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 mod internal; 20 pub mod ordering; 21 mod predefine; 22 23 pub use internal::AtomicImpl; 24 pub use ordering::{Acquire, Full, Relaxed, Release}; 25 pub(crate) use internal::{AtomicArithmeticOps, AtomicBasicOps, AtomicExchangeOps}; 26 27 use crate::build_error; 28 use internal::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 /// Types that support atomic add operations. 116 /// 117 /// # Safety 118 /// 119 // TODO: Properly defines `wrapping_add` in the following comment. 120 /// `wrapping_add` any value of type `Self::Repr::Delta` obtained by [`Self::rhs_into_delta()`] to 121 /// any value of type `Self::Repr` obtained through transmuting a value of type `Self` to must 122 /// yield a value with a bit pattern also valid for `Self`. 123 pub unsafe trait AtomicAdd<Rhs = Self>: AtomicType { 124 /// Converts `Rhs` into the `Delta` type of the atomic implementation. 125 fn rhs_into_delta(rhs: Rhs) -> <Self::Repr as AtomicImpl>::Delta; 126 } 127 128 #[inline(always)] 129 const fn into_repr<T: AtomicType>(v: T) -> T::Repr { 130 // SAFETY: Per the safety requirement of `AtomicType`, `T` is round-trip transmutable to 131 // `T::Repr`, therefore the transmute operation is sound. 132 unsafe { core::mem::transmute_copy(&v) } 133 } 134 135 /// # Safety 136 /// 137 /// `r` must be a valid bit pattern of `T`. 138 #[inline(always)] 139 const unsafe fn from_repr<T: AtomicType>(r: T::Repr) -> T { 140 // SAFETY: Per the safety requirement of the function, the transmute operation is sound. 141 unsafe { core::mem::transmute_copy(&r) } 142 } 143 144 impl<T: AtomicType> Atomic<T> { 145 /// Creates a new atomic `T`. 146 pub const fn new(v: T) -> Self { 147 // INVARIANT: Per the safety requirement of `AtomicType`, `into_repr(v)` is a valid `T`. 148 Self(AtomicRepr::new(into_repr(v))) 149 } 150 151 /// Creates a reference to an atomic `T` from a pointer of `T`. 152 /// 153 /// This usually is used when communicating with C side or manipulating a C struct, see 154 /// examples below. 155 /// 156 /// # Safety 157 /// 158 /// - `ptr` is aligned to `align_of::<T>()`. 159 /// - `ptr` is valid for reads and writes for `'a`. 160 /// - For the duration of `'a`, other accesses to `*ptr` must not cause data races (defined 161 /// by [`LKMM`]) against atomic operations on the returned reference. Note that if all other 162 /// accesses are atomic, then this safety requirement is trivially fulfilled. 163 /// 164 /// [`LKMM`]: srctree/tools/memory-model 165 /// 166 /// # Examples 167 /// 168 /// Using [`Atomic::from_ptr()`] combined with [`Atomic::load()`] or [`Atomic::store()`] can 169 /// achieve the same functionality as `READ_ONCE()`/`smp_load_acquire()` or 170 /// `WRITE_ONCE()`/`smp_store_release()` in C side: 171 /// 172 /// ``` 173 /// # use kernel::types::Opaque; 174 /// use kernel::sync::atomic::{Atomic, Relaxed, Release}; 175 /// 176 /// // Assume there is a C struct `foo`. 177 /// mod cbindings { 178 /// #[repr(C)] 179 /// pub(crate) struct foo { 180 /// pub(crate) a: i32, 181 /// pub(crate) b: i32 182 /// } 183 /// } 184 /// 185 /// let tmp = Opaque::new(cbindings::foo { a: 1, b: 2 }); 186 /// 187 /// // struct foo *foo_ptr = ..; 188 /// let foo_ptr = tmp.get(); 189 /// 190 /// // SAFETY: `foo_ptr` is valid, and `.a` is in bounds. 191 /// let foo_a_ptr = unsafe { &raw mut (*foo_ptr).a }; 192 /// 193 /// // a = READ_ONCE(foo_ptr->a); 194 /// // 195 /// // SAFETY: `foo_a_ptr` is valid for read, and all other accesses on it is atomic, so no 196 /// // data race. 197 /// let a = unsafe { Atomic::from_ptr(foo_a_ptr) }.load(Relaxed); 198 /// # assert_eq!