linux-kernel - Re: [RFC 3/5] rust: sync: Arc: Introduces Arc::get

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Date:   Sat, 04 Feb 2023 19:51:50 +0100
From:   "Vincenzo Palazzo" <vincenzopalazzodev@...il.com>
To:     "Boqun Feng" <boqun.feng@...il.com>,
        <rust-for-linux@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Cc:     "Will Deacon" <will@...nel.org>,
        "Peter Zijlstra" <peterz@...radead.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>
Subject: Re: [RFC 3/5] rust: sync: Arc: Introduces Arc::get_inner() helper

On Thu Feb 2, 2023 at 12:22 AM CET, Boqun Feng wrote:
> Getting a `&ArcInner<T>` should always be safe from a `Arc<T>`,
> therefore add this helper function to avoid duplicate unsafe
> `ptr.as_ref()`.
>
> Signed-off-by: Boqun Feng <boqun.feng@...il.com>
> ---

Reviwed-by: Vincenzo Palazzo <vincenzopalazzodev@...il.com>

>  rust/kernel/sync/arc.rs | 19 ++++++++++---------
>  1 file changed, 10 insertions(+), 9 deletions(-)
>
> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> index fbfceaa3096e..4d8de20c996f 100644
> --- a/rust/kernel/sync/arc.rs
> +++ b/rust/kernel/sync/arc.rs
> @@ -201,6 +201,13 @@ impl<T: ?Sized> Arc<T> {
>          // reference can be created.
>          unsafe { ArcBorrow::new(self.ptr) }
>      }
> +
> +    /// Returns a reference to the internal [`ArcInner`].
> +    fn get_inner(&self) -> &ArcInner<T> {
> +        // SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
> +        // safe to dereference it.
> +        unsafe { self.ptr.as_ref() }
> +    }
>  }
>  
>  impl<T: 'static> ForeignOwnable for Arc<T> {
> @@ -233,9 +240,7 @@ impl<T: ?Sized> Deref for Arc<T> {
>      type Target = T;
>  
>      fn deref(&self) -> &Self::Target {
> -        // SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
> -        // safe to dereference it.
> -        unsafe { &self.ptr.as_ref().data }
> +        &self.get_inner().data
>      }
>  }
>  
> @@ -244,7 +249,7 @@ impl<T: ?Sized> Clone for Arc<T> {
>          // INVARIANT: C `refcount_inc` saturates the refcount, so it cannot overflow to zero.
>          // SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
>          // safe to increment the refcount.
> -        unsafe { bindings::refcount_inc(self.ptr.as_ref().refcount.get()) };
> +        unsafe { bindings::refcount_inc(self.get_inner().refcount.get()) };
>  
>          // SAFETY: We just incremented the refcount. This increment is now owned by the new `Arc`.
>          unsafe { Self::from_inner(self.ptr) }
> @@ -253,11 +258,7 @@ impl<T: ?Sized> Clone for Arc<T> {
>  
>  impl<T: ?Sized> Drop for Arc<T> {
>      fn drop(&mut self) {
> -        // SAFETY: By the type invariant, there is necessarily a reference to the object. We cannot
> -        // touch `refcount` after it's decremented to a non-zero value because another thread/CPU
> -        // may concurrently decrement it to zero and free it. It is ok to have a raw pointer to
> -        // freed/invalid memory as long as it is never dereferenced.
> -        let refcount = unsafe { self.ptr.as_ref() }.refcount.get();
> +        let refcount = self.get_inner().refcount.get();
>  
>          // INVARIANT: If the refcount reaches zero, there are no other instances of `Arc`, and
>          // this instance is being dropped, so the broken invariant is not observable.
> -- 
> 2.39.1