-
Notifications
You must be signed in to change notification settings - Fork 1.8k
Commit
This commit does not belong to any branch on this repository, and may belong to a fork outside of the repository.
Introduce
SegmentQueueSynchronizer abstraction for synchronization …
…primitives
- Loading branch information
Showing
5 changed files
with
1,207 additions
and
176 deletions.
There are no files selected for viewing
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
359 changes: 359 additions & 0 deletions
359
kotlinx-coroutines-core/common/src/internal/SegmentQueueSynchronizer.kt
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,359 @@ | ||
| /* | ||
| * Copyright 2016-2020 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 kotlinx.coroutines.internal.SegmentQueueSynchronizer.ResumeMode.ASYNC | ||
| import kotlinx.coroutines.internal.SegmentQueueSynchronizer.ResumeMode.SYNC | ||
| import kotlinx.coroutines.internal.SegmentQueueSynchronizer.CancellationMode.* | ||
| import kotlinx.coroutines.sync.* | ||
| import kotlin.coroutines.* | ||
| import kotlin.native.concurrent.* | ||
|
|
||
| /** | ||
| * [SegmentQueueSynchronizer] is an abstraction for implementing _fair_ synchronization | ||
| * and communication primitives that maintains a FIFO queue of waiting requests. | ||
| * The two main functions it provides: | ||
| * + [suspend] that stores the specified waiter into the queue, and | ||
| * + [resume] function that tries to retrieve and resume the first waiter with the specified value. | ||
| * | ||
| * One may consider the structure as an infinite array with two counters that reference the next cells | ||
| * for enqueueing a continuation in [suspend] and for retrieving one in [tryResume]. To be short, when | ||
| * [suspend] is invoked, it increments the corresponding counter via fast `Fetch-And-Add` and stores the | ||
| * continuation into the cell. At the same time, [tryResume] increments its own counter and comes to the | ||
| * corresponding cell. | ||
| * | ||
| * Since [suspend] can store [CancellableContinuation]-s, it is possible for [tryResume] to fail if the | ||
| * continuation is already cancelled. In this case, most of the algorithms retry the whole operation. | ||
| * However, some solutions may invoke [tryResume] until it succeeds, so that [SegmentQueueSynchronizer] | ||
| * is provided with a nice short-cut [resume], which also efficiently skips consecutive cancelled continuations. | ||
| * | ||
| * The typical implementations via [SegmentQueueSynchronizer] perform some synchronization at first, | ||
| * (e.g., [Semaphore] modifies the number of available permits), and invoke [suspend] or [tryResume] | ||
| * after that. Following this pattern, it is possible in a concurrent environment that [tryResume] | ||
| * is executed before [suspend] (similarly to the race between `park` and `unpark` for threads), | ||
| * so that [tryResume] comes to an empty cell. This race can be solved with two [strategies][Mode]: | ||
| * [asynchronous][Mode.ASYNC] and [synchronous][Mode.SYNC]. | ||
| * In the [synchronous][Mode.ASYNC] mode, [tryResume] puts the element if the cell is empty | ||
| * and finishes, the next [suspend] comes to this cell and simply grabs the element without suspension. | ||
| * At the same time, in the [synchronous][Mode.SYNC] mode, [tryResume] waits in a bounded spin-loop | ||
| * until the put element is taken, marking the cell as broken if it is not after all. In this case both | ||
