Jakub Kotur | 5dd645c | 2020-12-21 17:28:14 +0100 | [diff] [blame] | 1 | use std::cell::{Cell, UnsafeCell}; |
| 2 | use std::cmp; |
| 3 | use std::fmt; |
| 4 | use std::iter::FromIterator; |
| 5 | use std::marker::PhantomData; |
| 6 | use std::mem::{self, MaybeUninit}; |
| 7 | use std::ptr; |
| 8 | use std::sync::atomic::{self, AtomicIsize, AtomicPtr, AtomicUsize, Ordering}; |
| 9 | use std::sync::Arc; |
| 10 | |
| 11 | use crate::epoch::{self, Atomic, Owned}; |
| 12 | use crate::utils::{Backoff, CachePadded}; |
| 13 | |
| 14 | // Minimum buffer capacity. |
| 15 | const MIN_CAP: usize = 64; |
| 16 | // Maximum number of tasks that can be stolen in `steal_batch()` and `steal_batch_and_pop()`. |
| 17 | const MAX_BATCH: usize = 32; |
| 18 | // If a buffer of at least this size is retired, thread-local garbage is flushed so that it gets |
| 19 | // deallocated as soon as possible. |
| 20 | const FLUSH_THRESHOLD_BYTES: usize = 1 << 10; |
| 21 | |
| 22 | /// A buffer that holds tasks in a worker queue. |
| 23 | /// |
| 24 | /// This is just a pointer to the buffer and its length - dropping an instance of this struct will |
| 25 | /// *not* deallocate the buffer. |
| 26 | struct Buffer<T> { |
| 27 | /// Pointer to the allocated memory. |
| 28 | ptr: *mut T, |
| 29 | |
| 30 | /// Capacity of the buffer. Always a power of two. |
| 31 | cap: usize, |
| 32 | } |
| 33 | |
| 34 | unsafe impl<T> Send for Buffer<T> {} |
| 35 | |
| 36 | impl<T> Buffer<T> { |
| 37 | /// Allocates a new buffer with the specified capacity. |
| 38 | fn alloc(cap: usize) -> Buffer<T> { |
| 39 | debug_assert_eq!(cap, cap.next_power_of_two()); |
| 40 | |
| 41 | let mut v = Vec::with_capacity(cap); |
| 42 | let ptr = v.as_mut_ptr(); |
| 43 | mem::forget(v); |
| 44 | |
| 45 | Buffer { ptr, cap } |
| 46 | } |
| 47 | |
| 48 | /// Deallocates the buffer. |
| 49 | unsafe fn dealloc(self) { |
| 50 | drop(Vec::from_raw_parts(self.ptr, 0, self.cap)); |
| 51 | } |
| 52 | |
| 53 | /// Returns a pointer to the task at the specified `index`. |
| 54 | unsafe fn at(&self, index: isize) -> *mut T { |
| 55 | // `self.cap` is always a power of two. |
| 56 | self.ptr.offset(index & (self.cap - 1) as isize) |
| 57 | } |
| 58 | |
| 59 | /// Writes `task` into the specified `index`. |
| 60 | /// |
| 61 | /// This method might be concurrently called with another `read` at the same index, which is |
| 62 | /// technically speaking a data race and therefore UB. We should use an atomic store here, but |
| 63 | /// that would be more expensive and difficult to implement generically for all types `T`. |
| 64 | /// Hence, as a hack, we use a volatile write instead. |
| 65 | unsafe fn write(&self, index: isize, task: T) { |
| 66 | ptr::write_volatile(self.at(index), task) |
| 67 | } |
| 68 | |
| 69 | /// Reads a task from the specified `index`. |
| 70 | /// |
| 71 | /// This method might be concurrently called with another `write` at the same index, which is |
| 72 | /// technically speaking a data race and therefore UB. We should use an atomic load here, but |
| 73 | /// that would be more expensive and difficult to implement generically for all types `T`. |
| 74 | /// Hence, as a hack, we use a volatile write instead. |
| 75 | unsafe fn read(&self, index: isize) -> T { |
| 76 | ptr::read_volatile(self.at(index)) |
| 77 | } |
| 78 | } |
| 79 | |
| 80 | impl<T> Clone for Buffer<T> { |
| 81 | fn clone(&self) -> Buffer<T> { |
| 82 | Buffer { |
| 83 | ptr: self.ptr, |
| 84 | cap: self.cap, |
| 85 | } |
| 86 | } |
| 87 | } |
| 88 | |
| 89 | impl<T> Copy for Buffer<T> {} |
| 90 | |
| 91 | /// Internal queue data shared between the worker and stealers. |
| 92 | /// |
| 93 | /// The implementation is based on the following work: |
| 94 | /// |
| 95 | /// 1. [Chase and Lev. Dynamic circular work-stealing deque. SPAA 2005.][chase-lev] |
| 96 | /// 2. [Le, Pop, Cohen, and Nardelli. Correct and efficient work-stealing for weak memory models. |
| 97 | /// PPoPP 2013.][weak-mem] |
| 98 | /// 3. [Norris and Demsky. CDSchecker: checking concurrent data structures written with C/C++ |
| 99 | /// atomics. OOPSLA 2013.][checker] |
| 100 | /// |
| 101 | /// [chase-lev]: https://dl.acm.org/citation.cfm?id=1073974 |
| 102 | /// [weak-mem]: https://dl.acm.org/citation.cfm?id=2442524 |
| 103 | /// [checker]: https://dl.acm.org/citation.cfm?id=2509514 |
| 104 | struct Inner<T> { |
| 105 | /// The front index. |
| 106 | front: AtomicIsize, |
| 107 | |
| 108 | /// The back index. |
| 109 | back: AtomicIsize, |
| 110 | |
| 111 | /// The underlying buffer. |
| 112 | buffer: CachePadded<Atomic<Buffer<T>>>, |
| 113 | } |
| 114 | |
| 115 | impl<T> Drop for Inner<T> { |
| 116 | fn drop(&mut self) { |
| 117 | // Load the back index, front index, and buffer. |
| 118 | let b = self.back.load(Ordering::Relaxed); |
| 119 | let f = self.front.load(Ordering::Relaxed); |
| 120 | |
| 121 | unsafe { |
| 122 | let buffer = self.buffer.load(Ordering::Relaxed, epoch::unprotected()); |
| 123 | |
| 124 | // Go through the buffer from front to back and drop all tasks in the queue. |
| 125 | let mut i = f; |
| 126 | while i != b { |
| 127 | buffer.deref().at(i).drop_in_place(); |
| 128 | i = i.wrapping_add(1); |
| 129 | } |
| 130 | |
| 131 | // Free the memory allocated by the buffer. |
| 132 | buffer.into_owned().into_box().dealloc(); |
| 133 | } |
| 134 | } |
| 135 | } |
| 136 | |
| 137 | /// Worker queue flavor: FIFO or LIFO. |
| 138 | #[derive(Clone, Copy, Debug, Eq, PartialEq)] |
| 139 | enum Flavor { |
| 140 | /// The first-in first-out flavor. |
| 141 | Fifo, |
| 142 | |
| 143 | /// The last-in first-out flavor. |
| 144 | Lifo, |
| 145 | } |
| 146 | |
| 147 | /// A worker queue. |
| 148 | /// |
| 149 | /// This is a FIFO or LIFO queue that is owned by a single thread, but other threads may steal |
| 150 | /// tasks from it. Task schedulers typically create a single worker queue per thread. |
| 151 | /// |
| 152 | /// # Examples |
| 153 | /// |
| 154 | /// A FIFO worker: |
| 155 | /// |
| 156 | /// ``` |
| 157 | /// use crossbeam_deque::{Steal, Worker}; |
| 158 | /// |
| 159 | /// let w = Worker::new_fifo(); |
| 160 | /// let s = w.stealer(); |
| 161 | /// |
| 162 | /// w.push(1); |
| 163 | /// w.push(2); |
| 164 | /// w.push(3); |
| 165 | /// |
| 166 | /// assert_eq!(s.steal(), Steal::Success(1)); |
| 167 | /// assert_eq!(w.pop(), Some(2)); |
| 168 | /// assert_eq!(w.pop(), Some(3)); |
| 169 | /// ``` |
| 170 | /// |
| 171 | /// A LIFO worker: |
| 172 | /// |
| 173 | /// ``` |
| 174 | /// use crossbeam_deque::{Steal, Worker}; |
| 175 | /// |
| 176 | /// let w = Worker::new_lifo(); |
| 177 | /// let s = w.stealer(); |
| 178 | /// |
| 179 | /// w.push(1); |
| 180 | /// w.push(2); |
| 181 | /// w.push(3); |
| 182 | /// |
| 183 | /// assert_eq!(s.steal(), Steal::Success(1)); |
| 184 | /// assert_eq!(w.pop(), Some(3)); |
| 185 | /// assert_eq!(w.pop(), Some(2)); |
| 186 | /// ``` |
| 187 | pub struct Worker<T> { |
| 188 | /// A reference to the inner representation of the queue. |
| 189 | inner: Arc<CachePadded<Inner<T>>>, |
| 190 | |
| 191 | /// A copy of `inner.buffer` for quick access. |
| 192 | buffer: Cell<Buffer<T>>, |
| 193 | |
| 194 | /// The flavor of the queue. |
| 195 | flavor: Flavor, |
| 196 | |
| 197 | /// Indicates that the worker cannot be shared among threads. |
| 198 | _marker: PhantomData<*mut ()>, // !Send + !Sync |
| 199 | } |
| 200 | |
| 201 | unsafe impl<T: Send> Send for Worker<T> {} |
| 202 | |
| 203 | impl<T> Worker<T> { |
| 204 | /// Creates a FIFO worker queue. |
| 205 | /// |
| 206 | /// Tasks are pushed and popped from opposite ends. |
| 207 | /// |
| 208 | /// # Examples |
| 209 | /// |
| 210 | /// ``` |
| 211 | /// use crossbeam_deque::Worker; |
| 212 | /// |
| 213 | /// let w = Worker::<i32>::new_fifo(); |
| 214 | /// ``` |
| 215 | pub fn new_fifo() -> Worker<T> { |
| 216 | let buffer = Buffer::alloc(MIN_CAP); |
| 217 | |
| 218 | let inner = Arc::new(CachePadded::new(Inner { |
| 219 | front: AtomicIsize::new(0), |
| 220 | back: AtomicIsize::new(0), |
| 221 | buffer: CachePadded::new(Atomic::new(buffer)), |
| 222 | })); |
| 223 | |
| 224 | Worker { |
| 225 | inner, |
| 226 | buffer: Cell::new(buffer), |
| 227 | flavor: Flavor::Fifo, |
| 228 | _marker: PhantomData, |
| 229 | } |
| 230 | } |
| 231 | |
| 232 | /// Creates a LIFO worker queue. |
| 233 | /// |
| 234 | /// Tasks are pushed and popped from the same end. |
| 235 | /// |
| 236 | /// # Examples |
| 237 | /// |
| 238 | /// ``` |
| 239 | /// use crossbeam_deque::Worker; |
| 240 | /// |
| 241 | /// let w = Worker::<i32>::new_lifo(); |
| 242 | /// ``` |
| 243 | pub fn new_lifo() -> Worker<T> { |
| 244 | let buffer = Buffer::alloc(MIN_CAP); |
| 245 | |
| 246 | let inner = Arc::new(CachePadded::new(Inner { |
| 247 | front: AtomicIsize::new(0), |
| 248 | back: AtomicIsize::new(0), |
| 249 | buffer: CachePadded::new(Atomic::new(buffer)), |
| 250 | })); |
| 251 | |
| 252 | Worker { |
| 253 | inner, |
| 254 | buffer: Cell::new(buffer), |
| 255 | flavor: Flavor::Lifo, |
