linux-kernel - Re: [PATCH 5/5] rust: add improved version of `ForeignOwnable::borrow

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Message-ID: <87ttcsodv0.fsf@kernel.org>
Date: Thu, 31 Oct 2024 09:50:27 +0100
From: Andreas Hindborg <a.hindborg@...nel.org>
To: "Tamir Duberstein" <tamird@...il.com>
Cc: "Miguel Ojeda" <ojeda@...nel.org>,  "Alex Gaynor"
 <alex.gaynor@...il.com>,  "Boqun Feng" <boqun.feng@...il.com>,  "Gary Guo"
 <gary@...yguo.net>,  Björn Roy Baron
 <bjorn3_gh@...tonmail.com>,  "Benno
 Lossin" <benno.lossin@...ton.me>,  "Alice Ryhl" <aliceryhl@...gle.com>,
  "Trevor Gross" <tmgross@...ch.edu>,  "Danilo Krummrich"
 <dakr@...nel.org>,  <rust-for-linux@...r.kernel.org>,
  <linux-kernel@...r.kernel.org>,  "Martin Rodriguez Reboredo"
 <yakoyoku@...il.com>
Subject: Re: [PATCH 5/5] rust: add improved version of
 `ForeignOwnable::borrow_mut`

"Tamir Duberstein" <tamird@...il.com> writes:

> From: Alice Ryhl <aliceryhl@...gle.com>
>
> Previously, the `ForeignOwnable` trait had a method called `borrow_mut`
> that was intended to provide mutable access to the inner value. However,
> the method accidentally made it possible to change the address of the
> object being modified, which usually isn't what we want. (And when we
> want that, it can be done by calling `from_foreign` and `into_foreign`,
> like how the old `borrow_mut` was implemented.)
>
> In this patch, we introduce an alternate definition of `borrow_mut` that
> solves the previous problem. Conceptually, given a pointer type `P` that
> implements `ForeignOwnable`, the `borrow_mut` method gives you the same
> kind of access as an `&mut P` would, except that it does not let you
> change the pointer `P` itself.
>
> This is analogous to how the existing `borrow` method provides the same
> kind of access to the inner value as an `&P`.
>
> Note that for types like `Arc`, having an `&mut Arc<T>` only gives you
> immutable access to the inner `T`. This is because mutable references
> assume exclusive access, but there might be other handles to the same
> reference counted value, so the access isn't exclusive. The `Arc` type
> implements this by making `borrow_mut` return the same type as `borrow`.
>
> Signed-off-by: Alice Ryhl <aliceryhl@...gle.com>
> Reviewed-by: Boqun Feng <boqun.feng@...il.com>
> Reviewed-by: Benno Lossin <benno.lossin@...ton.me>
> Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@...il.com>


Reviewed-by: Andreas Hindborg <a.hindborg@...nel.org>

Best regards,
Andreas Hindborg