| * the current [tryResume] and the [suspend] that comes to this broken cell fail. | ||
| * | ||
| * Here is a state machine for cells. Note that only one [suspend] and at most one [tryResume] (or [resume]) operation | ||
| * can deal with each cell. | ||
| * | ||
| * +------+ `suspend` succeeds +------+ `tryResume` tries +------+ // if `cont.tryResume(..)` succeeds, then | ||
| * | NULL | -------------------> | cont | -------------------> | DONE | (`cont` IS RETRIEVED) // the corresponding `tryResume` succeeds gets | ||
| * +------+ +------+ to resume `cont` +------+ // as well, fails otherwise. | ||
| * | | | ||
| * | | The suspended request is cancelled and the continuation is | ||
| * | `tryResume` comes | replaced with a special `CANCELLED` token to avoid memory leaks. | ||
| * | to the cell before V | ||
| * | `suspend` and puts +-----------+ | ||
| * | the element into | CANCELLED | (`cont` IS CANCELLED, `tryResume` FAILS) | ||
| * | the cell, waiting +-----------+ | ||
| * | for `suspend` in | ||
| * | ASYNC mode. | ||
| * | | ||
| * | `suspend` gets +-------+ ( ELIMINATION HAPPENED, ) | ||
| * | +-----------------> | TAKEN | ( BOTH `tryResume` and ) | ||
| * V | the element +-------+ ( `suspend` SUCCEED ) | ||
| * +---------+ | | ||
| * | element | --< | ||
| * +---------+ | | ||
| * | | ||
| * | `tryResume` has waited a bounded time +--------+ | ||
| * +---------------------------------------> | BROKEN | (BOTH `suspend` AND `tryResume` FAIL) | ||
| * but `suspend` has not come +--------+ | ||
| * | ||
| * As for the infinite array implementation, it is organized as a linked list of [segments][SQSSegment]; | ||
| * each segment contains a fixed number of cells. To determine the cell for each [suspend] and [tryResume] | ||
| * operation, the algorithm reads the current [tail] or [head], increments [enqIdx] or [deqIdx], and | ||
| * finds the required segment starting from the initially read one. | ||
| */ | ||
| @InternalCoroutinesApi | ||
| internal abstract class SegmentQueueSynchronizer<T : Any> { | ||
| private val head: AtomicRef<SQSSegment> | ||
| private val deqIdx = atomic(0L) | ||
| private val tail: AtomicRef<SQSSegment> | ||
| private val enqIdx = atomic(0L) | ||
|
|
||
| init { | ||
| val s = SQSSegment(0, null, 2) | ||
| head = atomic(s) | ||
| tail = atomic(s) | ||
| } | ||
|
|
||
| /** | ||
| * Specifies whether [resume] should work in | ||
| * [synchronous][SYNC] or [asynchronous][ASYNC] mode. | ||
| */ | ||
| protected open val resumeMode: ResumeMode get() = SYNC | ||
|
|
||
| /** | ||
| * Specifies whether [resume] should fail on cancelled waiters ([SIMPLE]), | ||
| * or skip them in either [synchronous][SMART_SYNC] or [asynchronous][SMART_ASYNC] | ||
| * way; in the asynchronous skip mode [resume] may pass the element to the | ||
| * cancellation handler in order not to wait, so that the element can be "hung" for a while. | ||
| * Specifies whether [resume] should skip cancelled waiters (`true`) | ||
| * or fail in this case (`false`). By default, [resume] fails if | ||
| * comes to a cancelled waiter. | ||
| */ | ||
| protected open val cancellationMode: CancellationMode get() = SIMPLE | ||
|
|
||
| /** | ||
| * This function is invoked when waiter is cancelled. It returns | ||
| * `true` if the cancellation succeeds and no additional synchronization | ||
| * is required, so that the corresponding cell can be marked as [CANCELLED]. | ||
| * However, if a concurrent [resume] is already going to resume this waiter, | ||
