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use crate::loom::cell::UnsafeCell;
use crate::loom::sync::atomic::{AtomicPtr, AtomicUsize};
use std::alloc::Layout;
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 header fields.
header: BlockHeader<T>,
/// 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>,
}
/// Extra fields for a `Block<T>`.
struct BlockHeader<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>,
}
pub(crate) enum Read<T> {
Value(T),
Closed,
}
#[repr(transparent)]
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
}
generate_addr_of_methods! {
impl<T> Block<T> {
unsafe fn addr_of_header(self: NonNull<Self>) -> NonNull<BlockHeader<T>> {
&self.header
}
unsafe fn addr_of_values(self: NonNull<Self>) -> NonNull<Values<T>> {
&self.values
}
}
}
impl<T> Block<T> {
pub(crate) fn new(start_index: usize) -> Box<Block<T>> {
unsafe {
// Allocate the block on the heap.
// SAFETY: The size of the Block<T> is non-zero, since it is at least the size of the header.
let block = std::alloc::alloc(Layout::new::<Block<T>>()) as *mut Block<T>;
let block = match NonNull::new(block) {
Some(block) => block,
None => std::alloc::handle_alloc_error(Layout::new::<Block<T>>()),
};
// Write the header to the block.
Block::addr_of_header(block).as_ptr().write(BlockHeader {
// 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),
});
// Initialize the values array.
Values::initialize(Block::addr_of_values(block));
// Convert the pointer to a `Box`.
// Safety: The raw pointer was allocated using the global allocator, and with
// the layout for a `Block<T>`, so it's valid to convert it to box.
Box::from_raw(block.as_ptr())
}
}
/// 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.header.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.header.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.header.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()))
}
/// Returns true if *this* block has a value in the given slot.
///
/// Always returns false when given an index from a different block.
pub(crate) fn has_value(&self, slot_index: usize) -> bool {
if slot_index < self.header.start_index {
return false;
}
if slot_index >= self.header.start_index + super::BLOCK_CAP {
return false;
}
let offset = offset(slot_index);
let ready_bits = self.header.ready_slots.load(Acquire);
is_ready(ready_bits, offset)
}
/// 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.header.ready_slots.fetch_or(TX_CLOSED, Release);
}
pub(crate) unsafe fn is_closed(&self) -> bool {
let ready_bits = self.header.ready_slots.load(Acquire);
is_tx_closed(ready_bits)
}
/// 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.header.start_index = 0;
self.header.next = AtomicPtr::new(ptr::null_mut());
self.header.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 targeting a greater
// tail_position is guaranteed to not access this block.
self.header
.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.header.ready_slots.fetch_or(RELEASED, Release);
}
/// Mark a slot as ready
fn set_ready(&self, slot: usize) {
let mask = 1 << slot;
self.header.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.
pub(crate) fn is_final(&self) -> bool {
self.header.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.header.ready_slots.load(Acquire) {
None
} else {
Some(
self.header
.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.header.next.load(ordering));
debug_assert!(unsafe {
ret.map_or(true, |block| {
block.as_ref().header.start_index == self.header.start_index.wrapping_add(BLOCK_CAP)
})
});
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().header.start_index = self.header.start_index.wrapping_add(BLOCK_CAP);
let next_ptr = self
.header
.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 = Block::new(self.header.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.header
.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,
};
crate::loom::thread::yield_now();
}
}
}
/// Returns `true` if the specified 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> {
/// Initialize a `Values` struct from a pointer.
///
/// # Safety
///
/// The raw pointer must be valid for writing a `Values<T>`.
unsafe fn initialize(_value: NonNull<Values<T>>) {
// When fuzzing, `UnsafeCell` needs to be initialized.
if_loom! {
let p = _value.as_ptr() as *mut UnsafeCell<MaybeUninit<T>>;
for i in 0..BLOCK_CAP {
p.add(i)
.write(UnsafeCell::new(MaybeUninit::uninit()));
}
}
}
}
impl<T> ops::Index<usize> for Values<T> {
type Output = UnsafeCell<MaybeUninit<T>>;
fn index(&self, index: usize) -> &Self::Output {
self.0.index(index)
}
}
#[cfg(all(test, not(loom)))]
#[test]
fn assert_no_stack_overflow() {
struct Foo {
_a: [u8; 2_000_000],
}
assert_eq!(
Layout::new::<MaybeUninit<Block<Foo>>>(),
Layout::new::<Block<Foo>>()
);
let _block = Block::<Foo>::new(0);
}