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Message-ID: <20240918211211.73b1324e.gary@garyguo.net>
Date: Wed, 18 Sep 2024 21:12:11 +0100
From: Gary Guo <gary@...yguo.net>
To: Benno Lossin <benno.lossin@...ton.me>
Cc: Andreas Hindborg <a.hindborg@...nel.org>, 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 Wed, 18 Sep 2024 18:19:20 +0000
Benno Lossin <benno.lossin@...ton.me> wrote:
> 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@...nel.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
>
> (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...
Best,
Gary
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