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

[Alan Huang]

2024年9月30日 16:57，Jonas Oberhauser <jonas.oberhauser@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxx> wrote：
>>> 
>>> On 2024-09-28 17:49, Alan Stern wrote:
>>>> On Sat, Sep 28, 2024 at 11:32:18AM -0400, Mathieu Desnoyers wrote:
>>>>> On 2024-09-28 16:49, Alan Stern wrote:
>>>>>> On Sat, Sep 28, 2024 at 09:51:27AM -0400, Mathieu Desnoyers wrote:
>>>>>>> equality, which does not preserve address dependencies and allows the
>>>>>>> following misordering speculations:
>>>>>>> 
>>>>>>> - If @b is a constant, the compiler can issue the loads which depend
>>>>>>>    on @a before loading @a.
>>>>>>> - If @b is a register populated by a prior load, weakly-ordered
>>>>>>>    CPUs can speculate loads which depend on @a before loading @a.
>>>>>> 
>>>>>> It shouldn't matter whether @a and @b are constants, registers, or
>>>>>> anything else.  All that matters is that the compiler uses the wrong
>>>>>> one, which allows weakly ordered CPUs to speculate loads you wouldn't
>>>>>> expect it to, based on the source code alone.
>>>>> 
>>>>> I only partially agree here.
>>>>> 
>>>>> On weakly-ordered architectures, indeed we don't care whether the
>>>>> issue is caused by the compiler reordering the code (constant)
>>>>> or the CPU speculating the load (registers).
>>>>> 
>>>>> However, on strongly-ordered architectures, AFAIU, only the constant
>>>>> case is problematic (compiler reordering the dependent load), because
>>>> I thought you were trying to prevent the compiler from using one pointer
>>>> instead of the other, not trying to prevent it from reordering anything.
>>>> Isn't this the point the documentation wants to get across when it says
>>>> that comparing pointers can be dangerous?
>>> 
>>> 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?

> 
> However, doing
> 
> barrier_data(&b);
> if (a == b) {
>   barrier();
>   foo(*b);
> }
> 
> might maybe prevent it, because after the address of b is escaped, the compiler might no longer be allowed to just do b=a;, but I'm not sure if that is completely correct, since the compiler knows b==a and no other thread can be concurrently modifying a or b. Therefore, given that the compiler knows the hardware, it might know that assigning b=a would not cause any  race-related issues even if another thread was reading b concurrently.
> 
> Finally, it may be only a combination of barrier_data and making b volatile could be guaranteed to solve the issue, but the code will be very obscure compared to using ptr_eq.
> 
>  jonas