linux-kernel - Re: Question about the barrier() in hlist_nulls_for_each_entry

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Date:   Fri, 21 Jul 2023 22:27:04 +0800
From:   Alan Huang <mmpgouride@...il.com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Eric Dumazet <edumazet@...gle.com>, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, rcu@...r.kernel.org,
        "Paul E. McKenney" <paulmck@...nel.org>, roman.gushchin@...ux.dev
Subject: Re: Question about the barrier() in hlist_nulls_for_each_entry_rcu()


> 2023年7月21日 20:54，Joel Fernandes <joel@...lfernandes.org> 写道：
> 
> 
> 
>> On Jul 20, 2023, at 4:00 PM, Alan Huang <mmpgouride@...il.com> wrote:
>> 
>> 
>>> 2023年7月21日 03:22，Eric Dumazet <edumazet@...gle.com> 写道：
>>> 
>>>> On Thu, Jul 20, 2023 at 8:54 PM Alan Huang <mmpgouride@...il.com> wrote:
>>>> 
>>>> Hi,
>>>> 
>>>> I noticed a commit c87a124a5d5e(“net: force a reload of first item in hlist_nulls_for_each_entry_rcu”)
>>>> and a related discussion [1].
>>>> 
>>>> 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:
>>>> 
>>>>      Is that a compiler bug? If so, has this bug been fixed today, ten years later?
>>>> 
>>>>      What about READ_ONCE(ptr->field)?
>>> 
>>> Make sure sparse is happy.
>> 
>> It caused a problem without barrier(), and the deprecated ACCESS_ONCE() didn’t 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 
>> decides not to reload ptr->field?
> 
> 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.
> 
> But hey, if you want to float the idea…

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

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

With READ_ONCE (or the deprecated ACCESS_ONCE),
I don’t think a compiler should cache the value of head->first.

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