(a, 1); 199 /// 200 /// // smp_store_release(&foo_ptr->a, 2); 201 /// // 202 /// // SAFETY: `foo_a_ptr` is valid for writes, and all other accesses on it is atomic, so 203 /// // no data race. 204 /// unsafe { Atomic::from_ptr(foo_a_ptr) }.store(2, Release); 205 /// ``` 206 pub unsafe fn from_ptr<'a>(ptr: *mut T) -> &'a Self 207 where 208 T: Sync, 209 { 210 // CAST: `T` and `Atomic<T>` have the same size, alignment and bit validity. 211 // SAFETY: Per function safety requirement, `ptr` is a valid pointer and the object will 212 // live long enough. It's safe to return a `&Atomic<T>` because function safety requirement 213 // guarantees other accesses won't cause data races. 214 unsafe { &*ptr.cast::<Self>() } 215 } 216 217 /// Returns a pointer to the underlying atomic `T`. 218 /// 219 /// Note that use of the return pointer must not cause data races defined by [`LKMM`]. 220 /// 221 /// # Guarantees 222 /// 223 /// The returned pointer is valid and properly aligned (i.e. aligned to [`align_of::<T>()`]). 224 /// 225 /// [`LKMM`]: srctree/tools/memory-model 226 /// [`align_of::<T>()`]: core::mem::align_of 227 pub const fn as_ptr(&self) -> *mut T { 228 // GUARANTEE: Per the function guarantee of `AtomicRepr::as_ptr()`, the `self.0.as_ptr()` 229 // must be a valid and properly aligned pointer for `T::Repr`, and per the safety guarantee 230 // of `AtomicType`, it's a valid and properly aligned pointer of `T`. 231 self.0.as_ptr().cast() 232 } 233 234 /// Returns a mutable reference to the underlying atomic `T`. 235 /// 236 /// This is safe because the mutable reference of the atomic `T` guarantees exclusive access. 237 pub fn get_mut(&mut self) -> &mut T { 238 // CAST: `T` and `T::Repr` has the same size and alignment per the safety requirement of 239 // `AtomicType`, and per the type invariants `self.0` is a valid `T`, therefore the casting 240 // result is a valid pointer of `T`. 241 // SAFETY: The pointer is valid per the CAST comment above, and the mutable reference 242 // guarantees exclusive access. 243 unsafe { &mut *self.0.as_ptr().cast() } 244 } 245 } 246 247 impl<T: AtomicType> Atomic<T> 248 where 249 T::Repr: AtomicBasicOps, 250 { 251 /// Loads the value from the atomic `T`. 252 /// 253 /// # Examples 254 /// 255 /// ``` 256 /// use kernel::sync::atomic::{Atomic, Relaxed}; 257 /// 258 /// let x = Atomic::new(42i32); 259 /// 260 /// assert_eq!(42, x.load(Relaxed)); 261 /// 262 /// let x = Atomic::new(42i64); 263 /// 264 /// assert_eq!(42, x.load(Relaxed)); 265 /// ``` 266 #[doc(alias("atomic_read", "atomic64_read"))] 267 #[inline(always)] 268 pub fn load<Ordering: ordering::AcquireOrRelaxed>(&self, _: Ordering) -> T { 269 let v = { 270 match Ordering::TYPE { 271 OrderingType::Relaxed => T::Repr::atomic_read(&self.0), 272 OrderingType::Acquire => T::Repr::atomic_read_acquire(&self.0), 273 _ => build_error!("Wrong ordering"), 274 } 275 }; 276 277 // SAFETY: `v` comes from reading `self.0`, which is a valid `T` per the type invariants. 278 unsafe { from_repr(v) } 279 } 280 281 /// Stores a value to the atomic `T`. 282 /// 283 /// # Examples 284 /// 285 /// ``` 286 /// use kernel::sync::atomic::{Atomic, Relaxed}; 287 /// 288 /// let x = Atomic::new(42i32); 289 /// 290 /// assert_eq!(42, x.load(Relaxed)); 291 /// 292 /// x.store(43, Relaxed); 293 /// 294 /// assert_eq!(43, x.load(Relaxed)); 295 /// ``` 296 #[doc(alias("atomic_set", "atomic64_set"))] 297 #[inline(always)] 298 pub fn store<Ordering: ordering::ReleaseOrRelaxed>(&self, v: T, _: Ordering) { 299 let v = into_repr(v); 300 301 // INVARIANT: `v` is a valid `T`, and is stored to `self.0` by `atomic_set*()`. 302 match Ordering::TYPE { 303 OrderingType::Relaxed => T::Repr::atomic_set(&self.0, v), 304 OrderingType::Release => T::Repr::atomic_set_release(&self.0, v), 305 _ => build_error!("Wrong ordering"), 306 } 307 } 308 } 309 310 impl<T: AtomicType> Atomic<T> 311 where 312 T::Repr: AtomicExchangeOps, 313 { 314 /// Atomic exchange. 315 /// 316 /// Atomically updates `*self` to `v` and returns the old value of `*self`. 