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atomics.swift
595 lines (549 loc) · 27.2 KB
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atomics.swift
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//===----------------------------------------------------------------------===//
//
// This source file is part of the SwiftNIO open source project
//
// Copyright (c) 2017-2018 Apple Inc. and the SwiftNIO project authors
// Licensed under Apache License v2.0
//
// See LICENSE.txt for license information
// See CONTRIBUTORS.txt for the list of SwiftNIO project authors
//
// SPDX-License-Identifier: Apache-2.0
//
//===----------------------------------------------------------------------===//
import CNIOAtomics
#if canImport(Darwin)
import Darwin
fileprivate func sys_sched_yield() {
pthread_yield_np()
}
#elseif os(Windows)
import ucrt
import WinSDK
fileprivate func sys_sched_yield() {
Sleep(0)
}
#else
#if canImport(Glibc)
import Glibc
#elseif canImport(Musl)
import Musl
#elseif canImport(WASILibc)
import WASILibc
#else
#error("The concurrency atomics module was unable to identify your C library.")
#endif
fileprivate func sys_sched_yield() {
_ = sched_yield()
}
#endif
/// An atomic primitive object.
///
/// Before using `UnsafeEmbeddedAtomic`, please consider whether your needs can be met by `Atomic` instead.
/// `UnsafeEmbeddedAtomic` is a value type, but atomics are heap-allocated. Thus, it is only safe to
/// use `UnsafeEmbeddedAtomic` in situations where the atomic can be guaranteed to be cleaned up (via calling `destroy`).
/// If you cannot make these guarantees, use `Atomic` instead, which manages this for you.
///
/// Atomic objects support a wide range of atomic operations:
///
/// - Compare and swap
/// - Add
/// - Subtract
/// - Exchange
/// - Load current value
/// - Store current value
///
/// Atomic primitives are useful when building constructs that need to
/// communicate or cooperate across multiple threads. In the case of
/// SwiftNIO this usually involves communicating across multiple event loops.
@available(*, deprecated, message: "please use UnsafeAtomic from https://github.com/apple/swift-atomics instead")
public struct UnsafeEmbeddedAtomic<T: AtomicPrimitive> {
@usableFromInline
internal let value: OpaquePointer
/// Create an atomic object with `value`.
@inlinable
public init(value: T) {
self.value = T.atomic_create(value)
}
/// Atomically compares the value against `expected` and, if they are equal,
/// replaces the value with `desired`.
///
/// This implementation conforms to C11's `atomic_compare_exchange_strong`. This
/// means that the compare-and-swap will always succeed if `expected` is equal to
/// value. Additionally, it uses a *sequentially consistent ordering*. For more
/// details on atomic memory models, check the documentation for C11's
/// `stdatomic.h`.
///
/// - Parameter expected: The value that this object must currently hold for the
/// compare-and-swap to succeed.
/// - Parameter desired: The new value that this object will hold if the compare
/// succeeds.
/// - Returns: `True` if the exchange occurred, or `False` if `expected` did not
/// match the current value and so no exchange occurred.
@inlinable
public func compareAndExchange(expected: T, desired: T) -> Bool {
return T.atomic_compare_and_exchange(self.value, expected, desired)
}
/// Atomically adds `rhs` to this object.
///
/// This implementation uses a *relaxed* memory ordering. This guarantees nothing
/// more than that this operation is atomic: there is no guarantee that any other
/// event will be ordered before or after this one.
///
/// - Parameter rhs: The value to add to this object.
/// - Returns: The previous value of this object, before the addition occurred.
@discardableResult
@inlinable
public func add(_ rhs: T) -> T {
return T.atomic_add(self.value, rhs)
}
/// Atomically subtracts `rhs` from this object.
///
/// This implementation uses a *relaxed* memory ordering. This guarantees nothing
/// more than that this operation is atomic: there is no guarantee that any other
/// event will be ordered before or after this one.
///
/// - Parameter rhs: The value to subtract from this object.
/// - Returns: The previous value of this object, before the subtraction occurred.
