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

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Message-ID: <87zfo4i3nc.fsf@kernel.org>
Date: Thu, 19 Sep 2024 08:00:39 +0200
From: Andreas Hindborg <a.hindborg@...nel.org>
To: "Gary Guo" <gary@...yguo.net>
Cc: "Benno Lossin" <benno.lossin@...ton.me>,  "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`

"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?

Best regards,
Andreas