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01db99b272
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. Link: https://lore.kernel.org/rust-for-linux/20240418-b4-rbtree-v3-5-323e134390ce@google.com/ [1] Signed-off-by: Benno Lossin <benno.lossin@proton.me> Reviewed-by: Boqun Feng <boqun.feng@gmail.com> Reviewed-by: Alice Ryhl <aliceryhl@google.com> Link: https://lore.kernel.org/r/20240819112415.99810-1-benno.lossin@proton.me Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
86 lines
3.0 KiB
Rust
86 lines
3.0 KiB
Rust
// SPDX-License-Identifier: GPL-2.0
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//! Extensions to [`Box`] for fallible allocations.
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use super::{AllocError, Flags};
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use alloc::boxed::Box;
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use core::{mem::MaybeUninit, ptr, result::Result};
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/// Extensions to [`Box`].
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pub trait BoxExt<T>: Sized {
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/// Allocates a new box.
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///
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/// The allocation may fail, in which case an error is returned.
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fn new(x: T, flags: Flags) -> Result<Self, AllocError>;
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/// Allocates a new uninitialised box.
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///
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/// The allocation may fail, in which case an error is returned.
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fn new_uninit(flags: Flags) -> Result<Box<MaybeUninit<T>>, AllocError>;
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/// Drops the contents, but keeps the allocation.
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///
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/// # Examples
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///
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/// ```
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/// use kernel::alloc::{flags, box_ext::BoxExt};
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/// let value = Box::new([0; 32], flags::GFP_KERNEL)?;
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/// assert_eq!(*value, [0; 32]);
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/// let value = Box::drop_contents(value);
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/// // Now we can re-use `value`:
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/// let value = Box::write(value, [1; 32]);
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/// assert_eq!(*value, [1; 32]);
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/// # Ok::<(), Error>(())
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/// ```
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fn drop_contents(this: Self) -> Box<MaybeUninit<T>>;
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}
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impl<T> BoxExt<T> for Box<T> {
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fn new(x: T, flags: Flags) -> Result<Self, AllocError> {
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let b = <Self as BoxExt<_>>::new_uninit(flags)?;
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Ok(Box::write(b, x))
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}
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#[cfg(any(test, testlib))]
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fn new_uninit(_flags: Flags) -> Result<Box<MaybeUninit<T>>, AllocError> {
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Ok(Box::new_uninit())
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}
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#[cfg(not(any(test, testlib)))]
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fn new_uninit(flags: Flags) -> Result<Box<MaybeUninit<T>>, AllocError> {
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let ptr = if core::mem::size_of::<MaybeUninit<T>>() == 0 {
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core::ptr::NonNull::<_>::dangling().as_ptr()
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} else {
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let layout = core::alloc::Layout::new::<MaybeUninit<T>>();
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// SAFETY: Memory is being allocated (first arg is null). The only other source of
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// safety issues is sleeping on atomic context, which is addressed by klint. Lastly,
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// the type is not a SZT (checked above).
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let ptr =
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unsafe { super::allocator::krealloc_aligned(core::ptr::null_mut(), layout, flags) };
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if ptr.is_null() {
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return Err(AllocError);
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}
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ptr.cast::<MaybeUninit<T>>()
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};
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// SAFETY: For non-zero-sized types, we allocate above using the global allocator. For
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// zero-sized types, we use `NonNull::dangling`.
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Ok(unsafe { Box::from_raw(ptr) })
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}
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fn drop_contents(this: Self) -> Box<MaybeUninit<T>> {
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let ptr = Box::into_raw(this);
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// SAFETY: `ptr` is valid, because it came from `Box::into_raw`.
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unsafe { ptr::drop_in_place(ptr) };
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// CAST: `MaybeUninit<T>` is a transparent wrapper of `T`.
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let ptr = ptr.cast::<MaybeUninit<T>>();
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// SAFETY: `ptr` is valid for writes, because it came from `Box::into_raw` and it is valid for
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// reads, since the pointer came from `Box::into_raw` and the type is `MaybeUninit<T>`.
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unsafe { Box::from_raw(ptr) }
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}
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}
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