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[103.168.172.201]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-70497d3971dsm5105016d6.62.2025.07.09.23.01.02 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 09 Jul 2025 23:01:03 -0700 (PDT) Received: from phl-compute-06.internal (phl-compute-06.phl.internal [10.202.2.46]) by mailfauth.phl.internal (Postfix) with ESMTP id 88CDEF4006C; Thu, 10 Jul 2025 02:01:02 -0400 (EDT) Received: from phl-mailfrontend-01 ([10.202.2.162]) by phl-compute-06.internal (MEProxy); Thu, 10 Jul 2025 02:01:02 -0400 X-ME-Sender: X-ME-Received: X-ME-Proxy-Cause: gggruggvucftvghtrhhoucdtuddrgeeffedrtdefgdefleeijecutefuodetggdotefrod ftvfcurfhrohhfihhlvgemucfhrghsthforghilhdpuffrtefokffrpgfnqfghnecuuegr ihhlohhuthemuceftddtnecusecvtfgvtghiphhivghnthhsucdlqddutddtmdenucfjug hrpefhvfevufffkffojghfggfgsedtkeertdertddtnecuhfhrohhmpeeuohhquhhnucfh vghnghcuoegsohhquhhnrdhfvghnghesghhmrghilhdrtghomheqnecuggftrfgrthhtvg hrnhepgeeljeeitdehvdehgefgjeevfeejjeekgfevffeiueejhfeuiefggeeuheeggefg necuvehluhhsthgvrhfuihiivgeptdenucfrrghrrghmpehmrghilhhfrhhomhepsghoqh hunhdomhgvshhmthhprghuthhhphgvrhhsohhnrghlihhthidqieelvdeghedtieegqddu jeejkeehheehvddqsghoqhhunhdrfhgvnhhgpeepghhmrghilhdrtghomhesfhhigihmvg drnhgrmhgvpdhnsggprhgtphhtthhopedvjedpmhhouggvpehsmhhtphhouhhtpdhrtghp thhtoheplhhinhhugidqkhgvrhhnvghlsehvghgvrhdrkhgvrhhnvghlrdhorhhgpdhrtg hpthhtoheprhhushhtqdhfohhrqdhlihhnuhigsehvghgvrhdrkhgvrhhnvghlrdhorhhg pdhrtghpthhtoheplhhkmhhmsehlihhsthhsrdhlihhnuhigrdguvghvpdhrtghpthhtoh eplhhinhhugidqrghrtghhsehvghgvrhdrkhgvrhhnvghlrdhorhhgpdhrtghpthhtohep ohhjvggurgeskhgvrhhnvghlrdhorhhgpdhrtghpthhtoheprghlvgigrdhgrgihnhhorh esghhmrghilhdrtghomhdprhgtphhtthhopegsohhquhhnrdhfvghnghesghhmrghilhdr tghomhdprhgtphhtthhopehgrghrhiesghgrrhihghhuohdrnhgvthdprhgtphhtthhope gsjhhorhhnfegpghhhsehprhhothhonhhmrghilhdrtghomh X-ME-Proxy: Feedback-ID: iad51458e:Fastmail Received: by mail.messagingengine.com (Postfix) with ESMTPA; Thu, 10 Jul 2025 02:01:01 -0400 (EDT) From: Boqun Feng To: linux-kernel@vger.kernel.org, rust-for-linux@vger.kernel.org, lkmm@lists.linux.dev, linux-arch@vger.kernel.org Cc: "Miguel Ojeda" , "Alex Gaynor" , "Boqun Feng" , "Gary Guo" , =?UTF-8?q?Bj=C3=B6rn=20Roy=20Baron?= , "Benno Lossin" , "Andreas Hindborg" , "Alice Ryhl" , "Trevor Gross" , "Danilo Krummrich" , "Will Deacon" , "Peter Zijlstra" , "Mark Rutland" , "Wedson Almeida Filho" , "Viresh Kumar" , "Lyude Paul" , "Ingo Molnar" , "Mitchell Levy" , "Paul E. McKenney" , "Greg Kroah-Hartman" , "Linus Torvalds" , "Thomas Gleixner" , Alan Stern Subject: [PATCH v6 4/9] rust: sync: atomic: Add generic atomics Date: Wed, 9 Jul 2025 23:00:47 -0700 Message-Id: <20250710060052.11955-5-boqun.feng@gmail.com> X-Mailer: git-send-email 2.39.5 (Apple Git-154) In-Reply-To: <20250710060052.11955-1-boqun.feng@gmail.com> References: <20250710060052.11955-1-boqun.feng@gmail.com> Precedence: bulk X-Mailing-List: lkmm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit To provide using LKMM atomics for Rust code, a generic `Atomic` is added, currently `T` needs to be Send + Copy because these are the straightforward usages and all basic types support this. Implement `AllowAtomic` for `i32` and `i64`, and so far only basic operations load() and store() are introduced. Reviewed-by: Alice Ryhl Signed-off-by: Boqun Feng --- rust/kernel/sync/atomic.rs | 14 ++ rust/kernel/sync/atomic/generic.rs | 289 +++++++++++++++++++++++++++++ 2 files changed, 303 insertions(+) create mode 100644 rust/kernel/sync/atomic/generic.rs diff --git a/rust/kernel/sync/atomic.rs b/rust/kernel/sync/atomic.rs