| 256 | _marker: PhantomData, |
| 257 | } |
| 258 | } |
| 259 | |
| 260 | /// Creates a stealer for this queue. |
| 261 | /// |
| 262 | /// The returned stealer can be shared among threads and cloned. |
| 263 | /// |
| 264 | /// # Examples |
| 265 | /// |
| 266 | /// ``` |
| 267 | /// use crossbeam_deque::Worker; |
| 268 | /// |
| 269 | /// let w = Worker::<i32>::new_lifo(); |
| 270 | /// let s = w.stealer(); |
| 271 | /// ``` |
| 272 | pub fn stealer(&self) -> Stealer<T> { |
| 273 | Stealer { |
| 274 | inner: self.inner.clone(), |
| 275 | flavor: self.flavor, |
| 276 | } |
| 277 | } |
| 278 | |
| 279 | /// Resizes the internal buffer to the new capacity of `new_cap`. |
| 280 | #[cold] |
| 281 | unsafe fn resize(&self, new_cap: usize) { |
| 282 | // Load the back index, front index, and buffer. |
| 283 | let b = self.inner.back.load(Ordering::Relaxed); |
| 284 | let f = self.inner.front.load(Ordering::Relaxed); |
| 285 | let buffer = self.buffer.get(); |
| 286 | |
| 287 | // Allocate a new buffer and copy data from the old buffer to the new one. |
| 288 | let new = Buffer::alloc(new_cap); |
| 289 | let mut i = f; |
| 290 | while i != b { |
| 291 | ptr::copy_nonoverlapping(buffer.at(i), new.at(i), 1); |
| 292 | i = i.wrapping_add(1); |
| 293 | } |
| 294 | |
| 295 | let guard = &epoch::pin(); |
| 296 | |
| 297 | // Replace the old buffer with the new one. |
| 298 | self.buffer.replace(new); |
| 299 | let old = |
| 300 | self.inner |
| 301 | .buffer |
| 302 | .swap(Owned::new(new).into_shared(guard), Ordering::Release, guard); |
| 303 | |
| 304 | // Destroy the old buffer later. |
| 305 | guard.defer_unchecked(move || old.into_owned().into_box().dealloc()); |
| 306 | |
| 307 | // If the buffer is very large, then flush the thread-local garbage in order to deallocate |
| 308 | // it as soon as possible. |
| 309 | if mem::size_of::<T>() * new_cap >= FLUSH_THRESHOLD_BYTES { |
| 310 | guard.flush(); |
| 311 | } |
| 312 | } |
| 313 | |
| 314 | /// Reserves enough capacity so that `reserve_cap` tasks can be pushed without growing the |
| 315 | /// buffer. |
| 316 | fn reserve(&self, reserve_cap: usize) { |
| 317 | if reserve_cap > 0 { |
| 318 | // Compute the current length. |
| 319 | let b = self.inner.back.load(Ordering::Relaxed); |
| 320 | let f = self.inner.front.load(Ordering::SeqCst); |
| 321 | let len = b.wrapping_sub(f) as usize; |
| 322 | |
| 323 | // The current capacity. |
| 324 | let cap = self.buffer.get().cap; |
| 325 | |
| 326 | // Is there enough capacity to push `reserve_cap` tasks? |
| 327 | if cap - len < reserve_cap { |
| 328 | // Keep doubling the capacity as much as is needed. |
| 329 | let mut new_cap = cap * 2; |
| 330 | while new_cap - len < reserve_cap { |
| 331 | new_cap *= 2; |
| 332 | } |
| 333 | |
| 334 | // Resize the buffer. |
| 335 | unsafe { |
| 336 | self.resize(new_cap); |
| 337 | } |
| 338 | } |
| 339 | } |
| 340 | } |
| 341 | |
| 342 | /// Returns `true` if the queue is empty. |
| 343 | /// |
| 344 | /// ``` |
| 345 | /// use crossbeam_deque::Worker; |
| 346 | /// |
| 347 | /// let w = Worker::new_lifo(); |
| 348 | /// |
| 349 | /// assert!(w.is_empty()); |
| 350 | /// w.push(1); |
| 351 | /// assert!(!w.is_empty()); |
| 352 | /// ``` |
| 353 | pub fn is_empty(&self) -> bool { |
| 354 | let b = self.inner.back.load(Ordering::Relaxed); |
| 355 | let f = self.inner.front.load(Ordering::SeqCst); |
| 356 | b.wrapping_sub(f) <= 0 |
| 357 | } |
| 358 | |
| 359 | /// Returns the number of tasks in the deque. |
| 360 | /// |
| 361 | /// ``` |
| 362 | /// use crossbeam_deque::Worker; |
| 363 | /// |
| 364 | /// let w = Worker::new_lifo(); |
| 365 | /// |
| 366 | /// assert_eq!(w.len(), 0); |
| 367 | /// w.push(1); |
| 368 | /// assert_eq!(w.len(), 1); |
| 369 | /// w.push(1); |
| 370 | /// assert_eq!(w.len(), 2); |
| 371 | /// ``` |
| 372 | pub fn len(&self) -> usize { |
| 373 | let b = self.inner.back.load(Ordering::Relaxed); |
| 374 | let f = self.inner.front.load(Ordering::SeqCst); |
| 375 | b.wrapping_sub(f).max(0) as usize |
| 376 | } |
| 377 | |
| 378 | /// Pushes a task into the queue. |
| 379 | /// |
| 380 | /// # Examples |
| 381 | /// |
| 382 | /// ``` |
| 383 | /// use crossbeam_deque::Worker; |
| 384 | /// |
| 385 | /// let w = Worker::new_lifo(); |
| 386 | /// w.push(1); |
| 387 | /// w.push(2); |
| 388 | /// ``` |
| 389 | pub fn push(&self, task: T) { |
| 390 | // Load the back index, front index, and buffer. |
| 391 | let b = self.inner.back.load(Ordering::Relaxed); |
| 392 | let f = self.inner.front.load(Ordering::Acquire); |
| 393 | let mut buffer = self.buffer.get(); |
| 394 | |
| 395 | // Calculate the length of the queue. |
| 396 | let len = b.wrapping_sub(f); |
| 397 | |
| 398 | // Is the queue full? |
| 399 | if len >= buffer.cap as isize { |
| 400 | // Yes. Grow the underlying buffer. |
| 401 | unsafe { |
| 402 | self.resize(2 * buffer.cap); |
| 403 | } |
| 404 | buffer = self.buffer.get(); |
| 405 | } |
| 406 | |
| 407 | // Write `task` into the slot. |
| 408 | unsafe { |
| 409 | buffer.write(b, task); |
| 410 | } |
| 411 | |
| 412 | atomic::fence(Ordering::Release); |
| 413 | |
| 414 | // Increment the back index. |
| 415 | // |
| 416 | // This ordering could be `Relaxed`, but then thread sanitizer would falsely report data |
| 417 | // races because it doesn't understand fences. |
| 418 | self.inner.back.store(b.wrapping_add(1), Ordering::Release); |
| 419 | } |
| 420 | |
| 421 | /// Pops a task from the queue. |
| 422 | /// |
| 423 | /// # Examples |
| 424 | /// |
| 425 | /// ``` |
| 426 | /// use crossbeam_deque::Worker; |
| 427 | /// |
| 428 | /// let w = Worker::new_fifo(); |
| 429 | /// w.push(1); |
| 430 | /// w.push(2); |
| 431 | /// |
| 432 | /// assert_eq!(w.pop(), Some(1)); |
| 433 | /// assert_eq!(w.pop(), Some(2)); |
| 434 | /// assert_eq!(w.pop(), None); |
| 435 | /// ``` |
| 436 | pub fn pop(&self) -> Option<T> { |
| 437 | // Load the back and front index. |
| 438 | let b = self.inner.back.load(Ordering::Relaxed); |
| 439 | let f = self.inner.front.load(Ordering::Relaxed); |
| 440 | |
| 441 | // Calculate the length of the queue. |
| 442 | let len = b.wrapping_sub(f); |
| 443 | |
| 444 | // Is the queue empty? |
| 445 | if len <= 0 { |
| 446 | return None; |
| 447 | } |
| 448 | |
| 449 | match self.flavor { |
| 450 | // Pop from the front of the queue. |
| 451 | Flavor::Fifo => { |
| 452 | // Try incrementing the front index to pop the task. |
| 453 | let f = self.inner.front.fetch_add(1, Ordering::SeqCst); |
| 454 | let new_f = f.wrapping_add(1); |
| 455 | |
| 456 | if b.wrapping_sub(new_f) < 0 { |
| 457 | self.inner.front.store(f, Ordering::Relaxed); |
| 458 | return None; |
| 459 | } |
| 460 | |
| 461 | unsafe { |
| 462 | // Read the popped task. |
| 463 | let buffer = self.buffer.get(); |
| 464 | let task = buffer.read(f); |
| 465 | |
| 466 | // Shrink the buffer if `len - 1` is less than one fourth of the capacity. |
| 467 | if buffer.cap > MIN_CAP && len <= buffer.cap as isize / 4 { |
| 468 | self.resize(buffer.cap / 2); |
| 469 | } |
| 470 | |
| 471 | Some(task) |
| 472 | } |
| 473 | } |
| 474 | |
| 475 | // Pop from the back of the queue. |
| 476 | Flavor::Lifo => { |
| 477 | // Decrement the back index. |
| 478 | let b = b.wrapping_sub(1); |
| 479 | self.inner.back.store(b, Ordering::Relaxed); |
| 480 | |
| 481 | atomic::fence(Ordering::SeqCst); |
| 482 | |
| 483 | // Load the front index. |
| 484 | let f = self.inner.front.load(Ordering::Relaxed); |
| 485 | |
| 486 | // Compute the length after the back index was decremented. |
| 487 | let len = b.wrapping_sub(f); |
| 488 | |
| 489 | if len < 0 { |
| 490 | // The queue is empty. Restore the back index to the original task. |
| 491 | self.inner.back.store(b.wrapping_add(1), Ordering::Relaxed); |
| 492 | None |
| 493 | } else { |
| 494 | // Read the task to be popped. |
| 495 | let buffer = self.buffer.get(); |
| 496 | let mut task = unsafe { Some(buffer.read(b)) }; |
| 497 | |
| 498 | // Are we popping the last task from the queue? |
| 499 | if len == 0 { |
| 500 | // Try incrementing the front index. |
| 501 | if self |
| 502 | .inner |
| 503 | .front |
| 504 | .compare_exchange( |
| 505 | f, |
| 506 | f.wrapping_add(1), |
| 507 | Ordering::SeqCst, |
| 508 | Ordering::Relaxed, |
| 509 | ) |
| 510 | .is_err() |
| 511 | { |
| 512 | // Failed. We didn't pop anything. |
| 513 | mem::forget(task.take()); |
| 514 | } |
| 515 | |
| 516 | // Restore the back index to the original task. |
| 517 | self.inner.back.store(b.wrapping_add(1), Ordering::Relaxed); |
| 518 | } else { |
| 519 | // Shrink the buffer if `len` is less than one fourth of the capacity. |
| 520 | if buffer.cap > MIN_CAP && len < buffer.cap as isize / 4 { |
| 521 | unsafe { |
| 522 | self.resize(buffer.cap / 2); |
| 523 | } |
| 524 | } |
| 525 | } |
| 526 | |
| 527 | task |
| 528 | } |
| 529 | } |
| 530 | } |
| 531 | } |
| 532 | } |
| 533 | |
| 534 | impl<T> fmt::Debug for Worker<T> { |
| 535 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