@discardableResult
@inlinable
public func sub(_ rhs: T) -> T {
return T.atomic_sub(self.value, rhs)
}
/// Atomically exchanges `value` for the current value of this object.
///
/// This implementation uses a *relaxed* memory ordering. This guarantees nothing
/// more than that this operation is atomic: there is no guarantee that any other
/// event will be ordered before or after this one.
///
/// - Parameter value: The new value to set this object to.
/// - Returns: The value previously held by this object.
@inlinable
public func exchange(with value: T) -> T {
return T.atomic_exchange(self.value, value)
}
/// Atomically loads and returns the value of this object.
///
/// This implementation uses a *relaxed* memory ordering. This guarantees nothing
/// more than that this operation is atomic: there is no guarantee that any other
/// event will be ordered before or after this one.
///
/// - Returns: The value of this object
@inlinable
public func load() -> T {
return T.atomic_load(self.value)
}
/// Atomically replaces the value of this object with `value`.
///
/// This implementation uses a *relaxed* memory ordering. This guarantees nothing
/// more than that this operation is atomic: there is no guarantee that any other
/// event will be ordered before or after this one.
///
/// - Parameter value: The new value to set the object to.
@inlinable
public func store(_ value: T) -> Void {
T.atomic_store(self.value, value)
}
/// Destroy the atomic value.
///
/// This method is the source of the unsafety of this structure. This *must* be called, or you will leak memory with each
/// atomic.
public func destroy() {
T.atomic_destroy(self.value)
}
}
/// An encapsulation of an atomic primitive object.
///
/// Atomic objects support a wide range of atomic operations:
///
/// - Compare and swap
/// - Add
/// - Subtract
/// - Exchange
/// - Load current value
/// - Store current value
///
/// Atomic primitives are useful when building constructs that need to
/// communicate or cooperate across multiple threads. In the case of
/// SwiftNIO this usually involves communicating across multiple event loops.
///
/// By necessity, all atomic values are references: after all, it makes no
/// sense to talk about managing an atomic value when each time it's modified
/// the thread that modified it gets a local copy!
@available(*, deprecated, message:"please use ManagedAtomic from https://github.com/apple/swift-atomics instead")
public final class Atomic<T: AtomicPrimitive> {
@usableFromInline
internal let embedded: UnsafeEmbeddedAtomic<T>
/// Create an atomic object with `value`.
@inlinable
public init(value: T) {
self.embedded = UnsafeEmbeddedAtomic(value: value)
}
/// Atomically compares the value against `expected` and, if they are equal,
/// replaces the value with `desired`.
///
/// This implementation conforms to C11's `atomic_compare_exchange_strong`. This
/// means that the compare-and-swap will always succeed if `expected` is equal to
/// value. Additionally, it uses a *sequentially consistent ordering*. For more
/// details on atomic memory models, check the documentation for C11's
/// `stdatomic.h`.
///
/// - Parameter expected: The value that this object must currently hold for the
/// compare-and-swap to succeed.
/// - Parameter desired: The new value that this object will hold if the compare
/// succeeds.
/// - Returns: `True` if the exchange occurred, or `False` if `expected` did not
/// match the current value and so no exchange occurred.
@inlinable
public func compareAndExchange(expected: T, desired: T) -> Bool {
return self.embedded.compareAndExchange(expected: expected, desired: desired)
}
/// Atomically adds `rhs` to this object.
///
/// This implementation uses a *relaxed* memory ordering. This guarantees nothing
/// more than that this operation is atomic: there is no guarantee that any other
/// event will be ordered before or after this one.
///
/// - Parameter rhs: The value to add to this object.
/// - Returns: The previous value of this object, before the addition occurred.
@discardableResult
@inlinable
public func add(_ rhs: T) -> T {
return self.embedded.add(rhs)
}
/// Atomically subtracts `rhs` from this object.
///
/// This implementation uses a *relaxed* memory ordering. This guarantees nothing
/// more than that this operation is atomic: there is no guarantee that any other
/// event will be ordered before or after this one.