index e80ac049f36b..c5193c1c90fe 100644 --- a/rust/kernel/sync/atomic.rs +++ b/rust/kernel/sync/atomic.rs @@ -16,7 +16,21 @@ //! //! [`LKMM`]: srctree/tools/memory-model/ +pub mod generic; pub mod ops; pub mod ordering; +pub use generic::Atomic; pub use ordering::{Acquire, Full, Relaxed, Release}; + +// SAFETY: `i32` has the same size and alignment with itself, and is round-trip transmutable to +// itself. +unsafe impl generic::AllowAtomic for i32 { + type Repr = i32; +} + +// SAFETY: `i64` has the same size and alignment with itself, and is round-trip transmutable to +// itself. +unsafe impl generic::AllowAtomic for i64 { + type Repr = i64; +} diff --git a/rust/kernel/sync/atomic/generic.rs b/rust/kernel/sync/atomic/generic.rs new file mode 100644 index 000000000000..e044fe21b128 --- /dev/null +++ b/rust/kernel/sync/atomic/generic.rs @@ -0,0 +1,289 @@ +// SPDX-License-Identifier: GPL-2.0 + +//! Generic atomic primitives. + +use super::ops::*; +use super::ordering::*; +use crate::build_error; +use core::cell::UnsafeCell; + +/// A generic atomic variable. +/// +/// `T` must impl [`AllowAtomic`], that is, an [`AtomicImpl`] has to be chosen. +/// +/// # Examples +/// +/// A customized type stored in [`Atomic`]: +/// +/// ```rust +/// use kernel::sync::atomic::{generic::AllowAtomic, Atomic, Relaxed}; +/// +/// #[derive(Clone, Copy, PartialEq, Eq)] +/// #[repr(i32)] +/// enum State { +/// Uninit = 0, +/// Working = 1, +/// Done = 2, +/// }; +/// +/// // SAFETY: `State` and `i32` has the same size and alignment, and it's round-trip +/// // transmutable to `i32`. +/// unsafe impl AllowAtomic for State { +/// type Repr = i32; +/// } +/// +/// let s = Atomic::new(State::Uninit); +/// +/// assert_eq!(State::Uninit, s.load(Relaxed)); +/// ``` +/// +/// # Guarantees +/// +/// Doing an atomic operation while holding a reference of [`Self`] won't cause a data race, +/// this is guaranteed by the safety requirement of [`Self::from_ptr()`] and the extra safety +/// requirement of the usage on pointers returned by [`Self::as_ptr()`]. +#[repr(transparent)] +pub struct Atomic(UnsafeCell); + +// SAFETY: `Atomic` is safe to share among execution contexts because all accesses are atomic. +unsafe impl Sync for Atomic {} + +/// Types that support basic atomic operations. +/// +/// # Round-trip transmutability +/// +/// Implementing [`AllowAtomic`] requires that the type is round-trip transmutable to its +/// representation: +/// +/// - Any value of [`Self`] must be sound to [`transmute()`] to a [`Self::Repr`], and this also +/// means that a pointer to [`Self`] can be treated as a pointer to [`Self::Repr`] for reading. +/// - If a value of [`Self::Repr`] is a result a [`transmute()`] from a [`Self`], it must be +/// sound to [`transmute()`] the value back to a [`Self`]. +/// +/// This essentially means a valid bit pattern of `T: AllowAtomic` has to be a valid bit pattern +/// of `T::Repr`. This is needed because [`Atomic`] operates on `T::Repr` to +/// implement atomic operations on `T`. +/// +/// Note that this is more relaxed than bidirectional transmutability (i.e. [`transmute()`] is +/// always sound between `T` and `T::Repr`) because of the support for atomic variables over +/// unit-only enums: +/// +/// ``` +/// #[repr(i32)] +/// enum State { Init = 0, Working = 1, Done = 2, } +/// ``` +/// +/// # Safety +/// +/// - [`Self`] must have the same size and alignment as [`Self::Repr`]. +/// - [`Self`] and [`Self::Repr`] must have the [round-trip transmutability]. +/// +/// # Limitations +/// +/// Because C primitives are used to implement the atomic operations, and a C