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Message-ID: <20240731210206.2b56b432.gary@garyguo.net>
Date: Wed, 31 Jul 2024 21:02:06 +0100
From: Gary Guo <gary@...yguo.net>
To: Matt Gilbride <mattgilbride@...gle.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
Wedson Almeida Filho <wedsonaf@...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@...sung.com>, Alice Ryhl <aliceryhl@...gle.com>, Greg
Kroah-Hartman <gregkh@...uxfoundation.org>, "Arve Hjønnevåg" <arve@...roid.com>, Todd Kjos <tkjos@...roid.com>,
Martijn Coenen <maco@...roid.com>, Joel Fernandes <joel@...lfernandes.org>,
Carlos Llamas <cmllamas@...gle.com>, Suren Baghdasaryan
<surenb@...gle.com>, Christian Brauner <brauner@...nel.org>, Rob Landley
<rob@...dley.net>, Davidlohr Bueso <dave@...olabs.net>, Michel Lespinasse
<michel@...pinasse.org>, rust-for-linux@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 1/6] rust: kernel: add `drop_contents` to `BoxExt`
On Thu, 11 Jul 2024 16:20:57 +0000
Matt Gilbride <mattgilbride@...gle.com> wrote:
> From: Benno Lossin <benno.lossin@...ton.me>
>
> Sometimes (see [1]) it is necessary to drop the value inside of a
> `Box<T>`, but retain the allocation. For example to reuse the allocation
> in the future.
> Introduce a new function `drop_contents` that turns a `Box<T>` into
> `Box<MaybeUninit<T>>` by dropping the value.
>
> Signed-off-by: Benno Lossin <benno.lossin@...ton.me>
> Link: https://lore.kernel.org/rust-for-linux/20240418-b4-rbtree-v3-5-323e134390ce@google.com/ [1]
> ---
> rust/kernel/alloc/box_ext.rs | 24 +++++++++++++++++++++++-
> 1 file changed, 23 insertions(+), 1 deletion(-)
>
> diff --git a/rust/kernel/alloc/box_ext.rs b/rust/kernel/alloc/box_ext.rs
> index 829cb1c1cf9e..557895db4f48 100644
> --- a/rust/kernel/alloc/box_ext.rs
> +++ b/rust/kernel/alloc/box_ext.rs
> @@ -4,7 +4,7 @@
>
> use super::{AllocError, Flags};
> use alloc::boxed::Box;
> -use core::mem::MaybeUninit;
> +use core::{mem::MaybeUninit, ptr};
>
> /// Extensions to [`Box`].
> pub trait BoxExt<T>: Sized {
> @@ -17,6 +17,20 @@ pub trait BoxExt<T>: Sized {
> ///
> /// The allocation may fail, in which case an error is returned.
> fn new_uninit(flags: Flags) -> Result<Box<MaybeUninit<T>>, AllocError>;
> +
> + /// Drops the contents, but keeps the allocation.
> + ///
> + /// # Examples
> + ///
> + /// ```
> + /// use kernel::alloc::flags;
> + ///
> + /// let value = Box::new([0; 32], flags::GFP_KERNEL);
> + /// let value = value.unwrap().drop_contents();
> + /// // Now we can re-use `value`:
> + /// Box::write(value, [1; 32]);
> + /// ```
> + fn drop_contents(self) -> Box<MaybeUninit<T>>;
Should this have signature `fn drop_contents(this: Self) -> ...`? Box
types normally don't have methods to avoid shadowing functions that the
pointee may have.
> }
>
> impl<T> BoxExt<T> for Box<T> {
> @@ -53,4 +67,12 @@ fn new_uninit(flags: Flags) -> Result<Box<MaybeUninit<T>>, AllocError> {
> // zero-sized types, we use `NonNull::dangling`.
> Ok(unsafe { Box::from_raw(ptr) })
> }
> +
> + fn drop_contents(self) -> Box<MaybeUninit<T>> {
> + let ptr = Box::into_raw(self);
> + // SAFETY: `ptr` is valid, because it came from `Box::into_raw`.
> + unsafe { ptr::drop_in_place(ptr) };
> + // SAFETY: `ptr` is valid, because it came from `Box::into_raw`.
> + unsafe { Box::from_raw(ptr.cast()) }
> + }
> }
>
Best,
Gary
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