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

[Alice Ryhl]

On Thu, Sep 19, 2024 at 8:19 AM Andreas Hindborg <a.hindborg@xxxxxxxxxx> wrote:
>
> Andreas Hindborg <a.hindborg@xxxxxxxxxx> writes:
>
> > "Benno Lossin" <benno.lossin@xxxxxxxxx> 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@xxxxxxxxxx>
> >>> ---
> >>>  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.

Doing it this way has been pretty standard with std for a long time,
until the Arc::increment_strong_count and similar methods were added.
I think it's fine, though I would have used ManuallyDrop instead of
mem::forget.

Of course, in this particular case, using `ArcBorrow` is even better.

Alice