///
/// - Parameter rhs: The value to subtract from this object.
/// - Returns: The previous value of this object, before the subtraction occurred.
@discardableResult
@inlinable
public func sub(_ rhs: T) -> T {
return self.embedded.sub(rhs)
}
/// Atomically exchanges `value` for the current value of this object.
///
/// This implementation uses a *relaxed* memory ordering. This guarantees nothing
/// more than that this operation is atomic: there is no guarantee that any other
/// event will be ordered before or after this one.
///
/// - Parameter value: The new value to set this object to.
/// - Returns: The value previously held by this object.
@inlinable
public func exchange(with value: T) -> T {
return self.embedded.exchange(with: value)
}
/// Atomically loads and returns the value of this object.
///
/// This implementation uses a *relaxed* memory ordering. This guarantees nothing
/// more than that this operation is atomic: there is no guarantee that any other
/// event will be ordered before or after this one.
///
/// - Returns: The value of this object
@inlinable
public func load() -> T {
return self.embedded.load()
}
/// Atomically replaces the value of this object with `value`.
///
/// This implementation uses a *relaxed* memory ordering. This guarantees nothing
/// more than that this operation is atomic: there is no guarantee that any other
/// event will be ordered before or after this one.
///
/// - Parameter value: The new value to set the object to.
@inlinable
public func store(_ value: T) -> Void {
self.embedded.store(value)
}
deinit {
self.embedded.destroy()
}
}
/// The protocol that all types that can be made atomic must conform to.
///
/// **Do not add conformance to this protocol for arbitrary types**. Only a small range
/// of types have appropriate atomic operations supported by the CPU, and those types
/// already have conformances implemented.
public protocol AtomicPrimitive {
static var atomic_create: (Self) -> OpaquePointer { get }
static var atomic_destroy: (OpaquePointer) -> Void { get }
static var atomic_compare_and_exchange: (OpaquePointer, Self, Self) -> Bool { get }
static var atomic_add: (OpaquePointer, Self) -> Self { get }
static var atomic_sub: (OpaquePointer, Self) -> Self { get }
static var atomic_exchange: (OpaquePointer, Self) -> Self { get }
static var atomic_load: (OpaquePointer) -> Self { get }
static var atomic_store: (OpaquePointer, Self) -> Void { get }
}
extension Bool: AtomicPrimitive {
public static let atomic_create = catmc_atomic__Bool_create
public static let atomic_destroy = catmc_atomic__Bool_destroy
public static let atomic_compare_and_exchange = catmc_atomic__Bool_compare_and_exchange
public static let atomic_add = catmc_atomic__Bool_add
public static let atomic_sub = catmc_atomic__Bool_sub
public static let atomic_exchange = catmc_atomic__Bool_exchange
public static let atomic_load = catmc_atomic__Bool_load
public static let atomic_store = catmc_atomic__Bool_store
}
extension Int8: AtomicPrimitive {
public static let atomic_create = catmc_atomic_int_least8_t_create
public static let atomic_destroy = catmc_atomic_int_least8_t_destroy
public static let atomic_compare_and_exchange = catmc_atomic_int_least8_t_compare_and_exchange
public static let atomic_add = catmc_atomic_int_least8_t_add
public static let atomic_sub = catmc_atomic_int_least8_t_sub
public static let atomic_exchange = catmc_atomic_int_least8_t_exchange
public static let atomic_load = catmc_atomic_int_least8_t_load
public static let atomic_store = catmc_atomic_int_least8_t_store
}
extension UInt8: AtomicPrimitive {
public static let atomic_create = catmc_atomic_uint_least8_t_create
public static let atomic_destroy = catmc_atomic_uint_least8_t_destroy
public static let atomic_compare_and_exchange = catmc_atomic_uint_least8_t_compare_and_exchange
public static let atomic_add = catmc_atomic_uint_least8_t_add
