Re: Question about the barrier() in hlist_nulls_for_each_entry_rcu()

[Alan Huang]

> 2023年7月21日 03:22，Eric Dumazet <edumazet@xxxxxxxxxx> 写道：
> 
> On Thu, Jul 20, 2023 at 8:54 PM Alan Huang <mmpgouride@xxxxxxxxx> 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@xxxxxxxxxxxxxx/

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