| 536 | f.pad("Worker { .. }") |
| 537 | } |
| 538 | } |
| 539 | |
| 540 | /// A stealer handle of a worker queue. |
| 541 | /// |
| 542 | /// Stealers can be shared among threads. |
| 543 | /// |
| 544 | /// Task schedulers typically have a single worker queue per worker thread. |
| 545 | /// |
| 546 | /// # Examples |
| 547 | /// |
| 548 | /// ``` |
| 549 | /// use crossbeam_deque::{Steal, Worker}; |
| 550 | /// |
| 551 | /// let w = Worker::new_lifo(); |
| 552 | /// w.push(1); |
| 553 | /// w.push(2); |
| 554 | /// |
| 555 | /// let s = w.stealer(); |
| 556 | /// assert_eq!(s.steal(), Steal::Success(1)); |
| 557 | /// assert_eq!(s.steal(), Steal::Success(2)); |
| 558 | /// assert_eq!(s.steal(), Steal::Empty); |
| 559 | /// ``` |
| 560 | pub struct Stealer<T> { |
| 561 | /// A reference to the inner representation of the queue. |
| 562 | inner: Arc<CachePadded<Inner<T>>>, |
| 563 | |
| 564 | /// The flavor of the queue. |
| 565 | flavor: Flavor, |
| 566 | } |
| 567 | |
| 568 | unsafe impl<T: Send> Send for Stealer<T> {} |
| 569 | unsafe impl<T: Send> Sync for Stealer<T> {} |
| 570 | |
| 571 | impl<T> Stealer<T> { |
| 572 | /// Returns `true` if the queue is empty. |
| 573 | /// |
| 574 | /// ``` |
| 575 | /// use crossbeam_deque::Worker; |
| 576 | /// |
| 577 | /// let w = Worker::new_lifo(); |
| 578 | /// let s = w.stealer(); |
| 579 | /// |
| 580 | /// assert!(s.is_empty()); |
| 581 | /// w.push(1); |
| 582 | /// assert!(!s.is_empty()); |
| 583 | /// ``` |
| 584 | pub fn is_empty(&self) -> bool { |
| 585 | let f = self.inner.front.load(Ordering::Acquire); |
| 586 | atomic::fence(Ordering::SeqCst); |
| 587 | let b = self.inner.back.load(Ordering::Acquire); |
| 588 | b.wrapping_sub(f) <= 0 |
| 589 | } |
| 590 | |
Joel Galenson | f0b1773 | 2021-08-09 10:27:52 -0700 | [diff] [blame] | 591 | /// Returns the number of tasks in the deque. |
| 592 | /// |
| 593 | /// ``` |
| 594 | /// use crossbeam_deque::Worker; |
| 595 | /// |
| 596 | /// let w = Worker::new_lifo(); |
| 597 | /// let s = w.stealer(); |
| 598 | /// |
| 599 | /// assert_eq!(s.len(), 0); |
| 600 | /// w.push(1); |
| 601 | /// assert_eq!(s.len(), 1); |
| 602 | /// w.push(2); |
| 603 | /// assert_eq!(s.len(), 2); |
| 604 | /// ``` |
| 605 | pub fn len(&self) -> usize { |
| 606 | let f = self.inner.front.load(Ordering::Acquire); |
| 607 | atomic::fence(Ordering::SeqCst); |
| 608 | let b = self.inner.back.load(Ordering::Acquire); |
| 609 | b.wrapping_sub(f).max(0) as usize |
| 610 | } |
| 611 | |
Jakub Kotur | 5dd645c | 2020-12-21 17:28:14 +0100 | [diff] [blame] | 612 | /// Steals a task from the queue. |
| 613 | /// |
| 614 | /// # Examples |
| 615 | /// |
| 616 | /// ``` |
| 617 | /// use crossbeam_deque::{Steal, Worker}; |
| 618 | /// |
| 619 | /// let w = Worker::new_lifo(); |
| 620 | /// w.push(1); |
| 621 | /// w.push(2); |
| 622 | /// |
| 623 | /// let s = w.stealer(); |
| 624 | /// assert_eq!(s.steal(), Steal::Success(1)); |
| 625 | /// assert_eq!(s.steal(), Steal::Success(2)); |
| 626 | /// ``` |
| 627 | pub fn steal(&self) -> Steal<T> { |
| 628 | // Load the front index. |
| 629 | let f = self.inner.front.load(Ordering::Acquire); |
| 630 | |
| 631 | // A SeqCst fence is needed here. |
| 632 | // |
| 633 | // If the current thread is already pinned (reentrantly), we must manually issue the |
| 634 | // fence. Otherwise, the following pinning will issue the fence anyway, so we don't |
| 635 | // have to. |
| 636 | if epoch::is_pinned() { |
| 637 | atomic::fence(Ordering::SeqCst); |
| 638 | } |
| 639 | |
| 640 | let guard = &epoch::pin(); |
| 641 | |
| 642 | // Load the back index. |
| 643 | let b = self.inner.back.load(Ordering::Acquire); |
| 644 | |
| 645 | // Is the queue empty? |
| 646 | if b.wrapping_sub(f) <= 0 { |
| 647 | return Steal::Empty; |
| 648 | } |
| 649 | |
| 650 | // Load the buffer and read the task at the front. |
| 651 | let buffer = self.inner.buffer.load(Ordering::Acquire, guard); |
| 652 | let task = unsafe { buffer.deref().read(f) }; |
| 653 | |
| 654 | // Try incrementing the front index to steal the task. |
Joel Galenson | f0b1773 | 2021-08-09 10:27:52 -0700 | [diff] [blame] | 655 | // If the buffer has been swapped or the increment fails, we retry. |
| 656 | if self.inner.buffer.load(Ordering::Acquire, guard) != buffer |
| 657 | || self |
| 658 | .inner |
| 659 | .front |
| 660 | .compare_exchange(f, f.wrapping_add(1), Ordering::SeqCst, Ordering::Relaxed) |
| 661 | .is_err() |
Jakub Kotur | 5dd645c | 2020-12-21 17:28:14 +0100 | [diff] [blame] | 662 | { |
| 663 | // We didn't steal this task, forget it. |
| 664 | mem::forget(task); |
| 665 | return Steal::Retry; |
| 666 | } |
| 667 | |
| 668 | // Return the stolen task. |
| 669 | Steal::Success(task) |
| 670 | } |
| 671 | |
| 672 | /// Steals a batch of tasks and pushes them into another worker. |
| 673 | /// |
| 674 | /// How many tasks exactly will be stolen is not specified. That said, this method will try to |
| 675 | /// steal around half of the tasks in the queue, but also not more than some constant limit. |
| 676 | /// |
| 677 | /// # Examples |
| 678 | /// |
| 679 | /// ``` |
| 680 | /// use crossbeam_deque::Worker; |
| 681 | /// |
| 682 | /// let w1 = Worker::new_fifo(); |
| 683 | /// w1.push(1); |
| 684 | /// w1.push(2); |
| 685 | /// w1.push(3); |
| 686 | /// w1.push(4); |
| 687 | /// |
| 688 | /// let s = w1.stealer(); |
| 689 | /// let w2 = Worker::new_fifo(); |
| 690 | /// |
| 691 | /// let _ = s.steal_batch(&w2); |
| 692 | /// assert_eq!(w2.pop(), Some(1)); |
| 693 | /// assert_eq!(w2.pop(), Some(2)); |
| 694 | /// ``` |
| 695 | pub fn steal_batch(&self, dest: &Worker<T>) -> Steal<()> { |
| 696 | if Arc::ptr_eq(&self.inner, &dest.inner) { |
| 697 | if dest.is_empty() { |
| 698 | return Steal::Empty; |
| 699 | } else { |
| 700 | return Steal::Success(()); |
| 701 | } |
| 702 | } |
| 703 | |
| 704 | // Load the front index. |
| 705 | let mut f = self.inner.front.load(Ordering::Acquire); |
| 706 | |
| 707 | // A SeqCst fence is needed here. |
| 708 | // |
| 709 | // If the current thread is already pinned (reentrantly), we must manually issue the |
| 710 | // fence. Otherwise, the following pinning will issue the fence anyway, so we don't |
| 711 | // have to. |
| 712 | if epoch::is_pinned() { |
| 713 | atomic::fence(Ordering::SeqCst); |
| 714 | } |
| 715 | |
| 716 | let guard = &epoch::pin(); |
| 717 | |
| 718 | // Load the back index. |
| 719 | let b = self.inner.back.load(Ordering::Acquire); |
| 720 | |
| 721 | // Is the queue empty? |
| 722 | let len = b.wrapping_sub(f); |
| 723 | if len <= 0 { |
| 724 | return Steal::Empty; |
| 725 | } |
| 726 | |
| 727 | // Reserve capacity for the stolen batch. |
| 728 | let batch_size = cmp::min((len as usize + 1) / 2, MAX_BATCH); |
| 729 | dest.reserve(batch_size); |
| 730 | let mut batch_size = batch_size as isize; |
| 731 | |
| 732 | // Get the destination buffer and back index. |
| 733 | let dest_buffer = dest.buffer.get(); |
| 734 | let mut dest_b = dest.inner.back.load(Ordering::Relaxed); |
| 735 | |
| 736 | // Load the buffer. |
| 737 | let buffer = self.inner.buffer.load(Ordering::Acquire, guard); |
| 738 | |
| 739 | match self.flavor { |
| 740 | // Steal a batch of tasks from the front at once. |
| 741 | Flavor::Fifo => { |
| 742 | // Copy the batch from the source to the destination buffer. |
| 743 | match dest.flavor { |
| 744 | Flavor::Fifo => { |
| 745 | for i in 0..batch_size { |
| 746 | unsafe { |
| 747 | let task = buffer.deref().read(f.wrapping_add(i)); |
| 748 | dest_buffer.write(dest_b.wrapping_add(i), task); |
| 749 | } |
| 750 | } |
| 751 | } |
| 752 | Flavor::Lifo => { |
| 753 | for i in 0..batch_size { |
| 754 | unsafe { |
| 755 | let task = buffer.deref().read(f.wrapping_add(i)); |
| 756 | dest_buffer.write(dest_b.wrapping_add(batch_size - 1 - i), task); |
| 757 | } |
| 758 | } |
| 759 | } |
| 760 | } |
| 761 | |
| 762 | // Try incrementing the front index to steal the batch. |
Joel Galenson | f0b1773 | 2021-08-09 10:27:52 -0700 | [diff] [blame] | 763 | // If the buffer has been swapped or the increment fails, we retry. |
| 764 | if self.inner.buffer.load(Ordering::Acquire, guard) != buffer |
| 765 | || self |
| 766 | .inner |
| 767 | .front |
| 768 | .compare_exchange( |
| 769 | f, |
| 770 | f.wrapping_add(batch_size), |
| 771 | Ordering::SeqCst, |
| 772 | Ordering::Relaxed, |
| 773 | ) |
| 774 | .is_err() |
Jakub Kotur | 5dd645c | 2020-12-21 17:28:14 +0100 | [diff] [blame] | 775 | { |
| 776 | return Steal::Retry; |
| 777 | } |
| 778 | |
| 779 | dest_b = dest_b.wrapping_add(batch_size); |
| 780 | } |
| 781 | |
| 782 | // Steal a batch of tasks from the front one by one. |
| 783 | Flavor::Lifo => { |
Joel Galenson | f0b1773 | 2021-08-09 10:27:52 -0700 | [diff] [blame] | 784 | // This loop may modify the batch_size, which triggers a clippy lint warning. |
| 785 | // Use a new variable to avoid the warning, and to make it clear we aren't |
| 786 | // modifying the loop exit condition during iteration. |
| 787 | let original_batch_size = batch_size; |
