From: Alice Ryhl <aliceryhl@google.com>
To: rust-for-linux@vger.kernel.org
Cc: "Miguel Ojeda" <ojeda@kernel.org>,
"Wedson Almeida Filho" <wedsonaf@gmail.com>,
"Tejun Heo" <tj@kernel.org>,
"Lai Jiangshan" <jiangshanlai@gmail.com>,
"Alex Gaynor" <alex.gaynor@gmail.com>,
"Boqun Feng" <boqun.feng@gmail.com>,
"Gary Guo" <gary@garyguo.net>,
"Björn Roy Baron" <bjorn3_gh@protonmail.com>,
"Benno Lossin" <benno.lossin@proton.me>,
"Alice Ryhl" <aliceryhl@google.com>,
linux-kernel@vger.kernel.org, patches@lists.linux.dev,
"Wedson Almeida Filho" <walmeida@microsoft.com>,
"Martin Rodriguez Reboredo" <yakoyoku@gmail.com>
Subject: [PATCH v2 3/8] rust: sync: add `Arc::{from_raw, into_raw}`
Date: Thu, 1 Jun 2023 13:49:41 +0000 [thread overview]
Message-ID: <20230601134946.3887870-4-aliceryhl@google.com> (raw)
In-Reply-To: <20230601134946.3887870-1-aliceryhl@google.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: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
---
rust/kernel/lib.rs | 1 +
rust/kernel/sync/arc.rs | 42 ++++++++++++++++++++++++++++++++++++++++-
2 files changed, 42 insertions(+), 1 deletion(-)
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 7ea777b731e6..ad9142928fb1 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..684be9f73aca 100644
--- a/rust/kernel/sync/arc.rs
+++ b/rust/kernel/sync/arc.rs
@@ -24,7 +24,7 @@
};
use alloc::boxed::Box;
use core::{
- alloc::AllocError,
+ alloc::{AllocError, Layout},
fmt,
marker::{PhantomData, Unsize},
mem::{ManuallyDrop, MaybeUninit},
@@ -212,6 +212,46 @@ 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 = unsafe { Layout::for_value(&*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 };
+
+ // This preserves the metadata in the pointer, if any.
+ //
+ // Note that `*const T` and `*const ArcInner<T>` have the same metadata as documented at
+ // <https://doc.rust-lang.org/std/ptr/trait.Pointee.html>.
+ let metadata = core::ptr::metadata(ptr as *const ArcInner<T>);
+ let ptr = (ptr as *mut u8).wrapping_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.rc0.172.g3f132b7071-goog
next prev parent reply other threads:[~2023-06-01 13:53 UTC|newest]
Thread overview: 53+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-06-01 13:49 [PATCH v2 0/8] rust: workqueue: add bindings for the workqueue Alice Ryhl
2023-06-01 13:49 ` [PATCH v2 1/8] rust: workqueue: add low-level workqueue bindings Alice Ryhl
2023-06-01 14:29 ` Martin Rodriguez Reboredo
2023-06-02 14:39 ` Andreas Hindborg (Samsung)
2023-06-06 23:36 ` Boqun Feng
2023-06-07 15:18 ` Alice Ryhl
2023-06-11 15:45 ` Benno Lossin
2023-06-01 13:49 ` [PATCH v2 2/8] rust: add offset_of! macro Alice Ryhl
2023-06-01 17:17 ` Gary Guo
2023-06-02 10:33 ` Andreas Hindborg (Samsung)
2023-06-07 15:15 ` Alice Ryhl
2023-06-11 15:47 ` Benno Lossin
2023-06-01 13:49 ` Alice Ryhl [this message]
2023-06-01 17:26 ` [PATCH v2 3/8] rust: sync: add `Arc::{from_raw, into_raw}` Gary Guo
2023-06-02 10:51 ` Andreas Hindborg (Samsung)
2023-06-05 14:31 ` Gary Guo
2023-06-05 14:49 ` Andreas Hindborg (Samsung)
2023-06-05 15:00 ` Boqun Feng
2023-06-05 15:20 ` Boqun Feng
2023-06-05 18:34 ` Andreas Hindborg (Samsung)
2023-06-11 15:48 ` Benno Lossin
2023-06-01 13:49 ` [PATCH v2 4/8] rust: workqueue: define built-in queues Alice Ryhl
2023-06-01 17:30 ` Gary Guo
2023-06-01 17:52 ` Martin Rodriguez Reboredo
2023-06-02 8:32 ` Alice Ryhl
2023-06-02 14:46 ` Andreas Hindborg (Samsung)
2023-06-11 15:49 ` Benno Lossin
2023-06-01 13:49 ` [PATCH v2 5/8] rust: workqueue: add helper for defining work_struct fields Alice Ryhl
2023-06-01 14:50 ` Martin Rodriguez Reboredo
2023-06-01 18:44 ` Boqun Feng
2023-06-02 8:38 ` Alice Ryhl
2023-06-02 16:32 ` Boqun Feng
2023-06-01 21:09 ` Boqun Feng
2023-06-02 9:37 ` Alice Ryhl
2023-06-02 14:41 ` Andreas Hindborg (Samsung)
2023-06-11 15:59 ` Benno Lossin
2023-06-27 8:42 ` Alice Ryhl
2023-06-01 13:49 ` [PATCH v2 6/8] rust: workqueue: implement `WorkItemPointer` for pointer types Alice Ryhl
2023-06-01 14:51 ` Martin Rodriguez Reboredo
2023-06-02 14:42 ` Andreas Hindborg (Samsung)
2023-06-11 16:01 ` Benno Lossin
2023-06-01 13:49 ` [PATCH v2 7/8] rust: workqueue: add `try_spawn` helper method Alice Ryhl
2023-06-01 14:53 ` Martin Rodriguez Reboredo
2023-06-02 14:43 ` Andreas Hindborg (Samsung)
2023-06-11 16:10 ` Benno Lossin
2023-06-01 13:49 ` [PATCH v2 8/8] rust: workqueue: add examples Alice Ryhl
2023-06-01 14:58 ` Martin Rodriguez Reboredo
2023-06-01 17:32 ` Gary Guo
2023-06-02 9:39 ` Alice Ryhl
2023-06-02 14:44 ` Andreas Hindborg (Samsung)
2023-06-02 14:48 ` Andreas Hindborg (Samsung)
2023-06-11 16:15 ` Benno Lossin
2023-06-27 8:38 ` Alice Ryhl
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