Messages in this thread | | | Date | Wed, 28 Dec 2022 11:03:34 +0100 | From | Alice Ryhl <> | Subject | Re: [PATCH 1/7] rust: sync: add `Arc` for ref-counted allocations |
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On 12/28/22 07:03, Wedson Almeida Filho wrote: > This is a basic implementation of `Arc` backed by C's `refcount_t`. It > allows Rust code to idiomatically allocate memory that is ref-counted. > > Cc: Will Deacon <will@kernel.org> > Cc: Peter Zijlstra <peterz@infradead.org> > Cc: Boqun Feng <boqun.feng@gmail.com> > Cc: Mark Rutland <mark.rutland@arm.com> > Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Instead of Box::leak, it would be more idiomatic to use Box::into_raw, but both approaches will work.
Regards, Alice Ryhl
> --- > rust/bindings/bindings_helper.h | 1 + > rust/bindings/lib.rs | 1 + > rust/helpers.c | 19 ++++ > rust/kernel/lib.rs | 1 + > rust/kernel/sync.rs | 10 ++ > rust/kernel/sync/arc.rs | 157 ++++++++++++++++++++++++++++++++ > 6 files changed, 189 insertions(+) > create mode 100644 rust/kernel/sync.rs > create mode 100644 rust/kernel/sync/arc.rs > > diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h > index c48bc284214a..75d85bd6c592 100644 > --- a/rust/bindings/bindings_helper.h > +++ b/rust/bindings/bindings_helper.h > @@ -7,6 +7,7 @@ > */ > > #include <linux/slab.h> > +#include <linux/refcount.h> > > /* `bindgen` gets confused at certain things. */ > const gfp_t BINDINGS_GFP_KERNEL = GFP_KERNEL; > diff --git a/rust/bindings/lib.rs b/rust/bindings/lib.rs > index 6c50ee62c56b..7b246454e009 100644 > --- a/rust/bindings/lib.rs > +++ b/rust/bindings/lib.rs > @@ -41,6 +41,7 @@ mod bindings_raw { > #[allow(dead_code)] > mod bindings_helper { > // Import the generated bindings for types. > + use super::bindings_raw::*; > include!(concat!( > env!("OBJTREE"), > "/rust/bindings/bindings_helpers_generated.rs" > diff --git a/rust/helpers.c b/rust/helpers.c > index b4f15eee2ffd..09a4d93f9d62 100644 > --- a/rust/helpers.c > +++ b/rust/helpers.c > @@ -20,6 +20,7 @@ > > #include <linux/bug.h> > #include <linux/build_bug.h> > +#include <linux/refcount.h> > > __noreturn void rust_helper_BUG(void) > { > @@ -27,6 +28,24 @@ __noreturn void rust_helper_BUG(void) > } > EXPORT_SYMBOL_GPL(rust_helper_BUG); > > +refcount_t rust_helper_REFCOUNT_INIT(int n) > +{ > + return (refcount_t)REFCOUNT_INIT(n); > +} > +EXPORT_SYMBOL_GPL(rust_helper_REFCOUNT_INIT); > + > +void rust_helper_refcount_inc(refcount_t *r) > +{ > + refcount_inc(r); > +} > +EXPORT_SYMBOL_GPL(rust_helper_refcount_inc); > + > +bool rust_helper_refcount_dec_and_test(refcount_t *r) > +{ > + return refcount_dec_and_test(r); > +} > +EXPORT_SYMBOL_GPL(rust_helper_refcount_dec_and_test); > + > /* > * We use `bindgen`'s `--size_t-is-usize` option to bind the C `size_t` type > * as the Rust `usize` type, so we can use it in contexts where Rust > diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs > index 53040fa9e897..ace064a3702a 100644 > --- a/rust/kernel/lib.rs > +++ b/rust/kernel/lib.rs > @@ -31,6 +31,7 @@ mod static_assert; > #[doc(hidden)] > pub mod std_vendor; > pub mod str; > +pub mod sync; > pub mod types; > > #[doc(hidden)] > diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs > new file mode 100644 > index 000000000000..39b379dd548f > --- /dev/null > +++ b/rust/kernel/sync.rs > @@ -0,0 +1,10 @@ > +// SPDX-License-Identifier: GPL-2.0 > + > +//! Synchronisation primitives. > +//! > +//! This module contains the kernel APIs related to synchronisation that have been ported or > +//! wrapped for usage by Rust code in the kernel. > + > +mod arc; > + > +pub use arc::Arc; > diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs > new file mode 100644 > index 000000000000..22290eb5ab9b > --- /dev/null > +++ b/rust/kernel/sync/arc.rs > @@ -0,0 +1,157 @@ > +// SPDX-License-Identifier: GPL-2.0 > + > +//! A reference-counted pointer. > +//! > +//! This module implements a way for users to create reference-counted objects and pointers to > +//! them. Such a pointer automatically increments and decrements the count, and drops the > +//! underlying object when it reaches zero. It is also safe to use concurrently from multiple > +//! threads. > +//! > +//! It is different from the standard library's [`Arc`] in a few ways: > +//! 1. It is backed by the kernel's `refcount_t` type. > +//! 2. It does not support weak references, which allows it to be half the size. > +//! 3. It saturates the reference count instead of aborting when it goes over a threshold. > +//! 