1 // SPDX-License-Identifier: GPL-2.0 2 3 //! Atomic primitives. 4 //! 5 //! These primitives have the same semantics as their C counterparts: and the precise definitions of 6 //! semantics can be found at [`LKMM`]. Note that Linux Kernel Memory (Consistency) Model is the 7 //! only model for Rust code in kernel, and Rust's own atomics should be avoided. 8 //! 9 //! # Data races 10 //! 11 //! [`LKMM`] atomics have different rules regarding data races: 12 //! 13 //! - A normal write from C side is treated as an atomic write if 14 //! CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC=y. 15 //! - Mixed-size atomic accesses don't cause data races. 16 //! 17 //! [`LKMM`]: srctree/tools/memory-model/ 18 19 #[allow(dead_code, unreachable_pub)] 20 mod internal; 21 pub mod ordering; 22 mod predefine; 23 24 pub use internal::AtomicImpl; 25 pub use ordering::{Acquire, Full, Relaxed, Release}; 26 27 use crate::build_error; 28 use internal::{AtomicBasicOps, AtomicExchangeOps, AtomicRepr}; 29 use ordering::OrderingType; 30 31 /// A memory location which can be safely modified from multiple execution contexts. 32 /// 33 /// This has the same size, alignment and bit validity as the underlying type `T`. And it disables 34 /// niche optimization for the same reason as [`UnsafeCell`]. 35 /// 36 /// The atomic operations are implemented in a way that is fully compatible with the [Linux Kernel 37 /// Memory (Consistency) Model][LKMM], hence they should be modeled as the corresponding 38 /// [`LKMM`][LKMM] atomic primitives. With the help of [`Atomic::from_ptr()`] and 39 /// [`Atomic::as_ptr()`], this provides a way to interact with [C-side atomic operations] 40 /// (including those without the `atomic` prefix, e.g. `READ_ONCE()`, `WRITE_ONCE()`, 41 /// `smp_load_acquire()` and `smp_store_release()`). 42 /// 43 /// # Invariants 44 /// 45 /// `self.0` is a valid `T`. 46 /// 47 /// [`UnsafeCell`]: core::cell::UnsafeCell 48 /// [LKMM]: srctree/tools/memory-model/ 49 /// [C-side atomic operations]: srctree/Documentation/atomic_t.txt 50 #[repr(transparent)] 51 pub struct Atomic<T: AtomicType>(AtomicRepr<T::Repr>); 52 53 // SAFETY: `Atomic<T>` is safe to share among execution contexts because all accesses are atomic. 54 unsafe impl<T: AtomicType> Sync for Atomic<T> {} 55 56 /// Types that support basic atomic operations. 57 /// 58 /// # Round-trip transmutability 59 /// 60 /// `T` is round-trip transmutable to `U` if and only if both of these properties hold: 61 /// 62 /// - Any valid bit pattern for `T` is also a valid bit pattern for `U`. 63 /// - Transmuting (e.g. using [`transmute()`]) a value of type `T` to `U` and then to `T` again 64 /// yields a value that is in all aspects equivalent to the original value. 65 /// 66 /// # Safety 67 /// 68 /// - [`Self`] must have the same size and alignment as [`Self::Repr`]. 69 /// - [`Self`] must be [round-trip transmutable] to [`Self::Repr`]. 70 /// 71 /// Note that this is more relaxed than requiring the bi-directional transmutability (i.e. 72 /// [`transmute()`] is always sound between `U` and `T`) because of the support for atomic 73 /// variables over unit-only enums, see [Examples]. 