Re: [PATCH v2 04/14] rust: sync: add `Arc::clone_from_raw`

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

Andreas Hindborg <a.hindborg@kernel.org> writes:

> "Benno Lossin" <benno.lossin@proton.me> writes:
>
>> On 18.09.24 00:27, Andreas Hindborg wrote:
>>> Add a method to clone an arc from a pointer to the data managed by the
>>> `Arc`.
>>>
>>> Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
>>> ---
>>>  rust/kernel/sync/arc.rs | 20 ++++++++++++++++++++
>>>  1 file changed, 20 insertions(+)
>>>
>>> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
>>> index a57ea3e2b44c..2c95712d12a2 100644
>>> --- a/rust/kernel/sync/arc.rs
>>> +++ b/rust/kernel/sync/arc.rs
>>> @@ -282,6 +282,26 @@ pub unsafe fn from_raw(ptr: *const T) -> Self {
>>>          unsafe { Self::from_inner(ptr) }
>>>      }
>>>
>>> +    /// Clones an [`Arc`] instance from a pointer to the contained data.
>>> +    ///
>>> +    /// # Safety
>>> +    ///
>>> +    /// `ptr` must point to an allocation that is contained within a live [`Arc<T>`].
>>> +    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
>>
>
> We do not own
> a refcount on the Arc. For a short duration you will have a wrong
> refcount. If you have two Arcs and the refcount is 1, the ArcInner might
> be dropped after the first line of this suggestion, before you do clone,
> and then this is not sound.

Well, disregard that. This is why one should not reply to emails before
coffee in the morning.

Of course, a precondition for calling this function is that the arc
containing the data pointed to by `ptr` is live for the duration. So
what you wrote would work. But I still do not like having two `Arc`s in
existence with the wrong refcount. 

BR Andreas