4. It does not provide a `get_mut` method, so the ref counted object is pinned. > +//! > +//! [`Arc`]: https://doc.rust-lang.org/std/sync/struct.Arc.html > + > +use crate::{bindings, error::Result, types::Opaque}; > +use alloc::boxed::Box; > +use core::{marker::PhantomData, ops::Deref, ptr::NonNull}; > + > +/// A reference-counted pointer to an instance of `T`. > +/// > +/// The reference count is incremented when new instances of [`Arc`] are created, and decremented > +/// when they are dropped. When the count reaches zero, the underlying `T` is also dropped. > +/// > +/// # Invariants > +/// > +/// The reference count on an instance of [`Arc`] is always non-zero. > +/// The object pointed to by [`Arc`] is always pinned. > +/// > +/// # Examples > +/// > +/// ``` > +/// use kernel::sync::Arc; > +/// > +/// struct Example { > +/// a: u32, > +/// b: u32, > +/// } > +/// > +/// // Create a ref-counted instance of `Example`. > +/// let obj = Arc::try_new(Example { a: 10, b: 20 })?; > +/// > +/// // Get a new pointer to `obj` and increment the refcount. > +/// let cloned = obj.clone(); > +/// > +/// // Assert that both `obj` and `cloned` point to the same underlying object. > +/// assert!(core::ptr::eq(&*obj, &*cloned)); > +/// > +/// // Destroy `obj` and decrement its refcount. > +/// drop(obj); > +/// > +/// // Check that the values are still accessible through `cloned`. > +/// assert_eq!(cloned.a, 10); > +/// assert_eq!(cloned.b, 20); > +/// > +/// // The refcount drops to zero when `cloned` goes out of scope, and the memory is freed. > +/// ``` > +pub struct Arc<T: ?Sized> { > + ptr: NonNull<ArcInner<T>>, > + _p: PhantomData<ArcInner<T>>, > +} > + > +#[repr(C)] > +struct ArcInner<T: ?Sized> { > + refcount: Opaque<bindings::refcount_t>, > + data: T, > +} > + > +// SAFETY: It is safe to send `Arc<T>` to another thread when the underlying `T` is `Sync` because > +// it effectively means sharing `&T` (which is safe because `T` is `Sync`); additionally, it needs > +// `T` to be `Send` because any thread that has an `Arc<T>` may ultimately access `T` directly, for > +// example, when the reference count reaches zero and `T` is dropped. > +unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {} > + > +// SAFETY: It is safe to send `&Arc<T>` to another thread when the underlying `T` is `Sync` for the > +// same reason as above. `T` needs to be `Send` as well because a thread can clone an `&Arc<T>` > +// into an `Arc<T>`, which may lead to `T` being accessed by the same reasoning as above. > +unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {} > + > +impl<T> Arc<T> { > + /// Constructs a new reference counted instance of `T`. > + pub fn try_new(contents: T) -> Result<Self> { > + // INVARIANT: The refcount is initialised to a non-zero value. > + let value = ArcInner { > + // SAFETY: There are no safety requirements for this FFI call. > + refcount: Opaque::new(unsafe { bindings::REFCOUNT_INIT(1) }), > + data: contents, > + }; > + > + let inner = Box::try_new(value)?; > + > + // SAFETY: We just created `inner` with a reference count of 1, which is owned by the new > + // `Arc` object. > + Ok(unsafe { Self::from_inner(Box::leak(inner).into()) }) > + } > +} > + > +impl<T: ?Sized> Arc<T> { > + /// Constructs a new [`Arc`] from an existing [`ArcInner`]. > + /// > + /// # Safety > + /// > + /// The caller must ensure that `inner` points to a valid location and has a non-zero reference > + /// count, one of which will be owned by the new [`Arc`] instance. > + unsafe fn from_inner(inner: NonNull<ArcInner<T>>) -> Self { > + // INVARIANT: By the safety requirements, the invariants hold. > + Arc { > + ptr: inner, > + _p: PhantomData, > + } > + } > +} > + > +impl<T: ?Sized> Deref for Arc<T> { > + type Target = T; > + > + fn deref(&self) -> &Self::Target { > + // SAFETY: By the type invariant, there is necessarily a reference to the object, so it is > + // safe to dereference it. > + unsafe { &self.ptr.as_ref().data } > + } > +} > + > +impl<T: ?Sized> Clone for Arc<T> { > + fn clone(&self) -> Self { > + // INVARIANT: C `refcount_inc` saturates the refcount, so it cannot overflow to zero. > + // SAFETY: By the type invariant, there is necessarily a reference to the object, so it is > + // safe to increment the refcount. > + unsafe { bindings::refcount_inc(self.ptr.as_ref().refcount.get()) }; > + > + // SAFETY: We just incremented the refcount. This increment is now owned by the new `Arc`. > + unsafe { Self::from_inner(self.ptr) } > + } > +} > + > +impl<T: ?Sized> Drop for Arc<T> { > + fn drop(&mut self) { > + // SAFETY: By the type invariant, there is necessarily a reference to the object. We cannot > + // touch `refcount` after it's decremented to a non-zero value because another thread/CPU > + // may concurrently decrement it to zero and free it. It is ok to have a raw pointer to > + // freed/invalid memory as long as it is never dereferenced. > + let refcount = unsafe { self.ptr.as_ref() }.refcount.get(); > + > + // INVARIANT: If the refcount reaches zero, there are no other instances of `Arc`, and > + // this instance is being dropped, so the broken invariant is not observable. > + // SAFETY: Also by the type invariant, we are allowed to decrement the refcount. > + let is_zero = unsafe { bindings::refcount_dec_and_test(refcount) }; > + if is_zero { > + // The count reached zero, we must free the memory. > + // > + // SAFETY: The pointer was initialised from the result of `Box::leak`. > + unsafe { Box::from_raw(self.ptr.as_ptr()) }; > + } > + } > +}
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