function requires a +/// valid object of a type to operate on (i.e. no `MaybeUninit<_>`), hence at the Rust <-> C +/// surface, only types with no uninitialized bits can be passed. As a result, types like `(u8, +/// u16)` (a tuple with a `MaybeUninit` hole in it) are currently not supported. Note that +/// technically these types can be supported if some APIs are removed for them and the inner +/// implementation is tweaked, but the justification of support such a type is not strong enough at +/// the moment. This should be resolved if there is an implementation for `MaybeUninit` as +/// `AtomicImpl`. +/// +/// [`transmute()`]: core::mem::transmute +/// [round-trip transmutability]: AllowAtomic#round-trip-transmutability +pub unsafe trait AllowAtomic: Sized + Send + Copy { + /// The backing atomic implementation type. + type Repr: AtomicImpl; +} + +#[inline(always)] +const fn into_repr(v: T) -> T::Repr { + // SAFETY: Per the safety requirement of `AllowAtomic`, the transmute operation is sound. + unsafe { core::mem::transmute_copy(&v) } +} + +/// # Safety +/// +/// `r` must be a valid bit pattern of `T`. +#[inline(always)] +const unsafe fn from_repr(r: T::Repr) -> T { + // SAFETY: Per the safety requirement of the function, the transmute operation is sound. + unsafe { core::mem::transmute_copy(&r) } +} + +impl Atomic { + /// Creates a new atomic. + pub const fn new(v: T) -> Self { + Self(UnsafeCell::new(v)) + } + + /// Creates a reference to [`Self`] from a pointer. + /// + /// # Safety + /// + /// - `ptr` has to be a valid pointer. + /// - `ptr` has to be valid for both reads and writes for the whole lifetime `'a`. + /// - For the duration of `'a`, other accesses to the object cannot cause data races (defined + /// by [`LKMM`]) against atomic operations on the returned reference. Note that if all other + /// accesses are atomic, then this safety requirement is trivially fulfilled. + /// + /// [`LKMM`]: srctree/tools/memory-model + /// + /// # Examples + /// + /// Using [`Atomic::from_ptr()`] combined with [`Atomic::load()`] or [`Atomic::store()`] can + /// achieve the same functionality as `READ_ONCE()`/`smp_load_acquire()` or + /// `WRITE_ONCE()`/`smp_store_release()` in C side: + /// + /// ```rust + /// # use kernel::types::Opaque; + /// use kernel::sync::atomic::{Atomic, Relaxed, Release}; + /// + /// // Assume there is a C struct `Foo`. + /// mod cbindings { + /// #[repr(C)] + /// pub(crate) struct foo { pub(crate) a: i32, pub(crate) b: i32 } + /// } + /// + /// let tmp = Opaque::new(cbindings::foo { a: 1, b: 2}); + /// + /// // struct foo *foo_ptr = ..; + /// let foo_ptr = tmp.get(); + /// + /// // SAFETY: `foo_ptr` is a valid pointer, and `.a` is in bounds. + /// let foo_a_ptr = unsafe { &raw mut (*foo_ptr).a }; + /// + /// // a = READ_ONCE(foo_ptr->a); + /// // + /// // SAFETY: `foo_a_ptr` is a valid pointer for read, and all accesses on it is atomic, so no + /// // data race. + /// let a = unsafe { Atomic::from_ptr(foo_a_ptr) }.load(Relaxed); + /// # assert_eq!(a, 1); + /// + /// // smp_store_release(&foo_ptr->a, 2); + /// // + /// // SAFETY: `foo_a_ptr` is a valid pointer for write, and all accesses on it is atomic, so + /// // no data race. + /// unsafe { Atomic::from_ptr(foo_a_ptr) }.store(2, Release); + /// ``` + /// + /// However, this should be only used when communicating with C side or manipulating a C struct. + pub unsafe fn from_ptr<'a>(ptr: *mut T) -> &'a Self + where + T: Sync, + { + // CAST: `T` is transparent to `Atomic`. + // SAFETY: Per function safety requirement, `ptr` is a valid pointer and the object will + // live long