74 /// 75 /// # Limitations 76 /// 77 /// Because C primitives are used to implement the atomic operations, and a C function requires a 78 /// valid object of a type to operate on (i.e. no `MaybeUninit<_>`), hence at the Rust <-> C 79 /// surface, only types with all the bits initialized can be passed. As a result, types like `(u8, 80 /// u16)` (padding bytes are uninitialized) are currently not supported. 81 /// 82 /// # Examples 83 /// 84 /// A unit-only enum that implements [`AtomicType`]: 85 /// 86 /// ``` 87 /// use kernel::sync::atomic::{AtomicType, Atomic, Relaxed}; 88 /// 89 /// #[derive(Clone, Copy, PartialEq, Eq)] 90 /// #[repr(i32)] 91 /// enum State { 92 /// Uninit = 0, 93 /// Working = 1, 94 /// Done = 2, 95 /// }; 96 /// 97 /// // SAFETY: `State` and `i32` has the same size and alignment, and it's round-trip 98 /// // transmutable to `i32`. 99 /// unsafe impl AtomicType for State { 100 /// type Repr = i32; 101 /// } 102 /// 103 /// let s = Atomic::new(State::Uninit); 104 /// 105 /// assert_eq!(State::Uninit, s.load(Relaxed)); 106 /// ``` 107 /// [`transmute()`]: core::mem::transmute 108 /// [round-trip transmutable]: AtomicType#round-trip-transmutability 109 /// [Examples]: AtomicType#examples 110 pub unsafe trait AtomicType: Sized + Send + Copy { 111 /// The backing atomic implementation type. 112 type Repr: AtomicImpl; 113 } 114 115 #[inline(always)] 116 const fn into_repr<T: AtomicType>(v: T) -> T::Repr { 117 // SAFETY: Per the safety requirement of `AtomicType`, `T` is round-trip transmutable to 118 // `T::Repr`, therefore the transmute operation is sound. 119 unsafe { core::mem::transmute_copy(&v) } 120 } 121 122 /// # Safety 123 /// 124 /// `r` must be a valid bit pattern of `T`. 125 #[inline(always)] 126 const unsafe fn from_repr<T: AtomicType>(r: T::Repr) -> T { 127 // SAFETY: Per the safety requirement of the function, the transmute operation is sound. 128 unsafe { core::mem::transmute_copy(&r) } 129 } 130 131 impl<T: AtomicType> Atomic<T> { 132 /// Creates a new atomic `T`. 133 pub const fn new(v: T) -> Self { 134 // INVARIANT: Per the safety requirement of `AtomicType`, `into_repr(v)` is a valid `T`. 135 Self(AtomicRepr::new(into_repr(v))) 136 } 137 138 /// Creates a reference to an atomic `T` from a pointer of `T`. 139 /// 140 /// This usually is used when communicating with C side or manipulating a C struct, see 141 /// examples below. 142 /// 143 /// # Safety 144 /// 145 /// - `ptr` is aligned to `align_of::<T>()`. 146 /// - `ptr` is valid for reads and writes for `'a`. 147 /// - For the duration of `'a`, other accesses to `*ptr` must not cause data races (defined 148 /// by [`LKMM`]) against atomic operations on the returned reference. Note that if all other 149 /// accesses are atomic, then this safety requirement is trivially fulfilled. 150 /// 151 /// [`LKMM`]: srctree/tools/memory-model 152 /// 153 /// # Examples 154 /// 155 /// Using [`Atomic::from_ptr()`] combined with [`Atomic::load()`] or [`Atomic::store()`] can 156 /// achieve the same functionality as `READ_ONCE()`/`smp_load_acquire()` or 157 /// `WRITE_ONCE()`/`smp_store_release()` in C side: 158 /// 159 /// ``` 160 /// # use kernel::types::Opaque; 161 /// use kernel::sync::atomic::{Atomic, Relaxed, Release}; 162 /// 163 /// // Assume there is a C struct `foo`. 