linux-kernel - Re: [PATCH v2 2/3] rust: convert `Arc` to use `Refcount`

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Message-ID: <CAH5fLgjOfw2otu4FJ1XHhjsSx3Fj3iQA25Hh-Anz5jXaLamrAQ@mail.gmail.com>
Date: Tue, 14 Jan 2025 11:02:25 +0100
From: Alice Ryhl <aliceryhl@...gle.com>
To: Gary Guo <gary@...yguo.net>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
	Boqun Feng <boqun.feng@...il.com>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Trevor Gross <tmgross@...ch.edu>, Wedson Almeida Filho <walmeida@...rosoft.com>, 
	Valentin Obst <kernel@...entinobst.de>, Alex Mantel <alexmantel93@...lbox.org>, 
	Danilo Krummrich <dakr@...nel.org>, Will Deacon <will@...nel.org>, Peter Zijlstra <peterz@...radead.org>, 
	Mark Rutland <mark.rutland@....com>, rust-for-linux@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/3] rust: convert `Arc` to use `Refcount`

On Sat, Dec 21, 2024 at 7:31 PM Gary Guo <gary@...yguo.net> wrote:
>
> With `Refcount` type created, `Arc` can use `Refcount` instead of
> calling into FFI directly.
>
> Signed-off-by: Gary Guo <gary@...yguo.net>

[...]

> -    pub fn into_unique_or_drop(self) -> Option<Pin<UniqueArc<T>>> {
> +    pub fn into_unique_or_drop(this: Self) -> Option<Pin<UniqueArc<T>>> {
>          // We will manually manage the refcount in this method, so we disable the destructor.
> -        let me = ManuallyDrop::new(self);
> +        let this = ManuallyDrop::new(this);
>          // SAFETY: We own a refcount, so the pointer is still valid.
> -        let refcount = unsafe { me.ptr.as_ref() }.refcount.get();
> +        let refcount = unsafe { &this.ptr.as_ref().refcount };
>
> -        // If the refcount reaches a non-zero value, then we have destroyed this `Arc` and will
> -        // return without further touching the `Arc`. If the refcount reaches zero, then there are
> -        // no other arcs, and we can create a `UniqueArc`.
> -        //
> -        // SAFETY: We own a refcount, so the pointer is not dangling.
> -        let is_zero = unsafe { bindings::refcount_dec_and_test(refcount) };
> +        if !refcount.dec_not_one() {

This is wrong. The into_unique_or_drop function must establish an
acqrel ordering when a UniqueArc is created, but dec_not_one() does
not do so. You need to use refcount_dec_and_test() instead.

Alice