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

Alan Huang
2024-09-28T22:27:05Z

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.

>
> But it turns out that ptr_eq() is also a good tool to prevent the
> compiler from reordering loads in case where the comparison is
> done against a constant.
>
>>> CPU speculating the loads across the control dependency is not an
>>> issue.
>>>
>>> So am I tempted to keep examples that clearly state whether
>>> the issue is caused by compiler reordering instructions, or by
>>> CPU speculation.
>> Isn't it true that on strongly ordered CPUs, a compiler barrier is
>> sufficient to prevent the rcu_dereference() problem? So the whole idea
>> behind ptr_eq() is that it prevents the problem on all CPUs.
>
> Correct. But given that we have ptr_eq(), it's good to show how it
> equally prevents the compiler from reordering address-dependent loads
> (comparison with constant) *and* prevents the compiler from using
> one pointer rather than the other (comparison between two non-constant
> pointers) which affects speculation on weakly-ordered CPUs.
>
>> You can make your examples as specific as you like, but the fact remains
>> that ptr_eq() is meant to prevent situations where both:
>> The compiler uses the wrong pointer for a load, and
>> The CPU performs the load earlier than you want.
>> If either one of those doesn't hold then the problem won't arise.
>
> Correct.
>
> Thanks,
>
> Mathieu
>
>
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> https://www.efficios.com
>
>