Atomic pointer support is an important piece of synchronization algorithm, e.g. RCU, hence provide the support for that.
Note that instead of relying on atomic_long or the implementation of `Atomic<usize>`, a new set of helpers (atomic_ptr_*) is introduced for atomic pointer specifically, this is because ptr2int casting would lose the provenance of a pointer and even though in theory there are a few tricks the provenance can be restored, it'll still be a simpler implementation if C could provide atomic pointers directly. The side effects of this approach are: we don't have the arithmetic and logical operations for pointers yet and the current implementation only works on ARCH_SUPPORTS_ATOMIC_RMW architectures, but these are implementation issues and can be added later. Signed-off-by: Boqun Feng <[email protected]> --- rust/helpers/atomic_ext.c | 3 +++ rust/kernel/sync/atomic.rs | 12 +++++++++++- rust/kernel/sync/atomic/internal.rs | 21 +++++++++++++++------ rust/kernel/sync/atomic/predefine.rs | 23 +++++++++++++++++++++++ 4 files changed, 52 insertions(+), 7 deletions(-) diff --git a/rust/helpers/atomic_ext.c b/rust/helpers/atomic_ext.c index 240218e2e708..c267d5190529 100644 --- a/rust/helpers/atomic_ext.c +++ b/rust/helpers/atomic_ext.c @@ -36,6 +36,7 @@ __rust_helper void rust_helper_atomic_##tname##_set_release(type *ptr, type val) GEN_READ_SET_HELPERS(i8, s8) GEN_READ_SET_HELPERS(i16, s16) +GEN_READ_SET_HELPERS(ptr, const void *) /* * xchg helpers depend on ARCH_SUPPORTS_ATOMIC_RMW and on the @@ -59,6 +60,7 @@ rust_helper_atomic_##tname##_xchg##suffix(type *ptr, type new) \ GEN_XCHG_HELPERS(i8, s8) GEN_XCHG_HELPERS(i16, s16) +GEN_XCHG_HELPERS(ptr, const void *) /* * try_cmpxchg helpers depend on ARCH_SUPPORTS_ATOMIC_RMW and on the @@ -82,3 +84,4 @@ rust_helper_atomic_##tname##_try_cmpxchg##suffix(type *ptr, type *old, type new) GEN_TRY_CMPXCHG_HELPERS(i8, s8) GEN_TRY_CMPXCHG_HELPERS(i16, s16) +GEN_TRY_CMPXCHG_HELPERS(ptr, const void *) diff --git a/rust/kernel/sync/atomic.rs b/rust/kernel/sync/atomic.rs index 4aebeacb961a..4d2a5228c2e4 100644 --- a/rust/kernel/sync/atomic.rs +++ b/rust/kernel/sync/atomic.rs @@ -51,6 +51,10 @@ #[repr(transparent)] pub struct Atomic<T: AtomicType>(AtomicRepr<T::Repr>); +// SAFETY: `Atomic<T>` is safe to transfer between execution contexts because of the safety +// requirement of `AtomicType`. +unsafe impl<T: AtomicType> Send for Atomic<T> {} + // SAFETY: `Atomic<T>` is safe to share among execution contexts because all accesses are atomic. unsafe impl<T: AtomicType> Sync for Atomic<T> {} @@ -68,6 +72,11 @@ unsafe impl<T: AtomicType> Sync for Atomic<T> {} /// /// - [`Self`] must have the same size and alignment as [`Self::Repr`]. /// - [`Self`] must be [round-trip transmutable] to [`Self::Repr`]. +/// - [`Self`] must be safe to transfer between execution contexts, if it's [`Send`], this is +/// automatically satisfied. The exception is pointer types that are even though marked as +/// `!Send` (e.g. raw pointers and [`NonNull<T>`]) but requiring `unsafe` to do anything +/// meaningful on them. This is because transferring pointer values between execution contexts is +/// safe as long as the actual `unsafe` dereferencing is justified. /// /// Note that this is more relaxed than requiring the bi-directional transmutability (i.e. /// [`transmute()`] is always sound between `U` and `T`) because of the support for atomic @@ -108,7 +117,8 @@ unsafe impl<T: AtomicType> Sync for Atomic<T> {} /// [`transmute()`]: core::mem::transmute /// [round-trip transmutable]: AtomicType#round-trip-transmutability /// [Examples]: AtomicType#examples -pub unsafe trait AtomicType: Sized + Send + Copy { +/// [`NonNull<T>`]: core::ptr::NonNull +pub unsafe trait AtomicType: Sized + Copy { /// The backing atomic implementation type. type Repr: AtomicImpl; } diff --git a/rust/kernel/sync/atomic/internal.rs b/rust/kernel/sync/atomic/internal.rs index 0dac58bca2b3..93f5a7846645 100644 --- a/rust/kernel/sync/atomic/internal.rs +++ b/rust/kernel/sync/atomic/internal.rs @@ -7,6 +7,7 @@ use crate::bindings; use crate::macros::paste; use