| 788 | |
| 789 | for i in 0..original_batch_size { |
Jakub Kotur | 5dd645c | 2020-12-21 17:28:14 +0100 | [diff] [blame] | 790 | // If this is not the first steal, check whether the queue is empty. |
| 791 | if i > 0 { |
| 792 | // We've already got the current front index. Now execute the fence to |
| 793 | // synchronize with other threads. |
| 794 | atomic::fence(Ordering::SeqCst); |
| 795 | |
| 796 | // Load the back index. |
| 797 | let b = self.inner.back.load(Ordering::Acquire); |
| 798 | |
| 799 | // Is the queue empty? |
| 800 | if b.wrapping_sub(f) <= 0 { |
| 801 | batch_size = i; |
| 802 | break; |
| 803 | } |
| 804 | } |
| 805 | |
| 806 | // Read the task at the front. |
| 807 | let task = unsafe { buffer.deref().read(f) }; |
| 808 | |
| 809 | // Try incrementing the front index to steal the task. |
Joel Galenson | f0b1773 | 2021-08-09 10:27:52 -0700 | [diff] [blame] | 810 | // If the buffer has been swapped or the increment fails, we retry. |
| 811 | if self.inner.buffer.load(Ordering::Acquire, guard) != buffer |
| 812 | || self |
| 813 | .inner |
| 814 | .front |
| 815 | .compare_exchange( |
| 816 | f, |
| 817 | f.wrapping_add(1), |
| 818 | Ordering::SeqCst, |
| 819 | Ordering::Relaxed, |
| 820 | ) |
| 821 | .is_err() |
Jakub Kotur | 5dd645c | 2020-12-21 17:28:14 +0100 | [diff] [blame] | 822 | { |
| 823 | // We didn't steal this task, forget it and break from the loop. |
| 824 | mem::forget(task); |
| 825 | batch_size = i; |
| 826 | break; |
| 827 | } |
| 828 | |
| 829 | // Write the stolen task into the destination buffer. |
| 830 | unsafe { |
| 831 | dest_buffer.write(dest_b, task); |
| 832 | } |
| 833 | |
| 834 | // Move the source front index and the destination back index one step forward. |
| 835 | f = f.wrapping_add(1); |
| 836 | dest_b = dest_b.wrapping_add(1); |
| 837 | } |
| 838 | |
| 839 | // If we didn't steal anything, the operation needs to be retried. |
| 840 | if batch_size == 0 { |
| 841 | return Steal::Retry; |
| 842 | } |
| 843 | |
| 844 | // If stealing into a FIFO queue, stolen tasks need to be reversed. |
| 845 | if dest.flavor == Flavor::Fifo { |
| 846 | for i in 0..batch_size / 2 { |
| 847 | unsafe { |
| 848 | let i1 = dest_b.wrapping_sub(batch_size - i); |
| 849 | let i2 = dest_b.wrapping_sub(i + 1); |
| 850 | let t1 = dest_buffer.read(i1); |
| 851 | let t2 = dest_buffer.read(i2); |
| 852 | dest_buffer.write(i1, t2); |
| 853 | dest_buffer.write(i2, t1); |
| 854 | } |
| 855 | } |
| 856 | } |
| 857 | } |
| 858 | } |
| 859 | |
| 860 | atomic::fence(Ordering::Release); |
| 861 | |
| 862 | // Update the back index in the destination queue. |
| 863 | // |
| 864 | // This ordering could be `Relaxed`, but then thread sanitizer would falsely report data |
| 865 | // races because it doesn't understand fences. |
| 866 | dest.inner.back.store(dest_b, Ordering::Release); |
| 867 | |
| 868 | // Return with success. |
| 869 | Steal::Success(()) |
| 870 | } |
| 871 | |
| 872 | /// Steals a batch of tasks, pushes them into another worker, and pops a task from that worker. |
| 873 | /// |
| 874 | /// How many tasks exactly will be stolen is not specified. That said, this method will try to |
| 875 | /// steal around half of the tasks in the queue, but also not more than some constant limit. |
| 876 | /// |
| 877 | /// # Examples |
| 878 | /// |
| 879 | /// ``` |
| 880 | /// use crossbeam_deque::{Steal, Worker}; |
| 881 | /// |
| 882 | /// let w1 = Worker::new_fifo(); |
| 883 | /// w1.push(1); |
| 884 | /// w1.push(2); |
| 885 | /// w1.push(3); |
| 886 | /// w1.push(4); |
| 887 | /// |
| 888 | /// let s = w1.stealer(); |
| 889 | /// let w2 = Worker::new_fifo(); |
| 890 | /// |
| 891 | /// assert_eq!(s.steal_batch_and_pop(&w2), Steal::Success(1)); |
| 892 | /// assert_eq!(w2.pop(), Some(2)); |
| 893 | /// ``` |
| 894 | pub fn steal_batch_and_pop(&self, dest: &Worker<T>) -> Steal<T> { |
| 895 | if Arc::ptr_eq(&self.inner, &dest.inner) { |
| 896 | match dest.pop() { |
| 897 | None => return Steal::Empty, |
| 898 | Some(task) => return Steal::Success(task), |
| 899 | } |
| 900 | } |
| 901 | |
| 902 | // Load the front index. |
| 903 | let mut f = self.inner.front.load(Ordering::Acquire); |
| 904 | |
| 905 | // A SeqCst fence is needed here. |
| 906 | // |
| 907 | // If the current thread is already pinned (reentrantly), we must manually issue the |
| 908 | // fence. Otherwise, the following pinning will issue the fence anyway, so we don't |
| 909 | // have to. |
| 910 | if epoch::is_pinned() { |
| 911 | atomic::fence(Ordering::SeqCst); |
| 912 | } |
| 913 | |
| 914 | let guard = &epoch::pin(); |
| 915 | |
| 916 | // Load the back index. |
| 917 | let b = self.inner.back.load(Ordering::Acquire); |
| 918 | |
| 919 | // Is the queue empty? |
| 920 | let len = b.wrapping_sub(f); |
| 921 | if len <= 0 { |
| 922 | return Steal::Empty; |
| 923 | } |
| 924 | |
| 925 | // Reserve capacity for the stolen batch. |
| 926 | let batch_size = cmp::min((len as usize - 1) / 2, MAX_BATCH - 1); |
| 927 | dest.reserve(batch_size); |
| 928 | let mut batch_size = batch_size as isize; |
| 929 | |
| 930 | // Get the destination buffer and back index. |
| 931 | let dest_buffer = dest.buffer.get(); |
| 932 | let mut dest_b = dest.inner.back.load(Ordering::Relaxed); |
| 933 | |
| 934 | // Load the buffer |
| 935 | let buffer = self.inner.buffer.load(Ordering::Acquire, guard); |
| 936 | |
| 937 | // Read the task at the front. |
| 938 | let mut task = unsafe { buffer.deref().read(f) }; |
| 939 | |
| 940 | match self.flavor { |
| 941 | // Steal a batch of tasks from the front at once. |
| 942 | Flavor::Fifo => { |
| 943 | // Copy the batch from the source to the destination buffer. |
| 944 | match dest.flavor { |
| 945 | Flavor::Fifo => { |
| 946 | for i in 0..batch_size { |
| 947 | unsafe { |
| 948 | let task = buffer.deref().read(f.wrapping_add(i + 1)); |
| 949 | dest_buffer.write(dest_b.wrapping_add(i), task); |
| 950 | } |
| 951 | } |
| 952 | } |
| 953 | Flavor::Lifo => { |
| 954 | for i in 0..batch_size { |
| 955 | unsafe { |
| 956 | let task = buffer.deref().read(f.wrapping_add(i + 1)); |
| 957 | dest_buffer.write(dest_b.wrapping_add(batch_size - 1 - i), task); |
| 958 | } |
| 959 | } |
| 960 | } |
| 961 | } |
| 962 | |
Joel Galenson | f0b1773 | 2021-08-09 10:27:52 -0700 | [diff] [blame] | 963 | // Try incrementing the front index to steal the task. |
| 964 | // If the buffer has been swapped or the increment fails, we retry. |
| 965 | if self.inner.buffer.load(Ordering::Acquire, guard) != buffer |
| 966 | || self |
| 967 | .inner |
| 968 | .front |
| 969 | .compare_exchange( |
| 970 | f, |
| 971 | f.wrapping_add(batch_size + 1), |
| 972 | Ordering::SeqCst, |
| 973 | Ordering::Relaxed, |
| 974 | ) |
| 975 | .is_err() |
Jakub Kotur | 5dd645c | 2020-12-21 17:28:14 +0100 | [diff] [blame] | 976 | { |
| 977 | // We didn't steal this task, forget it. |
| 978 | mem::forget(task); |
| 979 | return Steal::Retry; |
| 980 | } |
| 981 | |
| 982 | dest_b = dest_b.wrapping_add(batch_size); |
| 983 | } |
| 984 | |
| 985 | // Steal a batch of tasks from the front one by one. |
| 986 | Flavor::Lifo => { |
| 987 | // Try incrementing the front index to steal the task. |
| 988 | if self |
| 989 | .inner |
| 990 | .front |
| 991 | .compare_exchange(f, f.wrapping_add(1), Ordering::SeqCst, Ordering::Relaxed) |
| 992 | .is_err() |
| 993 | { |
| 994 | // We didn't steal this task, forget it. |
| 995 | mem::forget(task); |
| 996 | return Steal::Retry; |
| 997 | } |
| 998 | |
| 999 | // Move the front index one step forward. |
| 1000 | f = f.wrapping_add(1); |
| 1001 | |
| 1002 | // Repeat the same procedure for the batch steals. |
Joel Galenson | f0b1773 | 2021-08-09 10:27:52 -0700 | [diff] [blame] | 1003 | // |
| 1004 | // This loop may modify the batch_size, which triggers a clippy lint warning. |
| 1005 | // Use a new variable to avoid the warning, and to make it clear we aren't |
| 1006 | // modifying the loop exit condition during iteration. |
| 1007 | let original_batch_size = batch_size; |
| 1008 | for i in 0..original_batch_size { |
Jakub Kotur | 5dd645c | 2020-12-21 17:28:14 +0100 | [diff] [blame] | 1009 | // We've already got the current front index. Now execute the fence to |
| 1010 | // synchronize with other threads. |
| 1011 | atomic::fence(Ordering::SeqCst); |
| 1012 | |
| 1013 | // Load the back index. |
| 1014 | let b = self.inner.back.load(Ordering::Acquire); |
| 1015 | |
| 1016 | // Is the queue empty? |
| 1017 | if b.wrapping_sub(f) <= 0 { |
| 1018 | batch_size = i; |
| 1019 | break; |
| 1020 | } |
| 1021 | |
| 1022 | // Read the task at the front. |
| 1023 | let tmp = unsafe { buffer.deref().read(f) }; |
| 1024 | |
| 1025 | // Try incrementing the front index to steal the task. |
Joel Galenson | f0b1773 | 2021-08-09 10:27:52 -0700 | [diff] [blame] | 1026 | // If the buffer has been swapped or the increment fails, we retry. |
| 1027 | if self.inner.buffer.load(Ordering::Acquire, guard) != buffer |
| 1028 | || self |
