Re: [PATCH 1/3] rust: revocable: implement Revocable::access()

[Benno Lossin <benno.lossin@proton.me>]

On Sat Apr 26, 2025 at 7:03 PM CEST, Christian Schrefl wrote:
> On 26.04.25 6:54 PM, Boqun Feng wrote:
>> On Sat, Apr 26, 2025 at 06:44:03PM +0200, Christian Schrefl wrote:
>>> On 26.04.25 3:30 PM, Danilo Krummrich wrote:
>>>> Implement an unsafe direct accessor for the data stored within the
>>>> Revocable.
>>>>
>>>> This is useful for cases where we can proof that the data stored within
>>>> the Revocable is not and cannot be revoked for the duration of the
>>>> lifetime of the returned reference.
>>>>
>>>> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
>>>> ---
>>>> The explicit lifetimes in access() probably don't serve a practical
>>>> purpose, but I found them to be useful for documentation purposes.
>>>> --->  rust/kernel/revocable.rs | 12 ++++++++++++
>>>>  1 file changed, 12 insertions(+)
>>>>
>>>> diff --git a/rust/kernel/revocable.rs b/rust/kernel/revocable.rs
>>>> index 971d0dc38d83..33535de141ce 100644
>>>> --- a/rust/kernel/revocable.rs
>>>> +++ b/rust/kernel/revocable.rs
>>>> @@ -139,6 +139,18 @@ pub fn try_access_with<R, F: FnOnce(&T) -> R>(&self, f: F) -> Option<R> {
>>>>          self.try_access().map(|t| f(&*t))
>>>>      }
>>>>  
>>>> +    /// Directly access the revocable wrapped object.
>>>> +    ///
>>>> +    /// # Safety
>>>> +    ///
>>>> +    /// The caller must ensure this [`Revocable`] instance hasn't been revoked and won't be revoked
>>>> +    /// for the duration of `'a`.
>>>> +    pub unsafe fn access<'a, 's: 'a>(&'s self) -> &'a T {
>>> I'm not sure if the `'s` lifetime really carries much meaning here.
>>> I find just (explicit) `'a` on both parameter and return value is clearer to me,
>>> but I'm not sure what others (particularly those not very familiar with rust)
>>> think of this.
>> 
>> Yeah, I don't think we need two lifetimes here, the following version
>> should be fine (with implicit lifetime):
>> 
>> 	pub unsafe fn access(&self) -> &T { ... }
>> 
>> , because if you do:
>> 
>> 	let revocable: &'1 Revocable = ...;
>> 	...
>> 	let t: &'2 T = unsafe { revocable.access() };
>> 
>> '1 should already outlive '2 (i.e. '1: '2).
>
> I understand that implicit lifetimes desugars to 
> effectively the same code, I just think that keeping
> a explicit 'a makes it a bit more obvious that the
> lifetimes need to be considered here.
>
> But I'm also fine with just implicit lifetimes here.

We elide lifetimes all over the place especially for methods taking
`&self` and returning `&T`. I don't think that it serves a purpose here.

---
Cheers,
Benno