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

Mathieu Desnoyers
2024-09-28T15:57:26Z

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.

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.