| * [onCancellation] returns `false` so that the element | ||
| * and `false` if the element should be resumed blah- | ||
| * | ||
| * true = the the element can stay in the queue | ||
| */ | ||
| open fun onCancellation() : Boolean = false | ||
|
|
||
| open fun tryReturnRefusedValue(value: T): Boolean = true | ||
|
|
||
| open fun returnValue(value: T) {} | ||
|
|
||
| private fun returnRefusedValue(value: T) { | ||
| if (tryReturnRefusedValue(value)) return | ||
| returnValue(value) | ||
| } | ||
|
|
||
| /** | ||
| * TODO | ||
| * Returns `false` if the received permit cannot be used and the calling operation should restart. | ||
| */ | ||
| @Suppress("UNCHECKED_CAST") | ||
| fun suspend(cont: Continuation<T>): Boolean { | ||
| val curTail = this.tail.value | ||
| val enqIdx = enqIdx.getAndIncrement() | ||
| val segment = this.tail.findSegmentAndMoveForward(id = enqIdx / SEGMENT_SIZE, startFrom = curTail, | ||
| createNewSegment = ::createSegment).segment // cannot be closed | ||
| val i = (enqIdx % SEGMENT_SIZE).toInt() | ||
| // the regular (fast) path -- if the cell is empty, try to install continuation | ||
| if (segment.cas(i, null, cont)) { // try to install the continuation | ||
| if (cont is CancellableContinuation<*>) { | ||
| cont.invokeOnCancellation(SQSCancellationHandler(segment, i).asHandler) | ||
| } | ||
| return true | ||
| } | ||
| // On CAS failure -- the cell must either contain a value or be broken. | ||
| // Try to grab the value. | ||
| val value = segment.get(i) | ||
| if (value !== BROKEN && segment.cas(i, value, TAKEN)) { // took the value eliminating suspend/tryResume pair | ||
| cont.resume(value as T) | ||
| return true | ||
| } | ||
| assert { segment.get(i) === BROKEN } // it must be broken in this case, no other way around it | ||
| return false // broken cell, need to retry on a different cell | ||
| } | ||
|
|
||
| /** | ||
| * Essentially, this is a short-cut for `while (!tryResume(..)) {}`, but | ||
| * works in O(1) without contention independently on how many | ||
| * [suspended][suspend] continuations has been cancelled. | ||
| */ | ||
| fun resume(value: T): Boolean { | ||
| val skipCancelled = cancellationMode != SIMPLE | ||
| while (true) { | ||
| when (tryResumeImpl(value, adjustDeqIdx = skipCancelled)) { | ||
| TRY_RESUME_SUCCESS -> return true | ||
| TRY_RESUME_FAIL_CANCELLED -> if (!skipCancelled) return false | ||
| TRY_RESUME_FAIL_BROKEN -> return false | ||
| } | ||
| } | ||
| } | ||
|
|
||
| @Suppress("UNCHECKED_CAST") | ||
| private fun tryResumeImpl(value: T, adjustDeqIdx: Boolean): Int { | ||
| val curHead = this.head.value | ||
| val deqIdx = deqIdx.getAndIncrement() | ||
| val id = deqIdx / SEGMENT_SIZE | ||
| val segment = this.head.findSegmentAndMoveForward(id, startFrom = curHead, | ||
| createNewSegment = ::createSegment).segment // cannot be closed | ||
| segment.cleanPrev() | ||
| if (segment.id > id) { | ||
| if (adjustDeqIdx) adjustDeqIdx(segment.id * SEGMENT_SIZE) | ||
| return TRY_RESUME_FAIL_CANCELLED | ||
| } | ||
| val i = (deqIdx % SEGMENT_SIZE).toInt() | ||
| modify_cell@while (true) { // modify the cell state | ||
| val cellState = segment.get(i) | ||
| when { | ||
| cellState === null -> { | ||
| if (!segment.cas(i, null, value)) continue@modify_cell | ||
| // Return immediately in the async mode | ||
| if (resumeMode == ASYNC) return TRY_RESUME_SUCCESS | ||
| // Acquire has not touched this cell yet, wait until it comes for a bounded time | ||