317 /// 318 /// # Examples 319 /// 320 /// ``` 321 /// use kernel::sync::atomic::{Atomic, Acquire, Relaxed}; 322 /// 323 /// let x = Atomic::new(42); 324 /// 325 /// assert_eq!(42, x.xchg(52, Acquire)); 326 /// assert_eq!(52, x.load(Relaxed)); 327 /// ``` 328 #[doc(alias("atomic_xchg", "atomic64_xchg", "swap"))] 329 #[inline(always)] 330 pub fn xchg<Ordering: ordering::Ordering>(&self, v: T, _: Ordering) -> T { 331 let v = into_repr(v); 332 333 // INVARIANT: `self.0` is a valid `T` after `atomic_xchg*()` because `v` is transmutable to 334 // `T`. 335 let ret = { 336 match Ordering::TYPE { 337 OrderingType::Full => T::Repr::atomic_xchg(&self.0, v), 338 OrderingType::Acquire => T::Repr::atomic_xchg_acquire(&self.0, v), 339 OrderingType::Release => T::Repr::atomic_xchg_release(&self.0, v), 340 OrderingType::Relaxed => T::Repr::atomic_xchg_relaxed(&self.0, v), 341 } 342 }; 343 344 // SAFETY: `ret` comes from reading `*self`, which is a valid `T` per type invariants. 345 unsafe { from_repr(ret) } 346 } 347 348 /// Atomic compare and exchange. 349 /// 350 /// If `*self` == `old`, atomically updates `*self` to `new`. Otherwise, `*self` is not 351 /// modified. 352 /// 353 /// Compare: The comparison is done via the byte level comparison between `*self` and `old`. 354 /// 355 /// Ordering: When succeeds, provides the corresponding ordering as the `Ordering` type 356 /// parameter indicates, and a failed one doesn't provide any ordering, the load part of a 357 /// failed cmpxchg is a [`Relaxed`] load. 358 /// 359 /// Returns `Ok(value)` if cmpxchg succeeds, and `value` is guaranteed to be equal to `old`, 360 /// otherwise returns `Err(value)`, and `value` is the current value of `*self`. 361 /// 362 /// # Examples 363 /// 364 /// ``` 365 /// use kernel::sync::atomic::{Atomic, Full, Relaxed}; 366 /// 367 /// let x = Atomic::new(42); 368 /// 369 /// // Checks whether cmpxchg succeeded. 370 /// let success = x.cmpxchg(52, 64, Relaxed).is_ok(); 371 /// # assert!(!success); 372 /// 373 /// // Checks whether cmpxchg failed. 374 /// let failure = x.cmpxchg(52, 64, Relaxed).is_err(); 375 /// # assert!(failure); 376 /// 377 /// // Uses the old value if failed, probably re-try cmpxchg. 378 /// match x.cmpxchg(52, 64, Relaxed) { 379 /// Ok(_) => { }, 380 /// Err(old) => { 381 /// // do something with `old`. 382 /// # assert_eq!(old, 42); 383 /// } 384 /// } 385 /// 386 /// // Uses the latest value regardlessly, same as atomic_cmpxchg() in C. 387 /// let latest = x.cmpxchg(42, 64, Full).unwrap_or_else(|old| old); 388 /// # assert_eq!(42, latest); 389 /// assert_eq!(64, x.load(Relaxed)); 390 /// ``` 391 /// 392 /// [`Relaxed`]: ordering::Relaxed 393 #[doc(alias( 394 "atomic_cmpxchg", 395 "atomic64_cmpxchg", 396 "atomic_try_cmpxchg", 397 "atomic64_try_cmpxchg", 398 "compare_exchange" 399 ))] 400 #[inline(always)] 401 pub fn cmpxchg<Ordering: ordering::Ordering>( 402 &self, 403 mut old: T, 404 new: T, 405 o: Ordering, 406 ) -> Result<T, T> { 407 // Note on code generation: 408 // 409 // try_cmpxchg() is used to implement cmpxchg(), and if the helper functions are inlined, 410 // the compiler is able to figure out that branch is not needed if the users don't care 411 // about whether the operation succeeds or not. One exception is on x86, due to commit 412 // 44fe84459faf ("locking/atomic: Fix atomic_try_cmpxchg() semantics"), the 413 // atomic_try_cmpxchg() on x86 has a branch even if the caller doesn't care about the 414 // success of cmpxchg and only wants to use the old value. For example, for code like: 415 // 416 // let latest = x.cmpxchg(42, 64, Full).unwrap_or_else(|old| old); 417 // 418 // It will still generate code: 419 // 420 // movl $0x40, %ecx 421 // movl $0x34, %eax 422 // lock 423 // cmpxchgl %ecx, 0x4(%rsp) 424 // jne 1f 425 // 2: 426 // ... 