| 1029 | .inner |
| 1030 | .front |
| 1031 | .compare_exchange( |
| 1032 | f, |
| 1033 | f.wrapping_add(1), |
| 1034 | Ordering::SeqCst, |
| 1035 | Ordering::Relaxed, |
| 1036 | ) |
| 1037 | .is_err() |
Jakub Kotur | 5dd645c | 2020-12-21 17:28:14 +0100 | [diff] [blame] | 1038 | { |
| 1039 | // We didn't steal this task, forget it and break from the loop. |
| 1040 | mem::forget(tmp); |
| 1041 | batch_size = i; |
| 1042 | break; |
| 1043 | } |
| 1044 | |
| 1045 | // Write the previously stolen task into the destination buffer. |
| 1046 | unsafe { |
| 1047 | dest_buffer.write(dest_b, mem::replace(&mut task, tmp)); |
| 1048 | } |
| 1049 | |
| 1050 | // Move the source front index and the destination back index one step forward. |
| 1051 | f = f.wrapping_add(1); |
| 1052 | dest_b = dest_b.wrapping_add(1); |
| 1053 | } |
| 1054 | |
| 1055 | // If stealing into a FIFO queue, stolen tasks need to be reversed. |
| 1056 | if dest.flavor == Flavor::Fifo { |
| 1057 | for i in 0..batch_size / 2 { |
| 1058 | unsafe { |
| 1059 | let i1 = dest_b.wrapping_sub(batch_size - i); |
| 1060 | let i2 = dest_b.wrapping_sub(i + 1); |
| 1061 | let t1 = dest_buffer.read(i1); |
| 1062 | let t2 = dest_buffer.read(i2); |
| 1063 | dest_buffer.write(i1, t2); |
| 1064 | dest_buffer.write(i2, t1); |
| 1065 | } |
| 1066 | } |
| 1067 | } |
| 1068 | } |
| 1069 | } |
| 1070 | |
| 1071 | atomic::fence(Ordering::Release); |
| 1072 | |
| 1073 | // Update the back index in the destination queue. |
| 1074 | // |
| 1075 | // This ordering could be `Relaxed`, but then thread sanitizer would falsely report data |
| 1076 | // races because it doesn't understand fences. |
| 1077 | dest.inner.back.store(dest_b, Ordering::Release); |
| 1078 | |
| 1079 | // Return with success. |
| 1080 | Steal::Success(task) |
| 1081 | } |
| 1082 | } |
| 1083 | |
| 1084 | impl<T> Clone for Stealer<T> { |
| 1085 | fn clone(&self) -> Stealer<T> { |
| 1086 | Stealer { |
| 1087 | inner: self.inner.clone(), |
| 1088 | flavor: self.flavor, |
| 1089 | } |
| 1090 | } |
| 1091 | } |
| 1092 | |
| 1093 | impl<T> fmt::Debug for Stealer<T> { |
| 1094 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
| 1095 | f.pad("Stealer { .. }") |
| 1096 | } |
| 1097 | } |
| 1098 | |
| 1099 | // Bits indicating the state of a slot: |
| 1100 | // * If a task has been written into the slot, `WRITE` is set. |
| 1101 | // * If a task has been read from the slot, `READ` is set. |
| 1102 | // * If the block is being destroyed, `DESTROY` is set. |
| 1103 | const WRITE: usize = 1; |
| 1104 | const READ: usize = 2; |
| 1105 | const DESTROY: usize = 4; |
| 1106 | |
| 1107 | // Each block covers one "lap" of indices. |
| 1108 | const LAP: usize = 64; |
| 1109 | // The maximum number of values a block can hold. |
| 1110 | const BLOCK_CAP: usize = LAP - 1; |
| 1111 | // How many lower bits are reserved for metadata. |
| 1112 | const SHIFT: usize = 1; |
| 1113 | // Indicates that the block is not the last one. |
| 1114 | const HAS_NEXT: usize = 1; |
| 1115 | |
| 1116 | /// A slot in a block. |
| 1117 | struct Slot<T> { |
| 1118 | /// The task. |
| 1119 | task: UnsafeCell<MaybeUninit<T>>, |
| 1120 | |
| 1121 | /// The state of the slot. |
| 1122 | state: AtomicUsize, |
| 1123 | } |
| 1124 | |
| 1125 | impl<T> Slot<T> { |
| 1126 | /// Waits until a task is written into the slot. |
| 1127 | fn wait_write(&self) { |
| 1128 | let backoff = Backoff::new(); |
| 1129 | while self.state.load(Ordering::Acquire) & WRITE == 0 { |
| 1130 | backoff.snooze(); |
| 1131 | } |
| 1132 | } |
| 1133 | } |
| 1134 | |
| 1135 | /// A block in a linked list. |
| 1136 | /// |
| 1137 | /// Each block in the list can hold up to `BLOCK_CAP` values. |
| 1138 | struct Block<T> { |
| 1139 | /// The next block in the linked list. |
| 1140 | next: AtomicPtr<Block<T>>, |
| 1141 | |
| 1142 | /// Slots for values. |
| 1143 | slots: [Slot<T>; BLOCK_CAP], |
| 1144 | } |
| 1145 | |
| 1146 | impl<T> Block<T> { |
| 1147 | /// Creates an empty block that starts at `start_index`. |
| 1148 | fn new() -> Block<T> { |
| 1149 | // SAFETY: This is safe because: |
| 1150 | // [1] `Block::next` (AtomicPtr) may be safely zero initialized. |
| 1151 | // [2] `Block::slots` (Array) may be safely zero initialized because of [3, 4]. |
| 1152 | // [3] `Slot::task` (UnsafeCell) may be safely zero initialized because it |
| 1153 | // holds a MaybeUninit. |
| 1154 | // [4] `Slot::state` (AtomicUsize) may be safely zero initialized. |
| 1155 | unsafe { MaybeUninit::zeroed().assume_init() } |
| 1156 | } |
| 1157 | |
| 1158 | /// Waits until the next pointer is set. |
| 1159 | fn wait_next(&self) -> *mut Block<T> { |
| 1160 | let backoff = Backoff::new(); |
| 1161 | loop { |
| 1162 | let next = self.next.load(Ordering::Acquire); |
| 1163 | if !next.is_null() { |
| 1164 | return next; |
| 1165 | } |
| 1166 | backoff.snooze(); |
| 1167 | } |
| 1168 | } |
| 1169 | |
| 1170 | /// Sets the `DESTROY` bit in slots starting from `start` and destroys the block. |
| 1171 | unsafe fn destroy(this: *mut Block<T>, count: usize) { |
| 1172 | // It is not necessary to set the `DESTROY` bit in the last slot because that slot has |
| 1173 | // begun destruction of the block. |
| 1174 | for i in (0..count).rev() { |
| 1175 | let slot = (*this).slots.get_unchecked(i); |
| 1176 | |
| 1177 | // Mark the `DESTROY` bit if a thread is still using the slot. |
| 1178 | if slot.state.load(Ordering::Acquire) & READ == 0 |
| 1179 | && slot.state.fetch_or(DESTROY, Ordering::AcqRel) & READ == 0 |
| 1180 | { |
| 1181 | // If a thread is still using the slot, it will continue destruction of the block. |
| 1182 | return; |
| 1183 | } |
| 1184 | } |
| 1185 | |
| 1186 | // No thread is using the block, now it is safe to destroy it. |
| 1187 | drop(Box::from_raw(this)); |
| 1188 | } |
| 1189 | } |
| 1190 | |
| 1191 | /// A position in a queue. |
| 1192 | struct Position<T> { |
| 1193 | /// The index in the queue. |
| 1194 | index: AtomicUsize, |
| 1195 | |
| 1196 | /// The block in the linked list. |
| 1197 | block: AtomicPtr<Block<T>>, |
| 1198 | } |
| 1199 | |
| 1200 | /// An injector queue. |
| 1201 | /// |
| 1202 | /// This is a FIFO queue that can be shared among multiple threads. Task schedulers typically have |
| 1203 | /// a single injector queue, which is the entry point for new tasks. |
| 1204 | /// |
| 1205 | /// # Examples |
| 1206 | /// |
| 1207 | /// ``` |
| 1208 | /// use crossbeam_deque::{Injector, Steal}; |
| 1209 | /// |
| 1210 | /// let q = Injector::new(); |
| 1211 | /// q.push(1); |
| 1212 | /// q.push(2); |
| 1213 | /// |
| 1214 | /// assert_eq!(q.steal(), Steal::Success(1)); |
| 1215 | /// assert_eq!(q.steal(), Steal::Success(2)); |
| 1216 | /// assert_eq!(q.steal(), Steal::Empty); |
| 1217 | /// ``` |
| 1218 | pub struct Injector<T> { |
| 1219 | /// The head of the queue. |
| 1220 | head: CachePadded<Position<T>>, |
| 1221 | |
| 1222 | /// The tail of the queue. |
| 1223 | tail: CachePadded<Position<T>>, |
| 1224 | |
| 1225 | /// Indicates that dropping a `Injector<T>` may drop values of type `T`. |
| 1226 | _marker: PhantomData<T>, |
| 1227 | } |
| 1228 | |
| 1229 | unsafe impl<T: Send> Send for Injector<T> {} |
| 1230 | unsafe impl<T: Send> Sync for Injector<T> {} |
| 1231 | |
| 1232 | impl<T> Default for Injector<T> { |
| 1233 | fn default() -> Self { |
| 1234 | let block = Box::into_raw(Box::new(Block::<T>::new())); |
| 1235 | Self { |
| 1236 | head: CachePadded::new(Position { |
| 1237 | block: AtomicPtr::new(block), |
| 1238 | index: AtomicUsize::new(0), |
| 1239 | }), |
| 1240 | tail: CachePadded::new(Position { |
| 1241 | block: AtomicPtr::new(block), |
| 1242 | index: AtomicUsize::new(0), |
| 1243 | }), |
| 1244 | _marker: PhantomData, |
| 1245 | } |
| 1246 | } |
| 1247 | } |
| 1248 | |
| 1249 | impl<T> Injector<T> { |
| 1250 | /// Creates a new injector queue. |
| 1251 | /// |
| 1252 | /// # Examples |
| 1253 | /// |
| 1254 | /// ``` |
| 1255 | /// use crossbeam_deque::Injector; |
| 1256 | /// |
| 1257 | /// let q = Injector::<i32>::new(); |
| 1258 | /// ``` |
| 1259 | pub fn new() -> Injector<T> { |
| 1260 | Self::default() |
| 1261 | } |
| 1262 | |
| 1263 | /// Pushes a task into the queue. |
| 1264 | /// |
| 1265 | /// # Examples |
| 1266 | /// |
| 1267 | /// ``` |
| 1268 | /// use crossbeam_deque::Injector; |
| 1269 | /// |
| 1270 | /// let w = Injector::new(); |
| 1271 | /// w.push(1); |
| 1272 | /// w.push(2); |
| 1273 | /// ``` |
| 1274 | pub fn push(&self, task: T) { |
| 1275 | let backoff = Backoff::new(); |
| 1276 | let mut tail = self.tail.index.load(Ordering::Acquire); |
| 1277 | let mut block = self.tail.block.load(Ordering::Acquire); |
| 1278 | let mut next_block = None; |
| 1279 | |
| 1280 | loop { |
| 1281 | // Calculate the offset of the index into the block. |
| 1282 | let offset = (tail >> SHIFT) % LAP; |
| 1283 | |
| 1284 | // If we reached the end of the block, wait until the next one is installed. |
| 1285 | if offset == BLOCK_CAP { |
| 1286 | backoff.snooze(); |
