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Coroutines support libraries for Kotlin
/*
* Copyright 2016-2021 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
*/
package kotlinx.coroutines.internal
import kotlinx.atomicfu.*
import kotlinx.coroutines.*
import kotlin.jvm.*
private typealias Core = LockFreeTaskQueueCore
/**
* Lock-free Multiply-Producer xxx-Consumer Queue for task scheduling purposes.
*
* **Note 1: This queue is NOT linearizable. It provides only quiescent consistency for its operations.**
* However, this guarantee is strong enough for task-scheduling purposes.
* In particular, the following execution is permitted for this queue, but is not permitted for a linearizable queue:
*
* ```
* Thread 1: addLast(1) = true, removeFirstOrNull() = null
* Thread 2: addLast(2) = 2 // this operation is concurrent with both operations in the first thread
* ```
*
* **Note 2: When this queue is used with multiple consumers (`singleConsumer == false`) this it is NOT lock-free.**
* In particular, consumer spins until producer finishes its operation in the case of near-empty queue.
* It is a very short window that could manifest itself rarely and only under specific load conditions,
* but it still deprives this algorithm of its lock-freedom.
*/
internal open class LockFreeTaskQueue(
singleConsumer: Boolean // true when there is only a single consumer (slightly faster & lock-free)
) {
private val _cur = atomic(Core(Core.INITIAL_CAPACITY, singleConsumer))
// Note: it is not atomic w.r.t. remove operation (remove can transiently fail when isEmpty is false)
val isEmpty: Boolean get() = _cur.value.isEmpty
val size: Int get() = _cur.value.size
fun close() {
_cur.loop { cur ->
if (cur.close()) return // closed this copy
_cur.compareAndSet(cur, cur.next()) // move to next
}
}
fun addLast(element: E): Boolean {
_cur.loop { cur ->
when (cur.addLast(element)) {
Core.ADD_SUCCESS -> return true
Core.ADD_CLOSED -> return false
Core.ADD_FROZEN -> _cur.compareAndSet(cur, cur.next()) // move to next
}
}
}
@Suppress("UNCHECKED_CAST")
fun removeFirstOrNull(): E? {
_cur.loop { cur ->
val result = cur.removeFirstOrNull()
if (result !== Core.REMOVE_FROZEN) return result as E?
_cur.compareAndSet(cur, cur.next())
}
}
// Used for validation in tests only
fun map(transform: (E) -> R): List = _cur.value.map(transform)
// Used for validation in tests only
fun isClosed(): Boolean = _cur.value.isClosed()
}
/**
* Lock-free Multiply-Producer xxx-Consumer Queue core.
* @see LockFreeTaskQueue
*/
internal class LockFreeTaskQueueCore(
private val capacity: Int,
private val singleConsumer: Boolean // true when there is only a single consumer (slightly faster)
) {
private val mask = capacity - 1
private val _next = atomic?>(null)
private val _state = atomic(0L)
private val array = atomicArrayOfNulls(capacity)
init {
check(mask <= MAX_CAPACITY_MASK)
check(capacity and mask == 0)
}
// Note: it is not atomic w.r.t. remove operation (remove can transiently fail when isEmpty is false)
val isEmpty: Boolean get() = _state.value.withState { head, tail -> head == tail }
val size: Int get() = _state.value.withState { head, tail -> (tail - head) and MAX_CAPACITY_MASK }
fun close(): Boolean {
_state.update { state ->
if (state and CLOSED_MASK != 0L) return true // ok - already closed
if (state and FROZEN_MASK != 0L) return false // frozen -- try next
state or CLOSED_MASK // try set closed bit
}
return true
}
// ADD_CLOSED | ADD_FROZEN | ADD_SUCCESS
fun addLast(element: E): Int {
_state.loop { state ->
if (state and (FROZEN_MASK or CLOSED_MASK) != 0L) return state.addFailReason() // cannot add
state.withState { head, tail ->
val mask = this.mask // manually move instance field to local for performance
// If queue is Single-Consumer then there could be one element beyond head that we cannot overwrite,
// so we check for full queue with an extra margin of one element
if ((tail + 2) and mask == head and mask) return ADD_FROZEN // overfull, so do freeze & copy
// If queue is Multi-Consumer then the consumer could still have not cleared element
// despite the above check for one free slot.
