[v3,2/9] rust: sync: add `Arc::{from_raw, into_raw}`
Commit Message
From: Wedson Almeida Filho <walmeida@microsoft.com>
These methods can be used to turn an `Arc` into a raw pointer and back,
in a way that preserves the metadata for fat pointers.
This is done using the unstable ptr_metadata feature [1]. However, it
could also be done using the unstable pointer_byte_offsets feature [2],
which is likely to have a shorter path to stabilization than
ptr_metadata.
Link: https://github.com/rust-lang/rust/issues/81513 [1]
Link: https://github.com/rust-lang/rust/issues/96283 [2]
Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
Co-developed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
---
v2 -> v3:
* Use `transmute_copy` to convert metadata.
* Use `add` instead of `wrapping_add`.
* Dropped Reviewed-by from Martin and Benno due to changes.
rust/kernel/lib.rs | 1 +
rust/kernel/sync/arc.rs | 49 +++++++++++++++++++++++++++++++++++++++--
2 files changed, 48 insertions(+), 2 deletions(-)
Comments
On 7/11/23 06:32, Alice Ryhl wrote:
> From: Wedson Almeida Filho <walmeida@microsoft.com>
>
> These methods can be used to turn an `Arc` into a raw pointer and back,
> in a way that preserves the metadata for fat pointers.
>
> This is done using the unstable ptr_metadata feature [1]. However, it
> could also be done using the unstable pointer_byte_offsets feature [2],
> which is likely to have a shorter path to stabilization than
> ptr_metadata.
>
> Link: https://github.com/rust-lang/rust/issues/81513 [1]
> Link: https://github.com/rust-lang/rust/issues/96283 [2]
> Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
> Co-developed-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> Reviewed-by: Gary Guo <gary@garyguo.net>
> ---
> [...]
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
> From: Wedson Almeida Filho <walmeida@microsoft.com>
>
> These methods can be used to turn an `Arc` into a raw pointer and back,
> in a way that preserves the metadata for fat pointers.
>
> This is done using the unstable ptr_metadata feature [1]. However, it
> could also be done using the unstable pointer_byte_offsets feature [2],
> which is likely to have a shorter path to stabilization than
> ptr_metadata.
>
> Link: https://github.com/rust-lang/rust/issues/81513 [1]
> Link: https://github.com/rust-lang/rust/issues/96283 [2]
> Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
> Co-developed-by: Alice Ryhl <aliceryhl@google.com>
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> Reviewed-by: Gary Guo <gary@garyguo.net>
LGTM, two small documentation nits below, with them fixed you can add:
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
> ---
> v2 -> v3:
> * Use `transmute_copy` to convert metadata.
> * Use `add` instead of `wrapping_add`.
> * Dropped Reviewed-by from Martin and Benno due to changes.
>
> rust/kernel/lib.rs | 1 +
> rust/kernel/sync/arc.rs | 49 +++++++++++++++++++++++++++++++++++++++--
> 2 files changed, 48 insertions(+), 2 deletions(-)
>
> diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
> index 788914fd935f..6963d11092c4 100644
> --- a/rust/kernel/lib.rs
> +++ b/rust/kernel/lib.rs
> @@ -17,6 +17,7 @@
> #![feature(const_refs_to_cell)]
> #![feature(dispatch_from_dyn)]
> #![feature(new_uninit)]
> +#![feature(ptr_metadata)]
> #![feature(receiver_trait)]
> #![feature(unsize)]
>
> diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
> index a89843cacaad..4046b534e53b 100644
> --- a/rust/kernel/sync/arc.rs
> +++ b/rust/kernel/sync/arc.rs
> @@ -24,13 +24,13 @@
> };
> use alloc::boxed::Box;
> use core::{
> - alloc::AllocError,
> + alloc::{AllocError, Layout},
> fmt,
> marker::{PhantomData, Unsize},
> mem::{ManuallyDrop, MaybeUninit},
> ops::{Deref, DerefMut},
> pin::Pin,
> - ptr::NonNull,
> + ptr::{NonNull, Pointee},
> };
> use macros::pin_data;
>
> @@ -212,6 +212,51 @@ unsafe fn from_inner(inner: NonNull<ArcInner<T>>) -> Self {
> }
> }
>
> + /// Convert the [`Arc`] into a raw pointer.