core::cell::UnsafeCell; +use ffi::c_void; mod private { /// Sealed trait marker to disable customized impls on atomic implementation traits. @@ -14,10 +15,11 @@ pub trait Sealed {} } // The C side supports atomic primitives only for `i32` and `i64` (`atomic_t` and `atomic64_t`), -// while the Rust side also layers provides atomic support for `i8` and `i16` -// on top of lower-level C primitives. +// while the Rust side also provides atomic support for `i8`, `i16` and `*const c_void` on top of +// lower-level C primitives. impl private::Sealed for i8 {} impl private::Sealed for i16 {} +impl private::Sealed for *const c_void {} impl private::Sealed for i32 {} impl private::Sealed for i64 {} @@ -26,10 +28,10 @@ impl private::Sealed for i64 {} /// This trait is sealed, and only types that map directly to the C side atomics /// or can be implemented with lower-level C primitives are allowed to implement this: /// -/// - `i8` and `i16` are implemented with lower-level C primitives. +/// - `i8`, `i16` and `*const c_void` are implemented with lower-level C primitives. /// - `i32` map to `atomic_t` /// - `i64` map to `atomic64_t` -pub trait AtomicImpl: Sized + Send + Copy + private::Sealed { +pub trait AtomicImpl: Sized + Copy + private::Sealed { /// The type of the delta in arithmetic or logical operations. /// /// For example, in `atomic_add(ptr, v)`, it's the type of `v`. Usually it's the same type of @@ -51,6 +53,13 @@ impl AtomicImpl for i16 { type Delta = Self; } +// The current helpers of load/store uses `{WRITE,READ}_ONCE()` hence the atomicity is only +// guaranteed against read-modify-write operations if the architecture supports native atomic RmW. +#[cfg(CONFIG_ARCH_SUPPORTS_ATOMIC_RMW)] +impl AtomicImpl for *const c_void { + type Delta = isize; +} + // `atomic_t` implements atomic operations on `i32`. impl AtomicImpl for i32 { type Delta = Self; @@ -262,7 +271,7 @@ macro_rules! declare_and_impl_atomic_methods { } declare_and_impl_atomic_methods!( - [ i8 => atomic_i8, i16 => atomic_i16, i32 => atomic, i64 => atomic64 ] + [ i8 => atomic_i8, i16 => atomic_i16, *const c_void => atomic_ptr, i32 => atomic, i64 => atomic64 ] /// Basic atomic operations pub trait AtomicBasicOps { /// Atomic read (load). @@ -280,7 +289,7 @@ fn set[release](a: &AtomicRepr<Self>, v: Self) { ); declare_and_impl_atomic_methods!( - [ i8 => atomic_i8, i16 => atomic_i16, i32 => atomic, i64 => atomic64 ] + [ i8 => atomic_i8, i16 => atomic_i16, *const c_void => atomic_ptr, i32 => atomic, i64 => atomic64 ] /// Exchange and compare-and-exchange atomic operations pub trait AtomicExchangeOps { /// Atomic exchange. diff --git a/rust/kernel/sync/atomic/predefine.rs b/rust/kernel/sync/atomic/predefine.rs index 42067c6a266c..1a4670d225b5 100644 --- a/rust/kernel/sync/atomic/predefine.rs +++ b/rust/kernel/sync/atomic/predefine.rs @@ -4,6 +4,7 @@ use crate::static_assert; use core::mem::{align_of, size_of}; +use ffi::c_void; // Ensure size and alignment requirements are checked. static_assert!(size_of::<bool>() == size_of::<i8>()); @@ -28,6 +29,16 @@ unsafe impl super::AtomicType for i16 { type Repr = i16; } +// SAFETY: +// +// - `*mut T` has the same size and alignment with `*const c_void`, and is round-trip +// transmutable to `*const c_void`. +// - `*mut T` is safe to transfer between execution contexts. See the safety requirement of +// [`AtomicType`]. +unsafe impl<T: Sized> super::AtomicType for *mut T { + type Repr = *const c_void; +} + // SAFETY: `i32` has the same size and alignment with itself, and is round-trip transmutable to // itself. unsafe impl super::AtomicType for i32 { @@ -215,4 +226,16 @@ fn atomic_bool_tests() { assert_eq!(false, x.load(Relaxed)); assert_eq!(Ok(false), x.cmpxchg(false, true, Full)); } + + #[test] + fn atomic_ptr_tests() { + let mut v = 42; + let mut u = 43; + let x = Atomic::new(&raw mut v); + + assert_eq!(x.load(Acquire), &raw mut v); + assert_eq!(x.cmpxchg(&raw mut u, &raw mut u, Relaxed), Err(&raw mut v)); + assert_eq!(x.cmpxchg(&raw mut v, &raw mut u, Relaxed), Ok(&raw mut v)); + assert_eq!(x.load(Relaxed), &raw mut u); + } } -- 2.51.0