if (!singleConsumer && array[tail and mask].value != null) {
// There are two options in this situation
// 1. Spin-wait until consumer clears the slot
// 2. Freeze & resize to avoid spinning
// We use heuristic here to avoid memory-overallocation
// Freeze & reallocate when queue is small or more than half of the queue is used
if (capacity < MIN_ADD_SPIN_CAPACITY || (tail - head) and MAX_CAPACITY_MASK > capacity shr 1) {
return ADD_FROZEN
}
// otherwise spin
return@loop
}
val newTail = (tail + 1) and MAX_CAPACITY_MASK
if (_state.compareAndSet(state, state.updateTail(newTail))) {
// successfully added
array[tail and mask].value = element
// could have been frozen & copied before this item was set -- correct it by filling placeholder
var cur = this
while(true) {
if (cur._state.value and FROZEN_MASK == 0L) break // all fine -- not frozen yet
cur = cur.next().fillPlaceholder(tail, element) ?: break
}
return ADD_SUCCESS // added successfully
}
}
}
}
private fun fillPlaceholder(index: Int, element: E): Core? {
val old = array[index and mask].value
/*
* addLast actions:
* 1) Commit tail slot
* 2) Write element to array slot
* 3) Check for array copy
*
* If copy happened between 2 and 3 then the consumer might have consumed our element,
* then another producer might have written its placeholder in our slot, so we should
* perform *unique* check that current placeholder is our to avoid overwriting another producer placeholder
*/
if (old is Placeholder && old.index == index) {
array[index and mask].value = element
// we've corrected missing element, should check if that propagated to further copies, just in case
return this
}
// it is Ok, no need for further action
return null
}
// REMOVE_FROZEN | null (EMPTY) | E (SUCCESS)
fun removeFirstOrNull(): Any? {
_state.loop { state ->
if (state and FROZEN_MASK != 0L) return REMOVE_FROZEN // frozen -- cannot modify
state.withState { head, tail ->
if ((tail and mask) == (head and mask)) return null // empty
val element = array[head and mask].value
if (element == null) {
// If queue is Single-Consumer, then element == null only when add has not finished yet
if (singleConsumer) return null // consider it not added yet
// retry (spin) until consumer adds it
return@loop
}
// element == Placeholder can only be when add has not finished yet
if (element is Placeholder) return null // consider it not added yet
// we cannot put null into array here, because copying thread could replace it with Placeholder and that is a disaster
val newHead = (head + 1) and MAX_CAPACITY_MASK
if (_state.compareAndSet(state, state.updateHead(newHead))) {
// Array could have been copied by another thread and it is perfectly fine, since only elements
// between head and tail were copied and there are no extra steps we should take here
array[head and mask].value = null // now can safely put null (state was updated)
return element // successfully removed in fast-path
}
// Multi-Consumer queue must retry this loop on CAS failure (another consumer might have removed element)
if (!singleConsumer) return@loop
// Single-consumer queue goes to slow-path for remove in case of interference
var cur = this
while (true) {
@Suppress("UNUSED_VALUE")
cur = cur.removeSlowPath(head, newHead) ?: return element
}
}
}
}
private fun removeSlowPath(oldHead: Int, newHead: Int): Core? {
_state.loop { state ->
state.withState { head, _ ->
assert { head == oldHead } // "This queue can have only one consumer"