> + ///
> + /// The raw pointer has ownership of the refcount that this Arc object owned.
> + pub fn into_raw(self) -> *const T {
> + let ptr = self.ptr.as_ptr();
> + core::mem::forget(self);
> + // SAFETY: The pointer is valid.
> + unsafe { core::ptr::addr_of!((*ptr).data) }
> + }
> +
> + /// Recreates an [`Arc`] instance previously deconstructed via [`Arc::into_raw`].
> + ///
> + /// This code relies on the `repr(C)` layout of structs as described in
> + /// <https://doc.rust-lang.org/reference/type-layout.html#reprc-structs>.
Why is this in the documentation? I feel like it should be a normal code
comment at the very start of the function.
> + ///
> + /// # Safety
> + ///
> + /// `ptr` must have been returned by a previous call to [`Arc::into_raw`]. Additionally, it
> + /// can only be called once for each previous call to [`Arc::into_raw`].
"it can only" -> "it must only"
--
Cheers,
Benno
> + pub unsafe fn from_raw(ptr: *const T) -> Self {
> + let refcount_layout = Layout::new::<bindings::refcount_t>();
> + // SAFETY: The caller guarantees that the pointer is valid.
> + let val_layout = Layout::for_value(unsafe { &*ptr });
> + // SAFETY: We're computing the layout of a real struct that existed when compiling this
> + // binary, so its layout is not so large that it can trigger arithmetic overflow.
> + let val_offset = unsafe { refcount_layout.extend(val_layout).unwrap_unchecked().1 };
> +
> + let metadata: <T as Pointee>::Metadata = core::ptr::metadata(ptr);
> + // SAFETY: The metadata of `T` and `ArcInner<T>` is the same because `ArcInner` is a struct
> + // with `T` as its last field.
> + //
> + // This is documented at:
> + // <https://doc.rust-lang.org/std/ptr/trait.Pointee.html>.
> + let metadata: <ArcInner<T> as Pointee>::Metadata =
> + unsafe { core::mem::transmute_copy(&metadata) };
> + // SAFETY: The pointer is in-bounds of an allocation both before and after offsetting the
> + // pointer, since it originates from a previous call to `Arc::into_raw` and is still valid.
> + let ptr = unsafe { (ptr as *mut u8).sub(val_offset) as *mut () };
> + let ptr = core::ptr::from_raw_parts_mut(ptr, metadata);
> +
> + // SAFETY: By the safety requirements we know that `ptr` came from `Arc::into_raw`, so the
> + // reference count held then will be owned by the new `Arc` object.
> + unsafe { Self::from_inner(NonNull::new_unchecked(ptr)) }
> + }
> +
> /// Returns an [`ArcBorrow`] from the given [`Arc`].
> ///
> /// This is useful when the argument of a function call is an [`ArcBorrow`] (e.g., in a method
> --
> 2.41.0.255.g8b1d071c50-goog
>
Benno Lossin <benno.lossin@proton.me> writes:
>> + /// This code relies on the `repr(C)` layout of structs as described in
>> + /// <https://doc.rust-lang.org/reference/type-layout.html#reprc-structs>.
>
> Why is this in the documentation? I feel like it should be a normal code
> comment at the very start of the function.
In fact, I think we can drop this comment entirely. The motivation
behind using `Layout::extend` for computing `val_offset` is that its
correctness does not rely on how the repr(C) layout algorithm works.
