netdev - Re: Question about the barrier() in hlist_nulls_for_each_entry

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Date: Thu, 3 Aug 2023 21:40:11 +0800
From: Alan Huang <mmpgouride@...il.com>
To: "Paul E. McKenney" <paulmck@...nel.org>
Cc: Joel Fernandes <joel@...lfernandes.org>,
 Eric Dumazet <edumazet@...gle.com>,
 linux-kernel@...r.kernel.org,
 netdev@...r.kernel.org,
 rcu@...r.kernel.org,
 roman.gushchin@...ux.dev
Subject: Re: Question about the barrier() in hlist_nulls_for_each_entry_rcu()


> 2023年8月1日 上午4:09，Paul E. McKenney <paulmck@...nel.org> 写道：
> 
> On Fri, Jul 21, 2023 at 10:27:04PM +0800, Alan Huang wrote:
>> 
>>> 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.
> 
> Apologies for the late reply!
> 
> 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. :) 

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

> 
> 							Thanx, Paul
> 
>>> 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