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SubjectRe: Question about the barrier() in hlist_nulls_for_each_entry_rcu()
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> 2023年7月21日 20:54,Joel Fernandes <joel@joelfernandes.org> 写道:
>
>
>
>> On Jul 20, 2023, at 4:00 PM, Alan Huang <mmpgouride@gmail.com> wrote:
>>
>> 
>>> 2023年7月21日 03:22,Eric Dumazet <edumazet@google.com> 写道:
>>>
>>>> On Thu, Jul 20, 2023 at 8:54 PM Alan Huang <mmpgouride@gmail.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
>>

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