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

[Andreas Hindborg <a.hindborg@kernel.org>]

"Benno Lossin" <lossin@kernel.org> writes:

> On Fri Jun 27, 2025 at 9:57 AM CEST, Andreas Hindborg wrote:
>> Andreas Hindborg <a.hindborg@kernel.org> writes:
>>> "Benno Lossin" <lossin@kernel.org> 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 :(

Actually, perhaps we could aim at merging this code without this
synchronization? The lack of synchronization is only a problem if we
support custom parsing. This patch set does not allow custom parsing
code, so it does not suffer this issue.


Best regards,
Andreas Hindborg