      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
    386
    387

    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>>,
            success: Ordering,
            failure: Ordering,
        ) -> Result<(), NonNull<Block<T>>> {
            block.as_mut().start_index = self.start_index.wrapping_add(BLOCK_CAP);

            let next_ptr = self
                .next
                .compare_exchange(ptr::null_mut(), block.as_ptr(), success, failure)
                .unwrap_or_else(|x| x);

            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_exchange(ptr::null_mut(), new_block.as_ptr(), AcqRel, Acquire)
                    .unwrap_or_else(|x| x),
            );

            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, Acquire) };

                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)
        }
    }