public static let atomic_sub = catmc_atomic_uint_least8_t_sub
public static let atomic_exchange = catmc_atomic_uint_least8_t_exchange
public static let atomic_load = catmc_atomic_uint_least8_t_load
public static let atomic_store = catmc_atomic_uint_least8_t_store
}
extension Int16: AtomicPrimitive {
public static let atomic_create = catmc_atomic_int_least16_t_create
public static let atomic_destroy = catmc_atomic_int_least16_t_destroy
public static let atomic_compare_and_exchange = catmc_atomic_int_least16_t_compare_and_exchange
public static let atomic_add = catmc_atomic_int_least16_t_add
public static let atomic_sub = catmc_atomic_int_least16_t_sub
public static let atomic_exchange = catmc_atomic_int_least16_t_exchange
public static let atomic_load = catmc_atomic_int_least16_t_load
public static let atomic_store = catmc_atomic_int_least16_t_store
}
extension UInt16: AtomicPrimitive {
public static let atomic_create = catmc_atomic_uint_least16_t_create
public static let atomic_destroy = catmc_atomic_uint_least16_t_destroy
public static let atomic_compare_and_exchange = catmc_atomic_uint_least16_t_compare_and_exchange
public static let atomic_add = catmc_atomic_uint_least16_t_add
public static let atomic_sub = catmc_atomic_uint_least16_t_sub
public static let atomic_exchange = catmc_atomic_uint_least16_t_exchange
public static let atomic_load = catmc_atomic_uint_least16_t_load
public static let atomic_store = catmc_atomic_uint_least16_t_store
}
extension Int32: AtomicPrimitive {
public static let atomic_create = catmc_atomic_int_least32_t_create
public static let atomic_destroy = catmc_atomic_int_least32_t_destroy
public static let atomic_compare_and_exchange = catmc_atomic_int_least32_t_compare_and_exchange
public static let atomic_add = catmc_atomic_int_least32_t_add
public static let atomic_sub = catmc_atomic_int_least32_t_sub
public static let atomic_exchange = catmc_atomic_int_least32_t_exchange
public static let atomic_load = catmc_atomic_int_least32_t_load
public static let atomic_store = catmc_atomic_int_least32_t_store
}
extension UInt32: AtomicPrimitive {
public static let atomic_create = catmc_atomic_uint_least32_t_create
public static let atomic_destroy = catmc_atomic_uint_least32_t_destroy
public static let atomic_compare_and_exchange = catmc_atomic_uint_least32_t_compare_and_exchange
public static let atomic_add = catmc_atomic_uint_least32_t_add
public static let atomic_sub = catmc_atomic_uint_least32_t_sub
public static let atomic_exchange = catmc_atomic_uint_least32_t_exchange
public static let atomic_load = catmc_atomic_uint_least32_t_load
public static let atomic_store = catmc_atomic_uint_least32_t_store
}
extension Int64: AtomicPrimitive {
public static let atomic_create = catmc_atomic_long_long_create
public static let atomic_destroy = catmc_atomic_long_long_destroy
public static let atomic_compare_and_exchange = catmc_atomic_long_long_compare_and_exchange
public static let atomic_add = catmc_atomic_long_long_add
public static let atomic_sub = catmc_atomic_long_long_sub
public static let atomic_exchange = catmc_atomic_long_long_exchange
public static let atomic_load = catmc_atomic_long_long_load
public static let atomic_store = catmc_atomic_long_long_store
}
extension UInt64: AtomicPrimitive {
public static let atomic_create = catmc_atomic_unsigned_long_long_create
public static let atomic_destroy = catmc_atomic_unsigned_long_long_destroy
public static let atomic_compare_and_exchange = catmc_atomic_unsigned_long_long_compare_and_exchange
public static let atomic_add = catmc_atomic_unsigned_long_long_add
public static let atomic_sub = catmc_atomic_unsigned_long_long_sub
public static let atomic_exchange = catmc_atomic_unsigned_long_long_exchange
public static let atomic_load = catmc_atomic_unsigned_long_long_load
public static let atomic_store = catmc_atomic_unsigned_long_long_store