(As opposed to how the previous implementation's correctness *does*
depend on knowing the repr(C) layout algorithm:
Layout::new::<ArcInner<()>>().align_to(align).unwrap().pad_to_align().size()
)
>> + ///
>> + /// # Safety
>> + ///
>> + /// `ptr` must have been returned by a previous call to [`Arc::into_raw`]. Additionally, it
>> + /// can only be called once for each previous call to [`Arc::into_raw`].
>
> "it can only" -> "it must only"
Sounds good. I'll change it to use "must" in the next version.
Alice
@@ -17,6 +17,7 @@
#![feature(const_refs_to_cell)]
#![feature(dispatch_from_dyn)]
#![feature(new_uninit)]
+#![feature(ptr_metadata)]
#![feature(receiver_trait)]
#![feature(unsize)]
@@ -24,13 +24,13 @@
};
use alloc::boxed::Box;
use core::{
- alloc::AllocError,
+ alloc::{AllocError, Layout},
fmt,
marker::{PhantomData, Unsize},
mem::{ManuallyDrop, MaybeUninit},
ops::{Deref, DerefMut},
pin::Pin,
- ptr::NonNull,
+ ptr::{NonNull, Pointee},
};
use macros::pin_data;
@@ -212,6 +212,51 @@ unsafe fn from_inner(inner: NonNull<ArcInner<T>>) -> Self {
}
}
+ /// Convert the [`Arc`] into a raw pointer.
+ ///
+ /// The raw pointer has ownership of the refcount that this Arc object owned.
+ pub fn into_raw(self) -> *const T {
+ let ptr = self.ptr.as_ptr();
+ core::mem::forget(self);
+ // SAFETY: The pointer is valid.
+ unsafe { core::ptr::addr_of!((*ptr).data) }
+ }
+
+ /// Recreates an [`Arc`] instance previously deconstructed via [`Arc::into_raw`].
+ ///
+ /// This code relies on the `repr(C)` layout of structs as described in
+ /// <https://doc.rust-lang.org/reference/type-layout.html#reprc-structs>.
+ ///
+ /// # Safety
+ ///
+ /// `ptr` must have been returned by a previous call to [`Arc::into_raw`]. Additionally, it
+ /// can only be called once for each previous call to [`Arc::into_raw`].
+ pub unsafe fn from_raw(ptr: *const T) -> Self {
+ let refcount_layout = Layout::new::<bindings::refcount_t>();
+ // SAFETY: The caller guarantees that the pointer is valid.
+ let val_layout = Layout::for_value(unsafe { &*ptr });
+ // SAFETY: We're computing the layout of a real struct that existed when compiling this
+ // binary, so its layout is not so large that it can trigger arithmetic overflow.
+ let val_offset = unsafe { refcount_layout.extend(val_layout).unwrap_unchecked().1 };
+
+ let metadata: <T as Pointee>::Metadata = core::ptr::metadata(ptr);
+ // SAFETY: The metadata of `T` and `ArcInner<T>` is the same because `ArcInner` is a struct
+ // with `T` as its last field.
+ //
+ // This is documented at:
+ // <https://doc.rust-lang.org/std/ptr/trait.Pointee.html>.
+ let metadata: <ArcInner<T> as Pointee>::Metadata =
+ unsafe { core::mem::transmute_copy(&metadata) };
+ // SAFETY: The pointer is in-bounds of an allocation both before and after offsetting the
+ // pointer, since it originates from a previous call to `Arc::into_raw` and is still valid.
+ let ptr = unsafe { (ptr as *mut u8).sub(val_offset) as *mut () };
+ let ptr = core::ptr::from_raw_parts_mut(ptr, metadata);
+
+ // SAFETY: By the safety requirements we know that `ptr` came from `Arc::into_raw`, so the
+ // reference count held then will be owned by the new `Arc` object.
+ unsafe { Self::from_inner(NonNull::new_unchecked(ptr)) }
+ }
+
/// Returns an [`ArcBorrow`] from the given [`Arc`].
///
/// This is useful when the argument of a function call is an [`ArcBorrow`] (e.g., in a method