From 9dc04365500340e6d60a996333d562af747337b1 Mon Sep 17 00:00:00 2001 From: Wedson Almeida Filho Date: Wed, 28 Dec 2022 06:03:40 +0000 Subject: [PATCH] rust: sync: add `Arc` for ref-counted allocations 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 Cc: Peter Zijlstra Cc: Boqun Feng Cc: Mark Rutland Signed-off-by: Wedson Almeida Filho Reviewed-by: Alice Ryhl Reviewed-by: Gary Guo Reviewed-by: Vincenzo Palazzo Acked-by: Boqun Feng Signed-off-by: Miguel Ojeda --- 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 c48bc28..75d85bd 100644 --- a/rust/bindings/bindings_helper.h +++ b/rust/bindings/bindings_helper.h @@ -7,6 +7,7 @@ */ #include +#include /* `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 6c50ee6..7b24645 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 b4f15ee..09a4d93 100644 --- a/rust/helpers.c +++ b/rust/helpers.c @@ -20,6 +20,7 @@ #include #include +#include __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 53040fa..ace064a 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 0000000..39b379d --- /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 0000000..22290eb --- /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 { + ptr: NonNull>, + _p: PhantomData>, +} + +#[repr(C)] +struct ArcInner { + refcount: Opaque, + data: T, +} + +// SAFETY: It is safe to send `Arc` 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` may ultimately access `T` directly, for +// example, when the reference count reaches zero and `T` is dropped. +unsafe impl Send for Arc {} + +// SAFETY: It is safe to send `&Arc` 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` +// into an `Arc`, which may lead to `T` being accessed by the same reasoning as above. +unsafe impl Sync for Arc {} + +impl Arc { + /// Constructs a new reference counted instance of `T`. + pub fn try_new(contents: T) -> Result { + // 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 Arc { + /// 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>) -> Self { + // INVARIANT: By the safety requirements, the invariants hold. + Arc { + ptr: inner, + _p: PhantomData, + } + } +} + +impl Deref for Arc { + 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 Clone for Arc { + 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 Drop for Arc { + 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()) }; + } + } +} -- 2.7.4