1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385
use crate::loom::cell::UnsafeCell; use crate::loom::sync::atomic::{AtomicPtr, AtomicUsize}; use crate::loom::thread; use std::mem::MaybeUninit; use std::ops; use std::ptr::{self, NonNull}; use std::sync::atomic::Ordering::{self, AcqRel, Acquire, Release}; /// A block in a linked list. /// /// Each block in the list can hold up to `BLOCK_CAP` messages. pub(crate) struct Block<T> { /// The start index of this block. /// /// Slots in this block have indices in `start_index .. start_index + BLOCK_CAP`. start_index: usize, /// The next block in the linked list. next: AtomicPtr<Block<T>>, /// Bitfield tracking slots that are ready to have their values consumed. ready_slots: AtomicUsize, /// The observed `tail_position` value *after* the block has been passed by /// `block_tail`. observed_tail_position: UnsafeCell<usize>, /// Array containing values pushed into the block. Values are stored in a /// continuous array in order to improve cache line behavior when reading. /// The values must be manually dropped. values: Values<T>, } pub(crate) enum Read<T> { Value(T), Closed, } struct Values<T>([UnsafeCell<MaybeUninit<T>>; BLOCK_CAP]); use super::BLOCK_CAP; /// Masks an index to get the block identifier const BLOCK_MASK: usize = !(BLOCK_CAP - 1); /// Masks an index to get the value offset in a block. const SLOT_MASK: usize = BLOCK_CAP - 1; /// Flag tracking that a block has gone through the sender's release routine. /// /// When this is set, the receiver may consider freeing the block. const RELEASED: usize = 1 << BLOCK_CAP; /// Flag tracking all senders dropped. /// /// When this flag is set, the send half of the channel has closed. const TX_CLOSED: usize = RELEASED << 1; /// Mask covering all bits used to track slot readiness. const READY_MASK: usize = RELEASED - 1; /// Returns the index of the first slot in the block referenced by `slot_index`. #[inline(always)] pub(crate) fn start_index(slot_index: usize) -> usize { BLOCK_MASK & slot_index } /// Returns the offset into the block referenced by `slot_index`. #[inline(always)] pub(crate) fn offset(slot_index: usize) -> usize { SLOT_MASK & slot_index } impl<T> Block<T> { pub(crate) fn new(start_index: usize) -> Block<T> { Block { // The absolute index in the channel of the first slot in the block. start_index, // Pointer to the next block in the linked list. next: AtomicPtr::new(ptr::null_mut()), ready_slots: AtomicUsize::new(0), observed_tail_position: UnsafeCell::new(0), // Value storage values: unsafe { Values::uninitialized() }, } } /// Returns `true` if the block matches the given index pub(crate) fn is_at_index(&self, index: usize) -> bool { debug_assert!(offset(index) == 0); self.start_index == index } /// Returns the number of blocks between `self` and the block at the /// specified index. /// /// `start_index` must represent a block *after* `self`. pub(crate) fn distance(&self, other_index: usize) -> usize { debug_assert!(offset(other_index) == 0); other_index.wrapping_sub(self.start_index) / BLOCK_CAP } /// Reads the value at the given offset. /// /// Returns `None` if the slot is empty. /// /// # Safety /// /// To maintain safety, the caller must ensure: /// /// * No concurrent access to the slot. pub(crate) unsafe fn read(&self, slot_index: usize) -> Option<Read<T>> { let offset = offset(slot_index); let ready_bits = self.ready_slots.load(Acquire); if !is_ready(ready_bits, offset) { if is_tx_closed(ready_bits) { return Some(Read::Closed); } return None; } // Get the value let value = self.values[offset].with(|ptr| ptr::read(ptr)); Some(Read::Value(value.assume_init())) } /// Writes a value to the block at the given offset. /// /// # Safety /// /// To maintain safety, the caller must ensure: /// /// * The slot is empty. /// * No concurrent access to the slot. pub(crate) unsafe fn write(&self, slot_index: usize, value: T) { // Get the offset into the block let slot_offset = offset(slot_index); self.values[slot_offset].with_mut(|ptr| { ptr::write(ptr, MaybeUninit::new(value)); }); // Release the value. After this point, the slot ref may no longer // be used. It is possible for the receiver to free the memory at // any point. self.set_ready(slot_offset); } /// Signal to the receiver that the sender half of the list is closed. pub(crate) unsafe fn tx_close(&self) { self.ready_slots.fetch_or(TX_CLOSED, Release); } /// Resets the block to a blank state. This enables reusing blocks in the /// channel. /// /// # Safety /// /// To maintain safety, the caller must ensure: /// /// * All slots are empty. /// * The caller holds a unique pointer to the block. pub(crate) unsafe fn reclaim(&mut self) { self.start_index = 0; self.next = AtomicPtr::new(ptr::null_mut()); self.ready_slots = AtomicUsize::new(0); } /// Releases the block to the rx half for freeing. /// /// This function is called by the tx half once it can be guaranteed that no /// more senders will attempt to access the block. /// /// # Safety /// /// To maintain safety, the caller must ensure: /// /// * The block will no longer be accessed by any sender. pub(crate) unsafe fn tx_release(&self, tail_position: usize) { // Track the observed tail_position. Any sender targetting a greater // tail_position is guaranteed to not access this block. self.observed_tail_position .with_mut(|ptr| *ptr = tail_position); // Set the released bit, signalling to the receiver that it is safe to // free the block's memory as soon as all slots **prior** to // `observed_tail_position` have been filled. self.ready_slots.fetch_or(RELEASED, Release); } /// Mark a slot as ready fn set_ready(&self, slot: usize) { let mask = 1 << slot; self.ready_slots.fetch_or(mask, Release); } /// Returns `true` when all slots have their `ready` bits set. /// /// This indicates that the block is in its final state and will no longer /// be mutated. /// /// # Implementation /// /// The implementation walks each slot checking the `ready` flag. It might /// be that it would make more sense to coalesce ready flags as bits in a /// single atomic cell. However, this could have negative impact on cache /// behavior as there would be many more mutations to a single slot. pub(crate) fn is_final(&self) -> bool { self.ready_slots.load(Acquire) & READY_MASK == READY_MASK } /// Returns the `observed_tail_position` value, if set pub(crate) fn observed_tail_position(&self) -> Option<usize> { if 0 == RELEASED & self.ready_slots.load(Acquire) { None } else { Some(self.observed_tail_position.with(|ptr| unsafe { *ptr })) } } /// Loads the next block pub(crate) fn load_next(&self, ordering: Ordering) -> Option<NonNull<Block<T>>> { let ret = NonNull::new(self.next.load(ordering)); debug_assert!(unsafe { ret.map(|block| block.as_ref().start_index == self.start_index.wrapping_add(BLOCK_CAP)) .unwrap_or(true) }); ret } /// Pushes `block` as the next block in the link. /// /// Returns Ok if successful, otherwise, a pointer to the next block in /// the list is returned. /// /// This requires that the next pointer is null. /// /// # Ordering /// /// This performs a compare-and-swap on `next` using AcqRel ordering. /// /// # Safety /// /// To maintain safety, the caller must ensure: /// /// * `block` is not freed until it has been removed from the list. pub(crate) unsafe fn try_push( &self, block: &mut NonNull<Block<T>>, ordering: Ordering, ) -> Result<(), NonNull<Block<T>>> { block.as_mut().start_index = self.start_index.wrapping_add(BLOCK_CAP); let next_ptr = self .next .compare_and_swap(ptr::null_mut(), block.as_ptr(), ordering); match NonNull::new(next_ptr) { Some(next_ptr) => Err(next_ptr), None => Ok(()), } } /// Grows the `Block` linked list by allocating and appending a new block. /// /// The next block in the linked list is returned. This may or may not be /// the one allocated by the function call. /// /// # Implementation /// /// It is assumed that `self.next` is null. A new block is allocated with /// `start_index` set to be the next block. A compare-and-swap is performed /// with AcqRel memory ordering. If the compare-and-swap is successful, the /// newly allocated block is released to other threads walking the block /// linked list. If the compare-and-swap fails, the current thread acquires /// the next block in the linked list, allowing the current thread to access /// the slots. pub(crate) fn grow(&self) -> NonNull<Block<T>> { // Create the new block. It is assumed that the block will become the // next one after `&self`. If this turns out to not be the case, // `start_index` is updated accordingly. let new_block = Box::new(Block::new(self.start_index + BLOCK_CAP)); let mut new_block = unsafe { NonNull::new_unchecked(Box::into_raw(new_block)) }; // Attempt to store the block. The first compare-and-swap attempt is // "unrolled" due to minor differences in logic // // `AcqRel` is used as the ordering **only** when attempting the // compare-and-swap on self.next. // // If the compare-and-swap fails, then the actual value of the cell is // returned from this function and accessed by the caller. Given this, // the memory must be acquired. // // `Release` ensures that the newly allocated block is available to // other threads acquiring the next pointer. let next = NonNull::new(self.next.compare_and_swap( ptr::null_mut(), new_block.as_ptr(), AcqRel, )); let next = match next { Some(next) => next, None => { // The compare-and-swap succeeded and the newly allocated block // is successfully pushed. return new_block; } }; // There already is a next block in the linked list. The newly allocated // block could be dropped and the discovered next block returned; // however, that would be wasteful. Instead, the linked list is walked // by repeatedly attempting to compare-and-swap the pointer into the // `next` register until the compare-and-swap succeed. // // Care is taken to update new_block's start_index field as appropriate. let mut curr = next; // TODO: Should this iteration be capped? loop { let actual = unsafe { curr.as_ref().try_push(&mut new_block, AcqRel) }; curr = match actual { Ok(_) => { return next; } Err(curr) => curr, }; // When running outside of loom, this calls `spin_loop_hint`. thread::yield_now(); } } } /// Returns `true` if the specificed slot has a value ready to be consumed. fn is_ready(bits: usize, slot: usize) -> bool { let mask = 1 << slot; mask == mask & bits } /// Returns `true` if the closed flag has been set. fn is_tx_closed(bits: usize) -> bool { TX_CLOSED == bits & TX_CLOSED } impl<T> Values<T> { unsafe fn uninitialized() -> Values<T> { let mut vals = MaybeUninit::uninit(); // When fuzzing, `UnsafeCell` needs to be initialized. if_loom! { let p = vals.as_mut_ptr() as *mut UnsafeCell<MaybeUninit<T>>; for i in 0..BLOCK_CAP { p.add(i) .write(UnsafeCell::new(MaybeUninit::uninit())); } } Values(vals.assume_init()) } } impl<T> ops::Index<usize> for Values<T> { type Output = UnsafeCell<MaybeUninit<T>>; fn index(&self, index: usize) -> &Self::Output { self.0.index(index) } }