if (state and FROZEN_MASK != 0L) {
// state was already frozen, so removed element was copied to next
return next() // continue to correct head in next
}
if (_state.compareAndSet(state, state.updateHead(newHead))) {
array[head and mask].value = null // now can safely put null (state was updated)
return null
}
}
}
}
fun next(): LockFreeTaskQueueCore = allocateOrGetNextCopy(markFrozen())
private fun markFrozen(): Long =
_state.updateAndGet { state ->
if (state and FROZEN_MASK != 0L) return state // already marked
state or FROZEN_MASK
}
private fun allocateOrGetNextCopy(state: Long): Core {
_next.loop { next ->
if (next != null) return next // already allocated & copied
_next.compareAndSet(null, allocateNextCopy(state))
}
}
private fun allocateNextCopy(state: Long): Core {
val next = LockFreeTaskQueueCore(capacity * 2, singleConsumer)
state.withState { head, tail ->
var index = head
while (index and mask != tail and mask) {
// replace nulls with placeholders on copy
val value = array[index and mask].value ?: Placeholder(index)
next.array[index and next.mask].value = value
index++
}
next._state.value = state wo FROZEN_MASK
}
return next
}
// Used for validation in tests only
fun map(transform: (E) -> R): List {
val res = ArrayList(capacity)
_state.value.withState { head, tail ->
var index = head
while (index and mask != tail and mask) {
// replace nulls with placeholders on copy
val element = array[index and mask].value
@Suppress("UNCHECKED_CAST")
if (element != null && element !is Placeholder) res.add(transform(element as E))
index++
}
}
return res
}
// Used for validation in tests only
fun isClosed(): Boolean = _state.value and CLOSED_MASK != 0L
// Instance of this class is placed into array when we have to copy array, but addLast is in progress --
// it had already reserved a slot in the array (with null) and have not yet put its value there.
// Placeholder keeps the actual index (not masked) to distinguish placeholders on different wraparounds of array
// Internal because of inlining
internal class Placeholder(@JvmField val index: Int)
@Suppress("PrivatePropertyName", "MemberVisibilityCanBePrivate")
internal companion object {
const val INITIAL_CAPACITY = 8
const val CAPACITY_BITS = 30
const val MAX_CAPACITY_MASK = (1 shl CAPACITY_BITS) - 1
const val HEAD_SHIFT = 0
const val HEAD_MASK = MAX_CAPACITY_MASK.toLong() shl HEAD_SHIFT
const val TAIL_SHIFT = HEAD_SHIFT + CAPACITY_BITS
const val TAIL_MASK = MAX_CAPACITY_MASK.toLong() shl TAIL_SHIFT
const val FROZEN_SHIFT = TAIL_SHIFT + CAPACITY_BITS
const val FROZEN_MASK = 1L shl FROZEN_SHIFT
const val CLOSED_SHIFT = FROZEN_SHIFT + 1
const val CLOSED_MASK = 1L shl CLOSED_SHIFT
const val MIN_ADD_SPIN_CAPACITY = 1024
@JvmField val REMOVE_FROZEN = Symbol("REMOVE_FROZEN")
const val ADD_SUCCESS = 0
const val ADD_FROZEN = 1
const val ADD_CLOSED = 2
infix fun Long.wo(other: Long) = this and other.inv()
fun Long.updateHead(newHead: Int) = (this wo HEAD_MASK) or (newHead.toLong() shl HEAD_SHIFT)
fun Long.updateTail(newTail: Int) = (this wo TAIL_MASK) or (newTail.toLong() shl TAIL_SHIFT)
inline fun Long.withState(block: (head: Int, tail: Int) -> T): T {
val head = ((this and HEAD_MASK) shr HEAD_SHIFT).toInt()
val tail = ((this and TAIL_MASK) shr TAIL_SHIFT).toInt()
return block(head, tail)
}
// FROZEN | CLOSED
fun Long.addFailReason(): Int = if (this and CLOSED_MASK != 0L) ADD_CLOSED else ADD_FROZEN
}
}