pattern you shared above:
>=20
> #include <stdlib.h>
>=20
> #define READ_ONCE(x) (*(volatile typeof(x) *)&(x))
> #define barrier() __asm__ __volatile__("": : :"memory")
>=20
> typedef struct list_head {
>    int first;
>    struct list_head *next;
> } list_head;
>=20
> int main() {
>    list_head *head =3D (list_head *)malloc(sizeof(list_head));
>    head->first =3D 1;
>    head->next =3D 0;
>=20
>    READ_ONCE(head->first);
>    barrier();

Thanks for your time!

However, what I'm trying to say here is that without this barrier(), GCC =
wouldn't cache this value either.

So, I removed the barrier() and tested, GCC didn=E2=80=99t cache the =
value of head->first.
(Only tested on x86-64 (all the possible versions of gcc that Compiler =
Explorer has) where the original issue occurred [1].)

Therefore, the commit message and the related discussion ten years ago =
is misleading.

Thanks again!

[1] https://lkml.org/lkml/2013/4/16/371


>    READ_ONCE(head->first);
>=20
>    free(head);
>    return 0;
> }
>=20
> On ARM 32-bit, 64-bit and x86_64, with -Os and then another experiment =
with -O2 on new gcc versions.