enough. It's safe to return a `&Atomic` because function safety requirement + // guarantees other accesses won't cause data races. + unsafe { &*ptr.cast::() } + } + + /// Returns a pointer to the underlying atomic variable. + /// + /// Extra safety requirement on using the return pointer: the operations done via the pointer + /// cannot cause data races defined by [`LKMM`]. + /// + /// [`LKMM`]: srctree/tools/memory-model + pub const fn as_ptr(&self) -> *mut T { + self.0.get() + } + + /// Returns a mutable reference to the underlying atomic variable. + /// + /// This is safe because the mutable reference of the atomic variable guarantees the exclusive + /// access. + pub fn get_mut(&mut self) -> &mut T { + // SAFETY: `self.as_ptr()` is a valid pointer to `T`. `&mut self` guarantees the exclusive + // access, so it's safe to reborrow mutably. + unsafe { &mut *self.as_ptr() } + } +} + +impl Atomic +where + T::Repr: AtomicHasBasicOps, +{ + /// Loads the value from the atomic variable. + /// + /// # Examples + /// + /// Simple usages: + /// + /// ```rust + /// use kernel::sync::atomic::{Atomic, Relaxed}; + /// + /// let x = Atomic::new(42i32); + /// + /// assert_eq!(42, x.load(Relaxed)); + /// + /// let x = Atomic::new(42i64); + /// + /// assert_eq!(42, x.load(Relaxed)); + /// ``` + #[doc(alias("atomic_read", "atomic64_read"))] + #[inline(always)] + pub fn load(&self, _: Ordering) -> T { + // CAST: Per the safety requirement of `AllowAtomic`, a valid pointer of `T` is also a + // valid pointer of `T::Repr`. + let a = self.as_ptr().cast::(); + + // SAFETY: + // - For calling the atomic_read*() function: + // - `a` is a valid pointer for the function per the CAST justification above. + // - Per the type guarantees, the following atomic operation won't cause data races. + // - For extra safety requirement of usage on pointers returned by `self.as_ptr()`: + // - Atomic operations are used here. + let v = unsafe { + match Ordering::TYPE { + OrderingType::Relaxed => T::Repr::atomic_read(a), + OrderingType::Acquire => T::Repr::atomic_read_acquire(a), + _ => build_error!("Wrong ordering"), + } + }; + + // SAFETY: The atomic variable holds a valid `T`, so `v` is a valid bit pattern of `T`, + // therefore it's safe to call `from_repr()`. + unsafe { from_repr(v) } + } + + /// Stores a value to the atomic variable. + /// + /// # Examples + /// + /// ```rust + /// use kernel::sync::atomic::{Atomic, Relaxed}; + /// + /// let x = Atomic::new(42i32); + /// + /// assert_eq!(42, x.load(Relaxed)); + /// + /// x.store(43, Relaxed); + /// + /// assert_eq!(43, x.load(Relaxed)); + /// ``` + #[doc(alias("atomic_set", "atomic64_set"))] + #[inline(always)] + pub fn store(&self, v: T, _: Ordering) { + let v = into_repr(v); + // CAST: Per the safety requirement of `AllowAtomic`, a valid pointer of `T` is also a + // valid pointer of `T::Repr`. + let a = self.as_ptr().cast::(); + + // SAFETY: + // - For calling the atomic_set*() function: + // - `a` is a valid pointer for the function per the CAST justification above. + // - Per the type guarantees, the following atomic operation won't cause data races. + // - For extra safety requirement of usage on pointers returned by `self.as_ptr()`: + // - Atomic operations are used here. + // - For the bit validity of `Atomic`: + // - `v` is a valid bit pattern of `T`, so it's sound to store it in an `Atomic`. + unsafe { + match Ordering::TYPE { + OrderingType::Relaxed => T::Repr::atomic_set(a, v), + OrderingType::Release => T::Repr::atomic_set_release(a, v), + _ => build_error!("Wrong ordering"), + } + }; + } +} -- 2.39.5 (Apple Git-154)