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

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Message-ID: <6dbec9b3-b1a2-4fe7-9861-6bc879d7332c@gmail.com>
Date: Sat, 26 Apr 2025 19:03:09 +0200
From: Christian Schrefl <chrisi.schrefl@...il.com>
To: Boqun Feng <boqun.feng@...il.com>
Cc: Danilo Krummrich <dakr@...nel.org>, gregkh@...uxfoundation.org,
 rafael@...nel.org, bhelgaas@...gle.com, kwilczynski@...nel.org,
 zhiw@...dia.com, cjia@...dia.com, jhubbard@...dia.com, bskeggs@...dia.com,
 acurrid@...dia.com, joelagnelf@...dia.com, ttabi@...dia.com,
 acourbot@...dia.com, ojeda@...nel.org, alex.gaynor@...il.com,
 gary@...yguo.net, bjorn3_gh@...tonmail.com, benno.lossin@...ton.me,
 a.hindborg@...nel.org, aliceryhl@...gle.com, tmgross@...ch.edu,
 linux-pci@...r.kernel.org, rust-for-linux@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] rust: revocable: implement Revocable::access()

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@...nel.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 +    /// The caller must ensure this [`Revocable`] instance hasn't been revoked and won't be revoked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.

Cheers
Christian