| 1287 | tail = self.tail.index.load(Ordering::Acquire); |
| 1288 | block = self.tail.block.load(Ordering::Acquire); |
| 1289 | continue; |
| 1290 | } |
| 1291 | |
| 1292 | // If we're going to have to install the next block, allocate it in advance in order to |
| 1293 | // make the wait for other threads as short as possible. |
| 1294 | if offset + 1 == BLOCK_CAP && next_block.is_none() { |
| 1295 | next_block = Some(Box::new(Block::<T>::new())); |
| 1296 | } |
| 1297 | |
| 1298 | let new_tail = tail + (1 << SHIFT); |
| 1299 | |
| 1300 | // Try advancing the tail forward. |
| 1301 | match self.tail.index.compare_exchange_weak( |
| 1302 | tail, |
| 1303 | new_tail, |
| 1304 | Ordering::SeqCst, |
| 1305 | Ordering::Acquire, |
| 1306 | ) { |
| 1307 | Ok(_) => unsafe { |
| 1308 | // If we've reached the end of the block, install the next one. |
| 1309 | if offset + 1 == BLOCK_CAP { |
| 1310 | let next_block = Box::into_raw(next_block.unwrap()); |
| 1311 | let next_index = new_tail.wrapping_add(1 << SHIFT); |
| 1312 | |
| 1313 | self.tail.block.store(next_block, Ordering::Release); |
| 1314 | self.tail.index.store(next_index, Ordering::Release); |
| 1315 | (*block).next.store(next_block, Ordering::Release); |
| 1316 | } |
| 1317 | |
| 1318 | // Write the task into the slot. |
| 1319 | let slot = (*block).slots.get_unchecked(offset); |
| 1320 | slot.task.get().write(MaybeUninit::new(task)); |
| 1321 | slot.state.fetch_or(WRITE, Ordering::Release); |
| 1322 | |
| 1323 | return; |
| 1324 | }, |
| 1325 | Err(t) => { |
| 1326 | tail = t; |
| 1327 | block = self.tail.block.load(Ordering::Acquire); |
| 1328 | backoff.spin(); |
| 1329 | } |
| 1330 | } |
| 1331 | } |
| 1332 | } |
| 1333 | |
| 1334 | /// Steals a task from the queue. |
| 1335 | /// |
| 1336 | /// # Examples |
| 1337 | /// |
| 1338 | /// ``` |
| 1339 | /// use crossbeam_deque::{Injector, Steal}; |
| 1340 | /// |
| 1341 | /// let q = Injector::new(); |
| 1342 | /// q.push(1); |
| 1343 | /// q.push(2); |
| 1344 | /// |
| 1345 | /// assert_eq!(q.steal(), Steal::Success(1)); |
| 1346 | /// assert_eq!(q.steal(), Steal::Success(2)); |
| 1347 | /// assert_eq!(q.steal(), Steal::Empty); |
| 1348 | /// ``` |
| 1349 | pub fn steal(&self) -> Steal<T> { |
| 1350 | let mut head; |
| 1351 | let mut block; |
| 1352 | let mut offset; |
| 1353 | |
| 1354 | let backoff = Backoff::new(); |
| 1355 | loop { |
| 1356 | head = self.head.index.load(Ordering::Acquire); |
| 1357 | block = self.head.block.load(Ordering::Acquire); |
| 1358 | |
| 1359 | // Calculate the offset of the index into the block. |
| 1360 | offset = (head >> SHIFT) % LAP; |
| 1361 | |
| 1362 | // If we reached the end of the block, wait until the next one is installed. |
| 1363 | if offset == BLOCK_CAP { |
| 1364 | backoff.snooze(); |
| 1365 | } else { |
| 1366 | break; |
| 1367 | } |
| 1368 | } |
| 1369 | |
| 1370 | let mut new_head = head + (1 << SHIFT); |
| 1371 | |
| 1372 | if new_head & HAS_NEXT == 0 { |
| 1373 | atomic::fence(Ordering::SeqCst); |
| 1374 | let tail = self.tail.index.load(Ordering::Relaxed); |
| 1375 | |
| 1376 | // If the tail equals the head, that means the queue is empty. |
| 1377 | if head >> SHIFT == tail >> SHIFT { |
| 1378 | return Steal::Empty; |
| 1379 | } |
| 1380 | |
| 1381 | // If head and tail are not in the same block, set `HAS_NEXT` in head. |
| 1382 | if (head >> SHIFT) / LAP != (tail >> SHIFT) / LAP { |
| 1383 | new_head |= HAS_NEXT; |
| 1384 | } |
| 1385 | } |
| 1386 | |
| 1387 | // Try moving the head index forward. |
| 1388 | if self |
| 1389 | .head |
| 1390 | .index |
| 1391 | .compare_exchange_weak(head, new_head, Ordering::SeqCst, Ordering::Acquire) |
| 1392 | .is_err() |
| 1393 | { |
| 1394 | return Steal::Retry; |
| 1395 | } |
| 1396 | |
| 1397 | unsafe { |
| 1398 | // If we've reached the end of the block, move to the next one. |
| 1399 | if offset + 1 == BLOCK_CAP { |
| 1400 | let next = (*block).wait_next(); |
| 1401 | let mut next_index = (new_head & !HAS_NEXT).wrapping_add(1 << SHIFT); |
| 1402 | if !(*next).next.load(Ordering::Relaxed).is_null() { |
| 1403 | next_index |= HAS_NEXT; |
| 1404 | } |
| 1405 | |
| 1406 | self.head.block.store(next, Ordering::Release); |
| 1407 | self.head.index.store(next_index, Ordering::Release); |
| 1408 | } |
| 1409 | |
| 1410 | // Read the task. |
| 1411 | let slot = (*block).slots.get_unchecked(offset); |
| 1412 | slot.wait_write(); |
| 1413 | let task = slot.task.get().read().assume_init(); |
| 1414 | |
| 1415 | // Destroy the block if we've reached the end, or if another thread wanted to destroy |
| 1416 | // but couldn't because we were busy reading from the slot. |
Joel Galenson | f0b1773 | 2021-08-09 10:27:52 -0700 | [diff] [blame] | 1417 | if (offset + 1 == BLOCK_CAP) |
| 1418 | || (slot.state.fetch_or(READ, Ordering::AcqRel) & DESTROY != 0) |
| 1419 | { |
Jakub Kotur | 5dd645c | 2020-12-21 17:28:14 +0100 | [diff] [blame] | 1420 | Block::destroy(block, offset); |
| 1421 | } |
| 1422 | |
| 1423 | Steal::Success(task) |
| 1424 | } |
| 1425 | } |
| 1426 | |
| 1427 | /// Steals a batch of tasks and pushes them into a worker. |
| 1428 | /// |
| 1429 | /// How many tasks exactly will be stolen is not specified. That said, this method will try to |
| 1430 | /// steal around half of the tasks in the queue, but also not more than some constant limit. |
| 1431 | /// |
| 1432 | /// # Examples |
| 1433 | /// |
| 1434 | /// ``` |
| 1435 | /// use crossbeam_deque::{Injector, Worker}; |
| 1436 | /// |
| 1437 | /// let q = Injector::new(); |
| 1438 | /// q.push(1); |
| 1439 | /// q.push(2); |
| 1440 | /// q.push(3); |
| 1441 | /// q.push(4); |
| 1442 | /// |
| 1443 | /// let w = Worker::new_fifo(); |
| 1444 | /// let _ = q.steal_batch(&w); |
| 1445 | /// assert_eq!(w.pop(), Some(1)); |
| 1446 | /// assert_eq!(w.pop(), Some(2)); |
| 1447 | /// ``` |
| 1448 | pub fn steal_batch(&self, dest: &Worker<T>) -> Steal<()> { |
| 1449 | let mut head; |
| 1450 | let mut block; |
| 1451 | let mut offset; |
| 1452 | |
| 1453 | let backoff = Backoff::new(); |
| 1454 | loop { |
| 1455 | head = self.head.index.load(Ordering::Acquire); |
| 1456 | block = self.head.block.load(Ordering::Acquire); |
| 1457 | |
| 1458 | // Calculate the offset of the index into the block. |
| 1459 | offset = (head >> SHIFT) % LAP; |
| 1460 | |
| 1461 | // If we reached the end of the block, wait until the next one is installed. |
| 1462 | if offset == BLOCK_CAP { |
| 1463 | backoff.snooze(); |
| 1464 | } else { |
| 1465 | break; |
| 1466 | } |
| 1467 | } |
| 1468 | |
| 1469 | let mut new_head = head; |
| 1470 | let advance; |
| 1471 | |
| 1472 | if new_head & HAS_NEXT == 0 { |
| 1473 | atomic::fence(Ordering::SeqCst); |
| 1474 | let tail = self.tail.index.load(Ordering::Relaxed); |
| 1475 | |
| 1476 | // If the tail equals the head, that means the queue is empty. |
| 1477 | if head >> SHIFT == tail >> SHIFT { |
| 1478 | return Steal::Empty; |
| 1479 | } |
| 1480 | |
| 1481 | // If head and tail are not in the same block, set `HAS_NEXT` in head. Also, calculate |
| 1482 | // the right batch size to steal. |
| 1483 | if (head >> SHIFT) / LAP != (tail >> SHIFT) / LAP { |
| 1484 | new_head |= HAS_NEXT; |
| 1485 | // We can steal all tasks till the end of the block. |
| 1486 | advance = (BLOCK_CAP - offset).min(MAX_BATCH); |
| 1487 | } else { |
| 1488 | let len = (tail - head) >> SHIFT; |
| 1489 | // Steal half of the available tasks. |
| 1490 | advance = ((len + 1) / 2).min(MAX_BATCH); |
| 1491 | } |
| 1492 | } else { |
| 1493 | // We can steal all tasks till the end of the block. |
| 1494 | advance = (BLOCK_CAP - offset).min(MAX_BATCH); |
| 1495 | } |
| 1496 | |
| 1497 | new_head += advance << SHIFT; |
| 1498 | let new_offset = offset + advance; |
| 1499 | |
| 1500 | // Try moving the head index forward. |
| 1501 | if self |
| 1502 | .head |
| 1503 | .index |
| 1504 | .compare_exchange_weak(head, new_head, Ordering::SeqCst, Ordering::Acquire) |
| 1505 | .is_err() |
| 1506 | { |
| 1507 | return Steal::Retry; |
| 1508 | } |
| 1509 | |
| 1510 | // Reserve capacity for the stolen batch. |
| 1511 | let batch_size = new_offset - offset; |
| 1512 | dest.reserve(batch_size); |
| 1513 | |
| 1514 | // Get the destination buffer and back index. |
| 1515 | let dest_buffer = dest.buffer.get(); |
| 1516 | let dest_b = dest.inner.back.load(Ordering::Relaxed); |
| 1517 | |
| 1518 | unsafe { |
| 1519 | // If we've reached the end of the block, move to the next one. |
| 1520 | if new_offset == BLOCK_CAP { |
| 1521 | let next = (*block).wait_next(); |
| 1522 | let mut next_index = (new_head & !HAS_NEXT).wrapping_add(1 << SHIFT); |
| 1523 | if !(*next).next.load(Ordering::Relaxed).is_null() { |
| 1524 | next_index |= HAS_NEXT; |
| 1525 | } |
| 1526 | |
| 1527 | self.head.block.store(next, Ordering::Release); |
| 1528 | self.head.index.store(next_index, Ordering::Release); |
| 1529 | } |
| 1530 | |
| 1531 | // Copy values from the injector into the destination queue. |
| 1532 | match dest.flavor { |
| 1533 | Flavor::Fifo => { |
