On Fri, Jul 21, 2023 at 10:27:04PM +0800, Alan Huang wrote:

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2023年7月21日 20:54，Joel Fernandes [off-list ref] 写道：

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On Jul 20, 2023, at 4:00 PM, Alan Huang [off-list ref] wrote:



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2023年7月21日 03:22，Eric Dumazet [off-list ref] 写道：

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On Thu, Jul 20, 2023 at 8:54 PM Alan Huang [off-list ref] 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/ (local)

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.

							Thanx, Paul

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Thanks,

- Joel

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Do you have a patch for review ?

Possibly next month. :)

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[1] https://lore.kernel.org/all/1369699930.3301.494.camel@edumazet-glaptop/ (local)

Thanks,
Alan