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

Jonas Oberhauser
2024-09-30T08:58:15Z

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：
>>
>> 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.

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