From:   Martin Rodriguez Reboredo <yakoyoku@gmail.com>
To:     lina@asahilina.net
Cc:     alex.gaynor@gmail.com, asahi@lists.linux.dev,
        bjorn3_gh@protonmail.com, boqun.feng@gmail.com, gary@garyguo.net,
        linux-kernel@vger.kernel.org, ojeda@kernel.org,
        rust-for-linux@vger.kernel.org, wedsonaf@gmail.com
Subject: Re: [PATCH 1/2] rust: sync: arc: implement Arc<dyn Any + Send + Sync>::downcast()
Date:   Fri, 24 Feb 2023 11:34:29 -0300
Message-Id: <20230224143429.754374-1-yakoyoku@gmail.com>
In-Reply-To: <20230224-rust-arc-v1-1-568eea613a41@asahilina.net>
References: <20230224-rust-arc-v1-1-568eea613a41@asahilina.net>
MIME-Version: 1.0
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On Fri, Feb 24, 2023 at 05:09:47PM +0900, Asahi Lina wrote:=0D
> This mirrors the standard library's alloc::sync::Arc::downcast().=0D
> =0D
> Based on the Rust standard library implementation, ver 1.62.0,=0D
> licensed under "Apache-2.0 OR MIT", from:=0D
> =0D
>     https://github.com/rust-lang/rust/tree/1.62.0/library/alloc/src=0D
> =0D
> For copyright details, please see:=0D
> =0D
>     https://github.com/rust-lang/rust/blob/1.62.0/COPYRIGHT=0D
> =0D
> Signed-off-by: Asahi Lina <lina@asahilina.net>=0D
> ---=0D
>  rust/kernel/sync/arc.rs | 22 ++++++++++++++++++++++=0D
>  1 file changed, 22 insertions(+)=0D
> =0D
> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs=0D
> index a94e303217c6..752bd7c4699e 100644=0D
> --- a/rust/kernel/sync/arc.rs=0D
> +++ b/rust/kernel/sync/arc.rs=0D
> @@ -22,6 +22,7 @@ use crate::{=0D
>  };=0D
>  use alloc::boxed::Box;=0D
>  use core::{=0D
> +    any::Any,=0D
>      fmt,=0D
>      marker::{PhantomData, Unsize},=0D
>      mem::{ManuallyDrop, MaybeUninit},=0D
> @@ -220,6 +221,27 @@ impl<T: 'static> ForeignOwnable for Arc<T> {=0D
>      }=0D
>  }=0D
>  =0D
> +impl Arc<dyn Any + Send + Sync> {=0D
> +    /// Attempt to downcast the `Arc<dyn Any + Send + Sync>` to a concre=
te type.=0D
> +    // Based on the Rust standard library implementation, ver 1.62.0, wh=
ich is=0D
> +    // Apache-2.0 OR MIT.=0D
> +    pub fn downcast<T>(self) -> core::result::Result<Arc<T>, Self>=0D
> +    where=0D
> +        T: Any + Send + Sync,=0D
> +    {=0D
> +        if (*self).is::<T>() {=0D
> +            // SAFETY: We have just checked that the type is correct, so=
 we can cast the pointer.=0D
> +            unsafe {=0D
> +                let ptr =3D self.ptr.cast::<ArcInner<T>>();=0D
> +                core::mem::forget(self);=0D
=0D
I see, we cast the inner pointer of the `Arc` to the target type which=0D
is wrapped in an `ArcInner` that is then used for another `Arc`. This is=0D
important as this new value now owns the pointer and, by consequence,=0D
it is safe to forget the passed `Arc` as it won't leak the pointer.=0D
Thus, this method is safe.=0D
=0D
> +                Ok(Arc::from_inner(ptr))=0D
> +            }=0D
> +        } else {=0D
> +            Err(self)=0D
> +        }=0D
> +    }=0D
> +}=0D
> +=0D
>  impl<T: ?Sized> Deref for Arc<T> {=0D
>      type Target =3D T;=0D
=0D
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>=0D