164 /// mod cbindings { 165 /// #[repr(C)] 166 /// pub(crate) struct foo { 167 /// pub(crate) a: i32, 168 /// pub(crate) b: i32 169 /// } 170 /// } 171 /// 172 /// let tmp = Opaque::new(cbindings::foo { a: 1, b: 2 }); 173 /// 174 /// // struct foo *foo_ptr = ..; 175 /// let foo_ptr = tmp.get(); 176 /// 177 /// // SAFETY: `foo_ptr` is valid, and `.a` is in bounds. 178 /// let foo_a_ptr = unsafe { &raw mut (*foo_ptr).a }; 179 /// 180 /// // a = READ_ONCE(foo_ptr->a); 181 /// // 182 /// // SAFETY: `foo_a_ptr` is valid for read, and all other accesses on it is atomic, so no 183 /// // data race. 184 /// let a = unsafe { Atomic::from_ptr(foo_a_ptr) }.load(Relaxed); 185 /// # assert_eq!(a, 1); 186 /// 187 /// // smp_store_release(&foo_ptr->a, 2); 188 /// // 189 /// // SAFETY: `foo_a_ptr` is valid for writes, and all other accesses on it is atomic, so 190 /// // no data race. 191 /// unsafe { Atomic::from_ptr(foo_a_ptr) }.store(2, Release); 192 /// ``` 193 pub unsafe fn from_ptr<'a>(ptr: *mut T) -> &'a Self 194 where 195 T: Sync, 196 { 197 // CAST: `T` and `Atomic<T>` have the same size, alignment and bit validity. 198 // SAFETY: Per function safety requirement, `ptr` is a valid pointer and the object will 199 // live long enough. It's safe to return a `&Atomic<T>` because function safety requirement 200 // guarantees other accesses won't cause data races. 201 unsafe { &*ptr.cast::<Self>() } 202 } 203 204 /// Returns a pointer to the underlying atomic `T`. 205 /// 206 /// Note that use of the return pointer must not cause data races defined by [`LKMM`]. 207 /// 208 /// # Guarantees 209 /// 210 /// The returned pointer is valid and properly aligned (i.e. aligned to [`align_of::<T>()`]). 211 /// 212 /// [`LKMM`]: srctree/tools/memory-model 213 /// [`align_of::<T>()`]: core::mem::align_of 214 pub const fn as_ptr(&self) -> *mut T { 215 // GUARANTEE: Per the function guarantee of `AtomicRepr::as_ptr()`, the `self.0.as_ptr()` 216 // must be a valid and properly aligned pointer for `T::Repr`, and per the safety guarantee 217 // of `AtomicType`, it's a valid and properly aligned pointer of `T`. 218 self.0.as_ptr().cast() 219 } 220 221 /// Returns a mutable reference to the underlying atomic `T`. 222 /// 223 /// This is safe because the mutable reference of the atomic `T` guarantees exclusive access. 224 pub fn get_mut(&mut self) -> &mut T { 225 // CAST: `T` and `T::Repr` has the same size and alignment per the safety requirement of 226 // `AtomicType`, and per the type invariants `self.0` is a valid `T`, therefore the casting 227 // result is a valid pointer of `T`. 228 // SAFETY: The pointer is valid per the CAST comment above, and the mutable reference 229 // guarantees exclusive access. 230 unsafe { &mut *self.0.as_ptr().cast() } 231 } 232 } 233 234 impl<T: AtomicType> Atomic<T> 235 where 236 T::Repr: AtomicBasicOps, 237 { 238 /// Loads the value from the atomic `T`. 239 /// 240 /// # Examples 241 /// 242 /// ``` 243 /// use kernel::sync::atomic::{Atomic, Relaxed}; 244 /// 245 /// let x = Atomic::new(42i32); 246 /// 247 /// assert_eq!(42, x.load(Relaxed)); 248 /// 249 /// let x = Atomic::new(42i64); 250 /// 251 /// assert_eq!(42, x.load(Relaxed)); 252 /// ``` 253 #[doc(alias("atomic_read", "atomic64_read"))] 254 #[inline(always)] 255 pub fn load<Ordering: ordering::AcquireOrRelaxed>(&self, _: Ordering) -> T { 256 let v = { 257 match Ordering::TYPE { 258 OrderingType::Relaxed => T::Repr::atomic_read(&self.0), 259 OrderingType::Acquire => T::Repr::atomic_read_acquire(&self.0), 260 _ => build_error!