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Message-ID: <20240816-b4-rbtree-v9-1-425b442af7e5@google.com>
Date: Fri, 16 Aug 2024 20:47:58 +0000
From: Matt Gilbride <mattgilbride@...gle.com>
To: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
Wedson Almeida Filho <wedsonaf@...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>,
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>
Cc: 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, Matt Gilbride <mattgilbride@...gle.com>
Subject: [PATCH v9 1/6] rust: kernel: add `drop_contents` to `BoxExt`
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]
Reviewed-by: Boqun Feng <boqun.feng@...il.com>
Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>
---
rust/kernel/alloc/box_ext.rs | 22 ++++++++++++++++++++++
1 file changed, 22 insertions(+)
diff --git a/rust/kernel/alloc/box_ext.rs b/rust/kernel/alloc/box_ext.rs
index 829cb1c1cf9e..6cf79f96d6c7 100644
--- a/rust/kernel/alloc/box_ext.rs
+++ b/rust/kernel/alloc/box_ext.rs
@@ -5,6 +5,8 @@
use super::{AllocError, Flags};
use alloc::boxed::Box;
use core::mem::MaybeUninit;
+use core::ptr;
+use core::result::Result;
/// Extensions to [`Box`].
pub trait BoxExt<T>: Sized {
@@ -17,6 +19,18 @@ 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
+ ///
+ /// ```
+ /// let value = Box::new([0; 32], flags::GFP_KERNEL)
+ /// let value = value.drop_contents();
+ /// // Now we can re-use `value`:
+ /// Box::write(value, [1; 32]);
+ /// ```
+ fn drop_contents(self) -> Box<MaybeUninit<T>>;
}
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()) }
+ }
}
--
2.46.0.184.g6999bdac58-goog
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