| 1534 | for i in 0..batch_size { |
| 1535 | // Read the task. |
| 1536 | let slot = (*block).slots.get_unchecked(offset + i); |
| 1537 | slot.wait_write(); |
| 1538 | let task = slot.task.get().read().assume_init(); |
| 1539 | |
| 1540 | // Write it into the destination queue. |
| 1541 | dest_buffer.write(dest_b.wrapping_add(i as isize), task); |
| 1542 | } |
| 1543 | } |
| 1544 | |
| 1545 | Flavor::Lifo => { |
| 1546 | for i in 0..batch_size { |
| 1547 | // Read the task. |
| 1548 | let slot = (*block).slots.get_unchecked(offset + i); |
| 1549 | slot.wait_write(); |
| 1550 | let task = slot.task.get().read().assume_init(); |
| 1551 | |
| 1552 | // Write it into the destination queue. |
| 1553 | dest_buffer.write(dest_b.wrapping_add((batch_size - 1 - i) as isize), task); |
| 1554 | } |
| 1555 | } |
| 1556 | } |
| 1557 | |
| 1558 | atomic::fence(Ordering::Release); |
| 1559 | |
| 1560 | // Update the back index in the destination queue. |
| 1561 | // |
| 1562 | // This ordering could be `Relaxed`, but then thread sanitizer would falsely report |
| 1563 | // data races because it doesn't understand fences. |
| 1564 | dest.inner |
| 1565 | .back |
| 1566 | .store(dest_b.wrapping_add(batch_size as isize), Ordering::Release); |
| 1567 | |
| 1568 | // Destroy the block if we've reached the end, or if another thread wanted to destroy |
| 1569 | // but couldn't because we were busy reading from the slot. |
| 1570 | if new_offset == BLOCK_CAP { |
| 1571 | Block::destroy(block, offset); |
| 1572 | } else { |
| 1573 | for i in offset..new_offset { |
| 1574 | let slot = (*block).slots.get_unchecked(i); |
| 1575 | |
| 1576 | if slot.state.fetch_or(READ, Ordering::AcqRel) & DESTROY != 0 { |
| 1577 | Block::destroy(block, offset); |
| 1578 | break; |
| 1579 | } |
| 1580 | } |
| 1581 | } |
| 1582 | |
| 1583 | Steal::Success(()) |
| 1584 | } |
| 1585 | } |
| 1586 | |
| 1587 | /// Steals a batch of tasks, pushes them into a worker, and pops a task from that worker. |
| 1588 | /// |
| 1589 | /// How many tasks exactly will be stolen is not specified. That said, this method will try to |
| 1590 | /// steal around half of the tasks in the queue, but also not more than some constant limit. |
| 1591 | /// |
| 1592 | /// # Examples |
| 1593 | /// |
| 1594 | /// ``` |
| 1595 | /// use crossbeam_deque::{Injector, Steal, Worker}; |
| 1596 | /// |
| 1597 | /// let q = Injector::new(); |
| 1598 | /// q.push(1); |
| 1599 | /// q.push(2); |
| 1600 | /// q.push(3); |
| 1601 | /// q.push(4); |
| 1602 | /// |
| 1603 | /// let w = Worker::new_fifo(); |
| 1604 | /// assert_eq!(q.steal_batch_and_pop(&w), Steal::Success(1)); |
| 1605 | /// assert_eq!(w.pop(), Some(2)); |
| 1606 | /// ``` |
| 1607 | pub fn steal_batch_and_pop(&self, dest: &Worker<T>) -> Steal<T> { |
| 1608 | let mut head; |
| 1609 | let mut block; |
| 1610 | let mut offset; |
| 1611 | |
| 1612 | let backoff = Backoff::new(); |
| 1613 | loop { |
| 1614 | head = self.head.index.load(Ordering::Acquire); |
| 1615 | block = self.head.block.load(Ordering::Acquire); |
| 1616 | |
| 1617 | // Calculate the offset of the index into the block. |
| 1618 | offset = (head >> SHIFT) % LAP; |
| 1619 | |
| 1620 | // If we reached the end of the block, wait until the next one is installed. |
| 1621 | if offset == BLOCK_CAP { |
| 1622 | backoff.snooze(); |
| 1623 | } else { |
| 1624 | break; |
| 1625 | } |
| 1626 | } |
| 1627 | |
| 1628 | let mut new_head = head; |
| 1629 | let advance; |
| 1630 | |
| 1631 | if new_head & HAS_NEXT == 0 { |
| 1632 | atomic::fence(Ordering::SeqCst); |
| 1633 | let tail = self.tail.index.load(Ordering::Relaxed); |
| 1634 | |
| 1635 | // If the tail equals the head, that means the queue is empty. |
| 1636 | if head >> SHIFT == tail >> SHIFT { |
| 1637 | return Steal::Empty; |
| 1638 | } |
| 1639 | |
| 1640 | // If head and tail are not in the same block, set `HAS_NEXT` in head. |
| 1641 | if (head >> SHIFT) / LAP != (tail >> SHIFT) / LAP { |
| 1642 | new_head |= HAS_NEXT; |
| 1643 | // We can steal all tasks till the end of the block. |
| 1644 | advance = (BLOCK_CAP - offset).min(MAX_BATCH + 1); |
| 1645 | } else { |
| 1646 | let len = (tail - head) >> SHIFT; |
| 1647 | // Steal half of the available tasks. |
| 1648 | advance = ((len + 1) / 2).min(MAX_BATCH + 1); |
| 1649 | } |
| 1650 | } else { |
| 1651 | // We can steal all tasks till the end of the block. |
| 1652 | advance = (BLOCK_CAP - offset).min(MAX_BATCH + 1); |
| 1653 | } |
| 1654 | |
| 1655 | new_head += advance << SHIFT; |
| 1656 | let new_offset = offset + advance; |
| 1657 | |
| 1658 | // Try moving the head index forward. |
| 1659 | if self |
| 1660 | .head |
| 1661 | .index |
| 1662 | .compare_exchange_weak(head, new_head, Ordering::SeqCst, Ordering::Acquire) |
| 1663 | .is_err() |
| 1664 | { |
| 1665 | return Steal::Retry; |
| 1666 | } |
| 1667 | |
| 1668 | // Reserve capacity for the stolen batch. |
| 1669 | let batch_size = new_offset - offset - 1; |
| 1670 | dest.reserve(batch_size); |
| 1671 | |
| 1672 | // Get the destination buffer and back index. |
| 1673 | let dest_buffer = dest.buffer.get(); |
| 1674 | let dest_b = dest.inner.back.load(Ordering::Relaxed); |
| 1675 | |
| 1676 | unsafe { |
| 1677 | // If we've reached the end of the block, move to the next one. |
| 1678 | if new_offset == BLOCK_CAP { |
| 1679 | let next = (*block).wait_next(); |
| 1680 | let mut next_index = (new_head & !HAS_NEXT).wrapping_add(1 << SHIFT); |
| 1681 | if !(*next).next.load(Ordering::Relaxed).is_null() { |
| 1682 | next_index |= HAS_NEXT; |
| 1683 | } |
| 1684 | |
| 1685 | self.head.block.store(next, Ordering::Release); |
| 1686 | self.head.index.store(next_index, Ordering::Release); |
| 1687 | } |
| 1688 | |
| 1689 | // Read the task. |
| 1690 | let slot = (*block).slots.get_unchecked(offset); |
| 1691 | slot.wait_write(); |
| 1692 | let task = slot.task.get().read().assume_init(); |
| 1693 | |
| 1694 | match dest.flavor { |
| 1695 | Flavor::Fifo => { |
| 1696 | // Copy values from the injector into the destination queue. |
| 1697 | for i in 0..batch_size { |
| 1698 | // Read the task. |
| 1699 | let slot = (*block).slots.get_unchecked(offset + i + 1); |
| 1700 | slot.wait_write(); |
| 1701 | let task = slot.task.get().read().assume_init(); |
| 1702 | |
| 1703 | // Write it into the destination queue. |
| 1704 | dest_buffer.write(dest_b.wrapping_add(i as isize), task); |
| 1705 | } |
| 1706 | } |
| 1707 | |
| 1708 | Flavor::Lifo => { |
| 1709 | // Copy values from the injector into the destination queue. |
| 1710 | for i in 0..batch_size { |
| 1711 | // Read the task. |
| 1712 | let slot = (*block).slots.get_unchecked(offset + i + 1); |
| 1713 | slot.wait_write(); |
| 1714 | let task = slot.task.get().read().assume_init(); |
| 1715 | |
| 1716 | // Write it into the destination queue. |
| 1717 | dest_buffer.write(dest_b.wrapping_add((batch_size - 1 - i) as isize), task); |
| 1718 | } |
| 1719 | } |
| 1720 | } |
| 1721 | |
| 1722 | atomic::fence(Ordering::Release); |
| 1723 | |
| 1724 | // Update the back index in the destination queue. |
| 1725 | // |
| 1726 | // This ordering could be `Relaxed`, but then thread sanitizer would falsely report |
| 1727 | // data races because it doesn't understand fences. |
| 1728 | dest.inner |
| 1729 | .back |
| 1730 | .store(dest_b.wrapping_add(batch_size as isize), Ordering::Release); |
| 1731 | |
| 1732 | // Destroy the block if we've reached the end, or if another thread wanted to destroy |
| 1733 | // but couldn't because we were busy reading from the slot. |
| 1734 | if new_offset == BLOCK_CAP { |
| 1735 | Block::destroy(block, offset); |
| 1736 | } else { |
| 1737 | for i in offset..new_offset { |
| 1738 | let slot = (*block).slots.get_unchecked(i); |
| 1739 | |
| 1740 | if slot.state.fetch_or(READ, Ordering::AcqRel) & DESTROY != 0 { |
| 1741 | Block::destroy(block, offset); |
| 1742 | break; |
| 1743 | } |
| 1744 | } |
| 1745 | } |
| 1746 | |
| 1747 | Steal::Success(task) |
| 1748 | } |
| 1749 | } |
| 1750 | |
| 1751 | /// Returns `true` if the queue is empty. |
| 1752 | /// |
| 1753 | /// # Examples |
| 1754 | /// |
| 1755 | /// ``` |
| 1756 | /// use crossbeam_deque::Injector; |
| 1757 | /// |
| 1758 | /// let q = Injector::new(); |
| 1759 | /// |
| 1760 | /// assert!(q.is_empty()); |
| 1761 | /// q.push(1); |
| 1762 | /// assert!(!q.is_empty()); |
| 1763 | /// ``` |
| 1764 | pub fn is_empty(&self) -> bool { |
| 1765 | let head = self.head.index.load(Ordering::SeqCst); |
| 1766 | let tail = self.tail.index.load(Ordering::SeqCst); |
| 1767 | head >> SHIFT == tail >> SHIFT |
| 1768 | } |
| 1769 | |
| 1770 | /// Returns the number of tasks in the queue. |
| 1771 | /// |
| 1772 | /// # Examples |
| 1773 | /// |
| 1774 | /// ``` |
| 1775 | /// use crossbeam_deque::Injector; |
| 1776 | /// |
| 1777 | /// let q = Injector::new(); |
| 1778 | /// |
| 1779 | /// assert_eq!(q.len(), 0); |
| 1780 | /// q.push(1); |
| 1781 | /// assert_eq!(q.len(), 1); |
| 1782 | /// q.push(1); |
| 1783 | /// assert_eq!(q.len(), 2); |
