Re: [PATCH v13 2/6] rust: introduce module_param module

[Benno Lossin]

On Fri Jun 27, 2025 at 9:57 AM CEST, Andreas Hindborg wrote:
> Andreas Hindborg <a.hindborg@xxxxxxxxxx> writes:
>> "Benno Lossin" <lossin@xxxxxxxxxx> writes:
>>> That's good to know, then let's try to go for something simple.
>>>
>>> I don't think that we can just use a `Mutex<T>`, because we don't have a
>>> way to create it at const time... I guess we could have
>>>
>>>     impl<T> Mutex<T>
>>>         /// # Safety
>>>         ///
>>>         /// The returned value needs to be pinned and then `init` needs
>>>         /// to be called before any other methods are called on this.
>>>         pub unsafe const fn const_new() -> Self;
>>>
>>>         pub unsafe fn init(&self);
>>>     }
>>>
>>> But that seems like a bad idea, because where would we call the `init`
>>> function? That also needs to be synchronized...
>>
>> Ah, that is unfortunate. The init function will not run before this, so
>> we would need a `Once` or an atomic anyway to initialize the lock.
>>
>> I am not sure if we are allowed to sleep during this, I would have to
>> check. But then we could use a spin lock.
>>
>> We will need the locking anyway, when we want to enable sysfs write
>> access to the parameters.
>>
>>>
>>> Maybe we can just like you said use an atomic bool?
>>
>> Sigh, I will have to check how far that series has come.
>>
>
> I think I am going to build some kind of `Once` feature on top of
> Boqun's atomic series [1], so that we can initialize a lock in these
> statics. We can't use `global_lock!`, because that depends on module
> init to initialize the lock before first use.

Sounds good, though we probably don't want to name it `Once`. Since it
is something that will be populated in the future, but not by some
random accessor, but rather a specific populator.

So maybe:

    pub struct Delayed<T> {
        dummy: T,
        real: Opaque<T>,
        populated: Atomic<bool>, // or Atomic<Flag>
        writing: Atomic<bool>, // or Atomic<Flag>
    }

    impl<T> Delayed<T> {
        pub fn new(dummy: T) -> Self {
            Self {
                dummy,
                real: Opaque::uninit(),
                populated: Atomic::new(false),
                writing: Atomic::new(false),
            }
        }

        pub fn get(&self) -> &T {
            if self.populated.load(Acquire) {
                unsafe { &*self.real.get() }
            } else {
                // maybe print a warning here?
                // or maybe let the user configure this in `new()`?
                &self.dummy
            }
        }

        pub fn populate(&self, value: T) {
            if self.writing.cmpxchg(false, true, Release) {
                unsafe { *self.real.get() = value };
                self.populated.store(true, Release);
            } else {
                pr_warn!("`Delayed<{}>` written to twice!\n", core::any::type_name::<T>());
            }
        }
    }

(no idea if the orderings are correct, I always have to think way to
much about that... especially since our atomics seem to only take one
ordering in compare_exchange?)

> As far as I can tell, atomics may not land in v6.17, so this series
> will probably not be ready for merge until v6.18 at the earliest.

Yeah, sorry about that :(

> Thanks for the input, Benno!

My pleasure!

---
Cheers,
Benno