| // The cell state can only transition from PERMIT to TAKEN by addAcquireToQueue | ||
| repeat(MAX_SPIN_CYCLES) { | ||
| if (segment.get(i) === TAKEN) return TRY_RESUME_SUCCESS | ||
| } | ||
| // Try to break the slot in order not to wait | ||
| return if (segment.cas(i, value, BROKEN)) TRY_RESUME_FAIL_BROKEN else TRY_RESUME_SUCCESS | ||
| } | ||
| cellState === CANCELLED -> { | ||
| return TRY_RESUME_FAIL_CANCELLED | ||
| } // the acquire was already cancelled | ||
| cellState === REFUSE -> { | ||
| returnRefusedValue(value) | ||
| return TRY_RESUME_SUCCESS | ||
| } | ||
| cellState is CancellableContinuation<*> -> { | ||
| val success = (cellState as CancellableContinuation<T>).tryResume0(value) | ||
| if (success) { | ||
| segment.set(i, DONE) | ||
| return TRY_RESUME_SUCCESS | ||
| } else { | ||
| when (cancellationMode) { | ||
| SIMPLE -> return TRY_RESUME_FAIL_CANCELLED | ||
| SMART_SYNC -> continue@modify_cell | ||
| SMART_ASYNC -> { | ||
| // Let the cancellation handler decide what to do with the element :) | ||
| val valueToStore: Any? = if (value is Continuation<*>) WrappedContinuationValue(value) else value | ||
| if (segment.cas(i, cellState, valueToStore)) return TRY_RESUME_SUCCESS | ||
| } | ||
| } | ||
| } | ||
| } | ||
| else -> { | ||
| (cellState as Continuation<T>).resume(value) | ||
| segment.set(i, DONE) | ||
| return TRY_RESUME_SUCCESS | ||
| } | ||
| } | ||
| } | ||
| } | ||
|
|
||
| private fun adjustDeqIdx(newValue: Long): Unit = deqIdx.loop { cur -> | ||
| if (cur >= newValue) return | ||
| if (deqIdx.compareAndSet(cur, newValue)) return | ||
| } | ||
|
|
||
| /** | ||
| * These modes define the strategy that [tryResume] and [resume] should | ||
| * use when they come to the cell before [suspend] and find it empty. | ||
| * In the [asynchronous][ASYNC] mode, they put the value into the slot, | ||
| * so that [suspend] grabs it and immediately resumes. However, | ||
| * this strategy produces an incorrect behavior when used for some | ||
| * data structures (e.g., for [Semaphore]), and the [synchronous][SYNC] | ||
| * mode is used in this case. Similarly to the asynchronous mode, | ||
| * [tryResume] and [resume] put the value into the cell, but do not finish | ||
| * after that. In opposite, they wait in a bounded spin-loop | ||
| * (see [MAX_SPIN_CYCLES]) until the value is taken, marking the cell | ||
| * as [broken][BROKEN] and failing if it is not, so that the corresponding | ||
| * [suspend] invocation finds the cell [broken][BROKEN] and fails as well. | ||
| */ | ||
| internal enum class ResumeMode { SYNC, ASYNC } | ||
|
|
||
| internal enum class CancellationMode { SIMPLE, SMART_SYNC, SMART_ASYNC } | ||
|
|
||
| private inner class SQSCancellationHandler( | ||
| private val segment: SQSSegment, | ||
| private val index: Int | ||
| ) : CancelHandler() { | ||
| override fun invoke(cause: Throwable?) { | ||
| if (cancellationMode === SIMPLE) { | ||
| segment.markCancelled(index) | ||
| return | ||
| } | ||
| val cancelled = onCancellation() | ||
| if (cancelled) { | ||
| val value = segment.markCancelled(index) ?: return | ||
| if (resume(value as T)) return | ||
| returnValue(value) | ||
| } else { | ||
| val value = segment.markRefused(index) ?: return | ||
| returnRefusedValue(value as T) | ||
| } | ||
| } | ||
|
|
||
| override fun toString() = "SQSCancellationHandler[$segment, $index]" | ||
| } | ||
| } | ||
|
|
||
| private fun <T> CancellableContinuation<T>.tryResume0(value: T): Boolean { | ||