("Wrong ordering"), 261 } 262 }; 263 264 // SAFETY: `v` comes from reading `self.0`, which is a valid `T` per the type invariants. 265 unsafe { from_repr(v) } 266 } 267 268 /// Stores a value to the atomic `T`. 269 /// 270 /// # Examples 271 /// 272 /// ``` 273 /// use kernel::sync::atomic::{Atomic, Relaxed}; 274 /// 275 /// let x = Atomic::new(42i32); 276 /// 277 /// assert_eq!(42, x.load(Relaxed)); 278 /// 279 /// x.store(43, Relaxed); 280 /// 281 /// assert_eq!(43, x.load(Relaxed)); 282 /// ``` 283 #[doc(alias("atomic_set", "atomic64_set"))] 284 #[inline(always)] 285 pub fn store<Ordering: ordering::ReleaseOrRelaxed>(&self, v: T, _: Ordering) { 286 let v = into_repr(v); 287 288 // INVARIANT: `v` is a valid `T`, and is stored to `self.0` by `atomic_set*()`. 289 match Ordering::TYPE { 290 OrderingType::Relaxed => T::Repr::atomic_set(&self.0, v), 291 OrderingType::Release => T::Repr::atomic_set_release(&self.0, v), 292 _ => build_error!("Wrong ordering"), 293 } 294 } 295 } 296 297 impl<T: AtomicType> Atomic<T> 298 where 299 T::Repr: AtomicExchangeOps, 300 { 301 /// Atomic exchange. 302 /// 303 /// Atomically updates `*self` to `v` and returns the old value of `*self`. 304 /// 305 /// # Examples 306 /// 307 /// ``` 308 /// use kernel::sync::atomic::{Atomic, Acquire, Relaxed}; 309 /// 310 /// let x = Atomic::new(42); 311 /// 312 /// assert_eq!(42, x.xchg(52, Acquire)); 313 /// assert_eq!(52, x.load(Relaxed)); 314 /// ``` 315 #[doc(alias("atomic_xchg", "atomic64_xchg", "swap"))] 316 #[inline(always)] 317 pub fn xchg<Ordering: ordering::Ordering>(&self, v: T, _: Ordering) -> T { 318 let v = into_repr(v); 319 320 // INVARIANT: `self.0` is a valid `T` after `atomic_xchg*()` because `v` is transmutable to 321 // `T`. 322 let ret = { 323 match Ordering::TYPE { 324 OrderingType::Full => T::Repr::atomic_xchg(&self.0, v), 325 OrderingType::Acquire => T::Repr::atomic_xchg_acquire(&self.0, v), 326 OrderingType::Release => T::Repr::atomic_xchg_release(&self.0, v), 327 OrderingType::Relaxed => T::Repr::atomic_xchg_relaxed(&self.0, v), 328 } 329 }; 330 331 // SAFETY: `ret` comes from reading `*self`, which is a valid `T` per type invariants. 332 unsafe { from_repr(ret) } 333 } 334 335 /// Atomic compare and exchange. 336 /// 337 /// If `*self` == `old`, atomically updates `*self` to `new`. Otherwise, `*self` is not 338 /// modified. 339 /// 340 /// Compare: The comparison is done via the byte level comparison between `*self` and `old`. 341 /// 342 /// Ordering: When succeeds, provides the corresponding ordering as the `Ordering` type 343 /// parameter indicates, and a failed one doesn't provide any ordering, the load part of a 344 /// failed cmpxchg is a [`Relaxed`] load. 345 /// 346 /// Returns `Ok(value)` if cmpxchg succeeds, and `value` is guaranteed to be equal to `old`, 347 /// otherwise returns `Err(value)`, and `value` is the current value of `*self`. 348 /// 349 /// # Examples 350 /// 351 /// ``` 352 /// use kernel::sync::atomic::{Atomic, Full, Relaxed}; 353 /// 354 /// let x = Atomic::new(42); 355 /// 356 /// // Checks whether cmpxchg succeeded. 357 /// let success = x.cmpxchg(52, 64, Relaxed).is_ok(); 358 /// # assert!(!success); 359 /// 360 /// // Checks whether cmpxchg failed. 361 /// let failure = x.cmpxchg(52, 64, Relaxed).is_err(); 362 /// # assert!