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

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Date: Fri, 21 Jul 2023 03:59:33 +0800
From: Alan Huang <mmpgouride@...il.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: 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 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?

> 
> Do you have a patch for review ?

Possibly next month. :)

> 
> 
>> 
>> 
>> [1] https://lore.kernel.org/all/1369699930.3301.494.camel@edumazet-glaptop/
>> 
>> Thanks,
>> Alan