427 // 1: movl %eax, %ecx 428 // jmp 2b 429 // 430 // This might be "fixed" by introducing a try_cmpxchg_exclusive() that knows the "*old" 431 // location in the C function is always safe to write. 432 if self.try_cmpxchg(&mut old, new, o) { 433 Ok(old) 434 } else { 435 Err(old) 436 } 437 } 438 439 /// Atomic compare and exchange and returns whether the operation succeeds. 440 /// 441 /// If `*self` == `old`, atomically updates `*self` to `new`. Otherwise, `*self` is not 442 /// modified, `*old` is updated to the current value of `*self`. 443 /// 444 /// "Compare" and "Ordering" part are the same as [`Atomic::cmpxchg()`]. 445 /// 446 /// Returns `true` means the cmpxchg succeeds otherwise returns `false`. 447 #[inline(always)] 448 fn try_cmpxchg<Ordering: ordering::Ordering>(&self, old: &mut T, new: T, _: Ordering) -> bool { 449 let mut tmp = into_repr(*old); 450 let new = into_repr(new); 451 452 // INVARIANT: `self.0` is a valid `T` after `atomic_try_cmpxchg*()` because `new` is 453 // transmutable to `T`. 454 let ret = { 455 match Ordering::TYPE { 456 OrderingType::Full => T::Repr::atomic_try_cmpxchg(&self.0, &mut tmp, new), 457 OrderingType::Acquire => { 458 T::Repr::atomic_try_cmpxchg_acquire(&self.0, &mut tmp, new) 459 } 460 OrderingType::Release => { 461 T::Repr::atomic_try_cmpxchg_release(&self.0, &mut tmp, new) 462 } 463 OrderingType::Relaxed => { 464 T::Repr::atomic_try_cmpxchg_relaxed(&self.0, &mut tmp, new) 465 } 466 } 467 }; 468 469 // SAFETY: `tmp` comes from reading `*self`, which is a valid `T` per type invariants. 470 *old = unsafe { from_repr(tmp) }; 471 472 ret 473 } 474 } 475 476 impl<T: AtomicType> Atomic<T> 477 where 478 T::Repr: AtomicArithmeticOps, 479 { 480 /// Atomic add. 481 /// 482 /// Atomically updates `*self` to `(*self).wrapping_add(v)`. 483 /// 484 /// # Examples 485 /// 486 /// ``` 487 /// use kernel::sync::atomic::{Atomic, Relaxed}; 488 /// 489 /// let x = Atomic::new(42); 490 /// 491 /// assert_eq!(42, x.load(Relaxed)); 492 /// 493 /// x.add(12, Relaxed); 494 /// 495 /// assert_eq!(54, x.load(Relaxed)); 496 /// ``` 497 #[inline(always)] 498 pub fn add<Rhs>(&self, v: Rhs, _: ordering::Relaxed) 499 where 500 T: AtomicAdd<Rhs>, 501 { 502 let v = T::rhs_into_delta(v); 503 504 // INVARIANT: `self.0` is a valid `T` after `atomic_add()` due to safety requirement of 505 // `AtomicAdd`. 506 T::Repr::atomic_add(&self.0, v); 507 } 508 509 /// Atomic fetch and add. 510 /// 511 /// Atomically updates `*self` to `(*self).wrapping_add(v)`, and returns the value of `*self` 512 /// before the update. 513 /// 514 /// # Examples 515 /// 516 /// ``` 517 /// use kernel::sync::atomic::{Atomic, Acquire, Full, Relaxed}; 518 /// 519 /// let x = Atomic::new(42); 520 /// 521 /// assert_eq!(42, x.load(Relaxed)); 522 /// 523 /// assert_eq!(54, { x.fetch_add(12, Acquire); x.load(Relaxed) }); 524 /// 525 /// let x = Atomic::new(42); 526 /// 527 /// assert_eq!(42, x.load(Relaxed)); 528 /// 529 /// assert_eq!(54, { x.fetch_add(12, Full); x.load(Relaxed) } ); 530 /// ``` 531 #[inline(always)] 532 pub fn fetch_add<Rhs, Ordering: ordering::Ordering>(&self, v: Rhs, _: Ordering) -> T 533 where 534 T: AtomicAdd<Rhs>, 535 { 536 let v = T::rhs_into_delta(v); 537 538 // INVARIANT: `self.0` is a valid `T` after `atomic_fetch_add*()` due to safety requirement 539 // of `AtomicAdd`. 540 let ret = { 541 match Ordering::TYPE { 542 OrderingType::Full => T::Repr::atomic_fetch_add(&self.0, v), 543 OrderingType::Acquire => T::Repr::atomic_fetch_add_acquire(&self.0, v), 544 OrderingType::Release => T::Repr::atomic_fetch_add_release(&self.0, v), 545 OrderingType::Relaxed => T::Repr::atomic_fetch_add_relaxed(&self.0, v), 546 } 547 }; 548 549 // SAFETY: `ret` comes from reading `self.0`, which is a valid `T` per type invariants. 550 unsafe { from_repr(ret) } 551 } 552 } 553