(failure); 363 /// 364 /// // Uses the old value if failed, probably re-try cmpxchg. 365 /// match x.cmpxchg(52, 64, Relaxed) { 366 /// Ok(_) => { }, 367 /// Err(old) => { 368 /// // do something with `old`. 369 /// # assert_eq!(old, 42); 370 /// } 371 /// } 372 /// 373 /// // Uses the latest value regardlessly, same as atomic_cmpxchg() in C. 374 /// let latest = x.cmpxchg(42, 64, Full).unwrap_or_else(|old| old); 375 /// # assert_eq!(42, latest); 376 /// assert_eq!(64, x.load(Relaxed)); 377 /// ``` 378 /// 379 /// [`Relaxed`]: ordering::Relaxed 380 #[doc(alias( 381 "atomic_cmpxchg", 382 "atomic64_cmpxchg", 383 "atomic_try_cmpxchg", 384 "atomic64_try_cmpxchg", 385 "compare_exchange" 386 ))] 387 #[inline(always)] 388 pub fn cmpxchg<Ordering: ordering::Ordering>( 389 &self, 390 mut old: T, 391 new: T, 392 o: Ordering, 393 ) -> Result<T, T> { 394 // Note on code generation: 395 // 396 // try_cmpxchg() is used to implement cmpxchg(), and if the helper functions are inlined, 397 // the compiler is able to figure out that branch is not needed if the users don't care 398 // about whether the operation succeeds or not. One exception is on x86, due to commit 399 // 44fe84459faf ("locking/atomic: Fix atomic_try_cmpxchg() semantics"), the 400 // atomic_try_cmpxchg() on x86 has a branch even if the caller doesn't care about the 401 // success of cmpxchg and only wants to use the old value. For example, for code like: 402 // 403 // let latest = x.cmpxchg(42, 64, Full).unwrap_or_else(|old| old); 404 // 405 // It will still generate code: 406 // 407 // movl $0x40, %ecx 408 // movl $0x34, %eax 409 // lock 410 // cmpxchgl %ecx, 0x4(%rsp) 411 // jne 1f 412 // 2: 413 // ... 414 // 1: movl %eax, %ecx 415 // jmp 2b 416 // 417 // This might be "fixed" by introducing a try_cmpxchg_exclusive() that knows the "*old" 418 // location in the C function is always safe to write. 419 if self.try_cmpxchg(&mut old, new, o) { 420 Ok(old) 421 } else { 422 Err(old) 423 } 424 } 425 426 /// Atomic compare and exchange and returns whether the operation succeeds. 427 /// 428 /// If `*self` == `old`, atomically updates `*self` to `new`. Otherwise, `*self` is not 429 /// modified, `*old` is updated to the current value of `*self`. 430 /// 431 /// "Compare" and "Ordering" part are the same as [`Atomic::cmpxchg()`]. 432 /// 433 /// Returns `true` means the cmpxchg succeeds otherwise returns `false`. 434 #[inline(always)] 435 fn try_cmpxchg<Ordering: ordering::Ordering>(&self, old: &mut T, new: T, _: Ordering) -> bool { 436 let mut tmp = into_repr(*old); 437 let new = into_repr(new); 438 439 // INVARIANT: `self.0` is a valid `T` after `atomic_try_cmpxchg*()` because `new` is 440 // transmutable to `T`. 441 let ret = { 442 match Ordering::TYPE { 443 OrderingType::Full => T::Repr::atomic_try_cmpxchg(&self.0, &mut tmp, new), 444 OrderingType::Acquire => { 445 T::Repr::atomic_try_cmpxchg_acquire(&self.0, &mut tmp, new) 446 } 447 OrderingType::Release => { 448 T::Repr::atomic_try_cmpxchg_release(&self.0, &mut tmp, new) 449 } 450 OrderingType::Relaxed => { 451 T::Repr::atomic_try_cmpxchg_relaxed(&self.0, &mut tmp, new) 452 } 453 } 454 }; 455 456 // SAFETY: `tmp` comes from reading `*self`, which is a valid `T` per type invariants. 457 *old = unsafe { from_repr(tmp) }; 458 459 ret 460 } 461 } 462