linux-kernel - Re: [PATCH v2 04/14] rust: sync: add `Arc::clone_from

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Message-ID: <19760254-bc0b-49e2-88a0-7088ab55d9b1@proton.me>
Date: Thu, 19 Sep 2024 14:15:02 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Andreas Hindborg <a.hindborg@...nel.org>, Gary Guo <gary@...yguo.net>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, Anna-Maria Behnsen <anna-maria@...utronix.de>, Frederic Weisbecker <frederic@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, Boqun Feng <boqun.feng@...il.com>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, Alice Ryhl <aliceryhl@...gle.com>, rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 04/14] rust: sync: add `Arc::clone_from_raw`

On 19.09.24 08:00, Andreas Hindborg wrote:
> "Gary Guo" <gary@...yguo.net> writes:
> 
>> On Wed, 18 Sep 2024 18:19:20 +0000
>> Benno Lossin <benno.lossin@...ton.me> wrote:
>>
>>> On 18.09.24 00:27, Andreas Hindborg wrote:
> [...]
>>>> +    pub unsafe fn clone_from_raw(ptr: *const T) -> Self {
>>>> +        // SAFETY: The caller promises that this pointer points to data
>>>> +        // contained in an `Arc` that is still valid.
>>>> +        let inner = unsafe { ArcInner::container_of(ptr).as_ref() };
>>>> +
>>>> +        // INVARIANT: C `refcount_inc` saturates the refcount, so it cannot
>>>> +        // overflow to zero. SAFETY: By the function safety requirement, there
>>>> +        // is necessarily a reference to the object, so it is safe to increment
>>>> +        // the refcount.
>>>> +        unsafe { bindings::refcount_inc(inner.refcount.get()) };
>>>> +
>>>> +        // SAFETY: We just incremented the refcount. This increment is now owned by the new `Arc`.
>>>> +        unsafe { Self::from_inner(inner.into()) }
>>>
>>> The implementation of this function looks a bit strange to me, how about
>>> this?:
>>>
>>>     // SAFETY: this function has the same safety requirements as `from_raw`.
>>>     let arc = unsafe { Self::from_raw(ptr) };
>>>     let clone = arc.clone();
>>>     // Prevent decrementing the refcount.
>>>     mem::forget(arc);
>>>     clone
>>>
>>> (of course you would need to change the safety requirements of
>>> `clone_from_raw` to point to `from_raw`)
>>
>> Wouldn't this function simply be
>>
>> 	// SAFETY: ...
>> 	let borrow = unsafe { ArcBorrow::from_raw(ptr) }
>>   	borrow.into()
>>
>> ?
>>
>> Maybe this function doesn't even need to exist...
> 
> Maybe that could work. But my use case does not satisfy the safety
> requirements on `ArcBorrow::from_raw`. The`Arc::into_raw` was not
> called. Perhaps we can update the requirements for that function?

If I understood the code correctly, you are essentially doing this:

    let arc = Arc::<T>::new(..);
    let ptr = Arc::as_ptr(&arc);
    let ptr = T::raw_get_timer(ptr);
    let ptr = Timer::raw_get(ptr);

    // ptr is now used by the timer subsystem to fire the timer

    let ptr = ptr.cast::<Timer>();
    let ptr = T::timer_container_of(ptr);
    let borrow = ArcBorrow::from_raw(ptr);
    let arc = borrow.into();

The only thing that we would have to change would be adding
`Arc::as_ptr` as a source in the `ArcBorrow::from_raw` safety
requirements.

---
Cheers,
Benno