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

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Message-Id: <1E0741E0-2BD9-4FA3-BA41-4E83315A10A8@joelfernandes.org>
Date:   Fri, 21 Jul 2023 08:54:17 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Alan Huang <mmpgouride@...il.com>
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()



> 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…

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
>