| val token = tryResume(value) ?: return false | ||
| completeResume(token) | ||
| return true | ||
| } | ||
|
|
||
| private fun createSegment(id: Long, prev: SQSSegment?) = SQSSegment(id, prev, 0) | ||
|
|
||
| private class SQSSegment(id: Long, prev: SQSSegment?, pointers: Int) : Segment<SQSSegment>(id, prev, pointers) { | ||
| val waiters = atomicArrayOfNulls<Any?>(SEGMENT_SIZE) | ||
| override val maxSlots: Int get() = SEGMENT_SIZE | ||
|
|
||
| @Suppress("NOTHING_TO_INLINE") | ||
| inline fun get(index: Int): Any? = waiters[index].value | ||
|
|
||
| @Suppress("NOTHING_TO_INLINE") | ||
| inline fun set(index: Int, value: Any?) { | ||
| waiters[index].value = value | ||
| } | ||
|
|
||
| @Suppress("NOTHING_TO_INLINE") | ||
| inline fun cas(index: Int, expected: Any?, value: Any?): Boolean = waiters[index].compareAndSet(expected, value) | ||
|
|
||
| @Suppress("NOTHING_TO_INLINE") | ||
| inline fun getAndSet(index: Int, value: Any?): Any? = waiters[index].getAndSet(value) | ||
|
|
||
| // Cleans the acquirer slot located by the specified index | ||
| // and removes this segment physically if all slots are cleaned. | ||
| fun markCancelled(index: Int): Any? = mark(index, CANCELLED).also { res -> | ||
| if (res == null) onSlotCleaned() | ||
| } | ||
|
|
||
| fun markRefused(index: Int): Any? = mark(index, REFUSE) | ||
|
|
||
| private fun mark(index: Int, marker: Any?): Any? = | ||
| when (val old = getAndSet(index, marker)) { | ||
| is Continuation<*> -> { | ||
| assert { if (old is CancellableContinuation<*>) old.isCancelled else true } | ||
| null | ||
| } | ||
| is WrappedContinuationValue -> old.cont | ||
| else -> old | ||
| } | ||
|
|
||
| override fun toString() = "SQSSegment[id=$id, hashCode=${hashCode()}]" | ||
| } | ||
|
|
||
| /** | ||
| * In the [smart asynchronous cancellation mode][SegmentQueueSynchronizer.CancellationMode.SMART_ASYNC] | ||
| * it is possible that [resume] comes to the cell with cancelled continuation and | ||
| * asynchronously puts its value into the cell, so that the cancellation handler decides whether | ||
| * this value should be used for resuming the next waiter or be refused. When this | ||
| * value is a continuation, it is hard to distinguish it with the one related to the cancelled | ||
| * waiter. Thus, such values are wrapped with [WrappedContinuationValue] in this case. Note, that the | ||
| * wrapper is required only in [SegmentQueueSynchronizer.CancellationMode.SMART_ASYNC] mode | ||
| * and is used in the asynchronous race resolution logic between cancellation and [resume] | ||
| * invocation; this way, it is used relatively rare. | ||
| */ | ||
| private class WrappedContinuationValue(val cont: Continuation<*>) | ||
|
|
||
| @SharedImmutable | ||
| private val SEGMENT_SIZE = systemProp("kotlinx.coroutines.sqs.segmentSize", 16) | ||
| @SharedImmutable | ||
| private val MAX_SPIN_CYCLES = systemProp("kotlinx.coroutines.sqs.maxSpinCycles", 100) | ||
| @SharedImmutable | ||
| private val TAKEN = Symbol("TAKEN") | ||
| @SharedImmutable | ||
| private val BROKEN = Symbol("BROKEN") | ||
| @SharedImmutable | ||
| private val CANCELLED = Symbol("CANCELLED") | ||
| @SharedImmutable | ||
| private val REFUSE = Symbol("REFUSE") | ||
| @SharedImmutable | ||
| private val DONE = Symbol("DONE") | ||
|
|
||
| private const val TRY_RESUME_SUCCESS = 0 | ||
| private const val TRY_RESUME_FAIL_CANCELLED = 1 | ||
| private const val TRY_RESUME_FAIL_BROKEN = 2 |
Oops, something went wrong.