| 1784 | /// ``` |
| 1785 | pub fn len(&self) -> usize { |
| 1786 | loop { |
| 1787 | // Load the tail index, then load the head index. |
| 1788 | let mut tail = self.tail.index.load(Ordering::SeqCst); |
| 1789 | let mut head = self.head.index.load(Ordering::SeqCst); |
| 1790 | |
| 1791 | // If the tail index didn't change, we've got consistent indices to work with. |
| 1792 | if self.tail.index.load(Ordering::SeqCst) == tail { |
| 1793 | // Erase the lower bits. |
| 1794 | tail &= !((1 << SHIFT) - 1); |
| 1795 | head &= !((1 << SHIFT) - 1); |
| 1796 | |
| 1797 | // Fix up indices if they fall onto block ends. |
| 1798 | if (tail >> SHIFT) & (LAP - 1) == LAP - 1 { |
| 1799 | tail = tail.wrapping_add(1 << SHIFT); |
| 1800 | } |
| 1801 | if (head >> SHIFT) & (LAP - 1) == LAP - 1 { |
| 1802 | head = head.wrapping_add(1 << SHIFT); |
| 1803 | } |
| 1804 | |
| 1805 | // Rotate indices so that head falls into the first block. |
| 1806 | let lap = (head >> SHIFT) / LAP; |
| 1807 | tail = tail.wrapping_sub((lap * LAP) << SHIFT); |
| 1808 | head = head.wrapping_sub((lap * LAP) << SHIFT); |
| 1809 | |
| 1810 | // Remove the lower bits. |
| 1811 | tail >>= SHIFT; |
| 1812 | head >>= SHIFT; |
| 1813 | |
| 1814 | // Return the difference minus the number of blocks between tail and head. |
| 1815 | return tail - head - tail / LAP; |
| 1816 | } |
| 1817 | } |
| 1818 | } |
| 1819 | } |
| 1820 | |
| 1821 | impl<T> Drop for Injector<T> { |
| 1822 | fn drop(&mut self) { |
| 1823 | let mut head = self.head.index.load(Ordering::Relaxed); |
| 1824 | let mut tail = self.tail.index.load(Ordering::Relaxed); |
| 1825 | let mut block = self.head.block.load(Ordering::Relaxed); |
| 1826 | |
| 1827 | // Erase the lower bits. |
| 1828 | head &= !((1 << SHIFT) - 1); |
| 1829 | tail &= !((1 << SHIFT) - 1); |
| 1830 | |
| 1831 | unsafe { |
| 1832 | // Drop all values between `head` and `tail` and deallocate the heap-allocated blocks. |
| 1833 | while head != tail { |
| 1834 | let offset = (head >> SHIFT) % LAP; |
| 1835 | |
| 1836 | if offset < BLOCK_CAP { |
| 1837 | // Drop the task in the slot. |
| 1838 | let slot = (*block).slots.get_unchecked(offset); |
| 1839 | let p = &mut *slot.task.get(); |
| 1840 | p.as_mut_ptr().drop_in_place(); |
| 1841 | } else { |
| 1842 | // Deallocate the block and move to the next one. |
| 1843 | let next = (*block).next.load(Ordering::Relaxed); |
| 1844 | drop(Box::from_raw(block)); |
| 1845 | block = next; |
| 1846 | } |
| 1847 | |
| 1848 | head = head.wrapping_add(1 << SHIFT); |
| 1849 | } |
| 1850 | |
| 1851 | // Deallocate the last remaining block. |
| 1852 | drop(Box::from_raw(block)); |
| 1853 | } |
| 1854 | } |
| 1855 | } |
| 1856 | |
| 1857 | impl<T> fmt::Debug for Injector<T> { |
| 1858 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
| 1859 | f.pad("Worker { .. }") |
| 1860 | } |
| 1861 | } |
| 1862 | |
| 1863 | /// Possible outcomes of a steal operation. |
| 1864 | /// |
| 1865 | /// # Examples |
| 1866 | /// |
| 1867 | /// There are lots of ways to chain results of steal operations together: |
| 1868 | /// |
| 1869 | /// ``` |
| 1870 | /// use crossbeam_deque::Steal::{self, Empty, Retry, Success}; |
| 1871 | /// |
| 1872 | /// let collect = |v: Vec<Steal<i32>>| v.into_iter().collect::<Steal<i32>>(); |
| 1873 | /// |
| 1874 | /// assert_eq!(collect(vec![Empty, Empty, Empty]), Empty); |
| 1875 | /// assert_eq!(collect(vec![Empty, Retry, Empty]), Retry); |
| 1876 | /// assert_eq!(collect(vec![Retry, Success(1), Empty]), Success(1)); |
| 1877 | /// |
| 1878 | /// assert_eq!(collect(vec![Empty, Empty]).or_else(|| Retry), Retry); |
| 1879 | /// assert_eq!(collect(vec![Retry, Empty]).or_else(|| Success(1)), Success(1)); |
| 1880 | /// ``` |
| 1881 | #[must_use] |
| 1882 | #[derive(PartialEq, Eq, Copy, Clone)] |
| 1883 | pub enum Steal<T> { |
| 1884 | /// The queue was empty at the time of stealing. |
| 1885 | Empty, |
| 1886 | |
| 1887 | /// At least one task was successfully stolen. |
| 1888 | Success(T), |
| 1889 | |
| 1890 | /// The steal operation needs to be retried. |
| 1891 | Retry, |
| 1892 | } |
| 1893 | |
| 1894 | impl<T> Steal<T> { |
| 1895 | /// Returns `true` if the queue was empty at the time of stealing. |
| 1896 | /// |
| 1897 | /// # Examples |
| 1898 | /// |
| 1899 | /// ``` |
| 1900 | /// use crossbeam_deque::Steal::{Empty, Retry, Success}; |
| 1901 | /// |
| 1902 | /// assert!(!Success(7).is_empty()); |
| 1903 | /// assert!(!Retry::<i32>.is_empty()); |
| 1904 | /// |
| 1905 | /// assert!(Empty::<i32>.is_empty()); |
| 1906 | /// ``` |
| 1907 | pub fn is_empty(&self) -> bool { |
| 1908 | match self { |
| 1909 | Steal::Empty => true, |
| 1910 | _ => false, |
| 1911 | } |
| 1912 | } |
| 1913 | |
| 1914 | /// Returns `true` if at least one task was stolen. |
| 1915 | /// |
| 1916 | /// # Examples |
| 1917 | /// |
| 1918 | /// ``` |
| 1919 | /// use crossbeam_deque::Steal::{Empty, Retry, Success}; |
| 1920 | /// |
| 1921 | /// assert!(!Empty::<i32>.is_success()); |
| 1922 | /// assert!(!Retry::<i32>.is_success()); |
| 1923 | /// |
| 1924 | /// assert!(Success(7).is_success()); |
| 1925 | /// ``` |
| 1926 | pub fn is_success(&self) -> bool { |
| 1927 | match self { |
| 1928 | Steal::Success(_) => true, |
| 1929 | _ => false, |
| 1930 | } |
| 1931 | } |
| 1932 | |
| 1933 | /// Returns `true` if the steal operation needs to be retried. |
| 1934 | /// |
| 1935 | /// # Examples |
| 1936 | /// |
| 1937 | /// ``` |
| 1938 | /// use crossbeam_deque::Steal::{Empty, Retry, Success}; |
| 1939 | /// |
| 1940 | /// assert!(!Empty::<i32>.is_retry()); |
| 1941 | /// assert!(!Success(7).is_retry()); |
| 1942 | /// |
| 1943 | /// assert!(Retry::<i32>.is_retry()); |
| 1944 | /// ``` |
| 1945 | pub fn is_retry(&self) -> bool { |
| 1946 | match self { |
| 1947 | Steal::Retry => true, |
| 1948 | _ => false, |
| 1949 | } |
| 1950 | } |
| 1951 | |
| 1952 | /// Returns the result of the operation, if successful. |
| 1953 | /// |
| 1954 | /// # Examples |
| 1955 | /// |
| 1956 | /// ``` |
| 1957 | /// use crossbeam_deque::Steal::{Empty, Retry, Success}; |
| 1958 | /// |
| 1959 | /// assert_eq!(Empty::<i32>.success(), None); |
| 1960 | /// assert_eq!(Retry::<i32>.success(), None); |
| 1961 | /// |
| 1962 | /// assert_eq!(Success(7).success(), Some(7)); |
| 1963 | /// ``` |
| 1964 | pub fn success(self) -> Option<T> { |
| 1965 | match self { |
| 1966 | Steal::Success(res) => Some(res), |
| 1967 | _ => None, |
| 1968 | } |
| 1969 | } |
| 1970 | |
| 1971 | /// If no task was stolen, attempts another steal operation. |
| 1972 | /// |
| 1973 | /// Returns this steal result if it is `Success`. Otherwise, closure `f` is invoked and then: |
| 1974 | /// |
| 1975 | /// * If the second steal resulted in `Success`, it is returned. |
| 1976 | /// * If both steals were unsuccessful but any resulted in `Retry`, then `Retry` is returned. |
| 1977 | /// * If both resulted in `None`, then `None` is returned. |
| 1978 | /// |
| 1979 | /// # Examples |
| 1980 | /// |
| 1981 | /// ``` |
| 1982 | /// use crossbeam_deque::Steal::{Empty, Retry, Success}; |
| 1983 | /// |
| 1984 | /// assert_eq!(Success(1).or_else(|| Success(2)), Success(1)); |
| 1985 | /// assert_eq!(Retry.or_else(|| Success(2)), Success(2)); |
| 1986 | /// |
| 1987 | /// assert_eq!(Retry.or_else(|| Empty), Retry::<i32>); |
| 1988 | /// assert_eq!(Empty.or_else(|| Retry), Retry::<i32>); |
| 1989 | /// |
| 1990 | /// assert_eq!(Empty.or_else(|| Empty), Empty::<i32>); |
| 1991 | /// ``` |
| 1992 | pub fn or_else<F>(self, f: F) -> Steal<T> |
| 1993 | where |
| 1994 | F: FnOnce() -> Steal<T>, |
| 1995 | { |
| 1996 | match self { |
| 1997 | Steal::Empty => f(), |
| 1998 | Steal::Success(_) => self, |
| 1999 | Steal::Retry => { |
| 2000 | if let Steal::Success(res) = f() { |
| 2001 | Steal::Success(res) |
| 2002 | } else { |
| 2003 | Steal::Retry |
| 2004 | } |
| 2005 | } |
| 2006 | } |
| 2007 | } |
| 2008 | } |
| 2009 | |
| 2010 | impl<T> fmt::Debug for Steal<T> { |
| 2011 | fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { |
| 2012 | match self { |
| 2013 | Steal::Empty => f.pad("Empty"), |
| 2014 | Steal::Success(_) => f.pad("Success(..)"), |
| 2015 | Steal::Retry => f.pad("Retry"), |
| 2016 | } |
| 2017 | } |
| 2018 | } |
| 2019 | |
| 2020 | impl<T> FromIterator<Steal<T>> for Steal<T> { |
| 2021 | /// Consumes items until a `Success` is found and returns it. |
| 2022 | /// |
| 2023 | /// If no `Success` was found, but there was at least one `Retry`, then returns `Retry`. |
| 2024 | /// Otherwise, `Empty` is returned. |
| 2025 | fn from_iter<I>(iter: I) -> Steal<T> |
| 2026 | where |
| 2027 | I: IntoIterator<Item = Steal<T>>, |
| 2028 | { |
| 2029 | let mut retry = false; |
| 2030 | for s in iter { |
| 2031 | match &s { |
| 2032 | Steal::Empty => {} |
| 2033 | Steal::Success(_) => return s, |
| 2034 | Steal::Retry => retry = true, |
| 2035 | } |
| 2036 | } |
| 2037 | |
| 2038 | if retry { |
| 2039 | Steal::Retry |
| 2040 | } else { |
| 2041 | Steal::Empty |
| 2042 | } |
| 2043 | } |
| 2044 | } |