linux-kernel - Re: [RFC 2/5] rust: sync: Arc: Introduces ArcInner::count()

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Date:   Thu, 2 Feb 2023 10:14:06 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Vincenzo Palazzo <vincenzopalazzodev@...il.com>
Subject: Re: [RFC 2/5] rust: sync: Arc: Introduces ArcInner::count()

On Wed, Feb 01, 2023 at 03:22:41PM -0800, Boqun Feng wrote:
> This allows reading the current count of a refcount in an `ArcInner`.
> 
> Signed-off-by: Boqun Feng <boqun.feng@...il.com>
> ---
>  rust/helpers.c          | 6 ++++++
>  rust/kernel/sync/arc.rs | 9 +++++++++
>  2 files changed, 15 insertions(+)
> 
> diff --git a/rust/helpers.c b/rust/helpers.c
> index 09a4d93f9d62..afc5f1a39fef 100644
> --- a/rust/helpers.c
> +++ b/rust/helpers.c
> @@ -46,6 +46,12 @@ bool rust_helper_refcount_dec_and_test(refcount_t *r)
>  }
>  EXPORT_SYMBOL_GPL(rust_helper_refcount_dec_and_test);
>  
> +unsigned int rust_helper_refcount_read(refcount_t *r)
> +{
> +	return refcount_read(r);
> +}
> +EXPORT_SYMBOL_GPL(rust_helper_refcount_read);
> +
>  /*
>   * We use `bindgen`'s `--size_t-is-usize` option to bind the C `size_t` type
>   * as the Rust `usize` type, so we can use it in contexts where Rust
> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> index fc680a4a795c..fbfceaa3096e 100644
> --- a/rust/kernel/sync/arc.rs
> +++ b/rust/kernel/sync/arc.rs
> @@ -127,6 +127,15 @@ struct ArcInner<T: ?Sized> {
>      data: T,
>  }
>  
> +impl<T: ?Sized> ArcInner<T> {
> +    /// Returns the current reference count of [`ArcInner`].
> +    fn count(&self) -> u32 {
> +        // SAFETY: `self.refcount.get()` is always a valid pointer, and `refcount_read()` is a
> +        // normal atomic read (i.e. no data race) only requiring on the address is valid.
> +        unsafe { bindings::refcount_read(self.refcount.get()) }
> +    }
> +}

This is completely unsafe vs concurrency. In order to enable correct
tracing of refcount manipulations we have the __refcount_*(.oldp) API.

Admittedly, I have no idea how to sanely wire that up in this thing, but
it seems odd to me to have this 'safe' language display fundamentally
unsafe data like this.