Re: [PATCH 1/2] compiler.h: Introduce ptr_eq() to preserve address dependency

Alan Huang
2024-09-30T10:12:55Z

On Sep 30, 2024, at 17:33, Jonas Oberhauser <jonas.oberhauser@huaweicloud.com> wrote:
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> Am 9/30/2024 um 11:27 AM schrieb Alan Huang:
>> 2024年9月30日 17:15，Alan Huang <mmpgouride@gmail.com> 写道：
>>>
>>> 2024年9月30日 16:57，Jonas Oberhauser <jonas.oberhauser@huaweicloud.com> 写道：
>>>>
>>>>
>>>>
>>>> Am 9/29/2024 um 12:26 AM schrieb Alan Huang:
>>>>> 2024年9月28日 23:55，Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote：
>>>>>>
>>>>>> The motivation for introducing ptr_eq() is indeed because the
>>>>>> compiler barrier is not sufficient to prevent the compiler from
>>>>>> using one pointer instead of the other.
>>>>> barrier_data(&b) prevents that.
>>>>
>>>> I don't think one barrier_data can garantuee preventing this, because right after doing the comparison, the compiler still could do b=a.
>>>>
>>>> In that case you would be guaranteed to use the value in b, but that value is not the value loaded into b originally but rather the value loaded into a, and hence your address dependency goes to the wrong load still.
>>>
>>> After barrier_data(&b), *b will be loaded from memory, you mean even if *b is loaded from memory, the address dependency goes to the wrong load still?
>> Sorry, *b should b.
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> That's exactly what I meant to say. In my understanding, it can happen like this:
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> a = READ_ONCE(*p);
> ...
> b = READ_ONCE(*p);
> if (a == b) {
> b = a; // inserted by compiler
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> barrier_data(&b);
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> foo(*b); // compiler definitely use the current value in b
> }
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> In the end, the address dependency is from the first load, and the CPU can speculatively (with register renaming, forwarding etc) execute
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> a = READ_ONCE(*p);
> b2 = a; // speculatively
> tmp = load *b2 // speculatively
> b1 = READ_ONCE(*p);
> if (a == b1) { // confirmed
> foo(tmp);
> }

I get it now, thanks for the explanation.

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> best wishes,
> jonas
>