}
#if os(Windows)
extension Int: AtomicPrimitive {
public static let atomic_create = catmc_atomic_intptr_t_create
public static let atomic_destroy = catmc_atomic_intptr_t_destroy
public static let atomic_compare_and_exchange = catmc_atomic_intptr_t_compare_and_exchange
public static let atomic_add = catmc_atomic_intptr_t_add
public static let atomic_sub = catmc_atomic_intptr_t_sub
public static let atomic_exchange = catmc_atomic_intptr_t_exchange
public static let atomic_load = catmc_atomic_intptr_t_load
public static let atomic_store = catmc_atomic_intptr_t_store
}
extension UInt: AtomicPrimitive {
public static let atomic_create = catmc_atomic_uintptr_t_create
public static let atomic_destroy = catmc_atomic_uintptr_t_destroy
public static let atomic_compare_and_exchange = catmc_atomic_uintptr_t_compare_and_exchange
public static let atomic_add = catmc_atomic_uintptr_t_add
public static let atomic_sub = catmc_atomic_uintptr_t_sub
public static let atomic_exchange = catmc_atomic_uintptr_t_exchange
public static let atomic_load = catmc_atomic_uintptr_t_load
public static let atomic_store = catmc_atomic_uintptr_t_store
}
#else
extension Int: AtomicPrimitive {
public static let atomic_create = catmc_atomic_long_create
public static let atomic_destroy = catmc_atomic_long_destroy
public static let atomic_compare_and_exchange = catmc_atomic_long_compare_and_exchange
public static let atomic_add = catmc_atomic_long_add
public static let atomic_sub = catmc_atomic_long_sub
public static let atomic_exchange = catmc_atomic_long_exchange
public static let atomic_load = catmc_atomic_long_load
public static let atomic_store = catmc_atomic_long_store
}
extension UInt: AtomicPrimitive {
public static let atomic_create = catmc_atomic_unsigned_long_create
public static let atomic_destroy = catmc_atomic_unsigned_long_destroy
public static let atomic_compare_and_exchange = catmc_atomic_unsigned_long_compare_and_exchange
public static let atomic_add = catmc_atomic_unsigned_long_add
public static let atomic_sub = catmc_atomic_unsigned_long_sub
public static let atomic_exchange = catmc_atomic_unsigned_long_exchange
public static let atomic_load = catmc_atomic_unsigned_long_load
public static let atomic_store = catmc_atomic_unsigned_long_store
}
#endif
/// `AtomicBox` is a heap-allocated box which allows lock-free access to an instance of a Swift class.
///
/// - warning: The use of `AtomicBox` should be avoided because it requires an implementation of a spin-lock
/// (more precisely a CAS loop) to operate correctly.
@available(*, deprecated, message: "AtomicBox is deprecated without replacement because the original implementation doesn't work.")
public final class AtomicBox<T: AnyObject> {
private let storage: NIOAtomic<UInt>
public init(value: T) {
let ptr = Unmanaged<T>.passRetained(value)
self.storage = NIOAtomic.makeAtomic(value: UInt(bitPattern: ptr.toOpaque()))
}
deinit {
let oldPtrBits = self.storage.exchange(with: 0xdeadbee)
let oldPtr = Unmanaged<T>.fromOpaque(UnsafeRawPointer(bitPattern: oldPtrBits)!)
oldPtr.release()
}
/// Atomically compares the value against `expected` and, if they are equal,
/// replaces the value with `desired`.
///
/// This implementation conforms to C11's `atomic_compare_exchange_strong`. This
/// means that the compare-and-swap will always succeed if `expected` is equal to
/// value. Additionally, it uses a *sequentially consistent ordering*. For more
/// details on atomic memory models, check the documentation for C11's
/// `stdatomic.h`.
///
///
/// - warning: The implementation of `exchange` contains a _Compare and Exchange loop_, ie. it may busy wait with
/// 100% CPU load.
///
/// - Parameter expected: The value that this object must currently hold for the
/// compare-and-swap to succeed.
/// - Parameter desired: The new value that this object will hold if the compare
/// succeeds.
/// - Returns: `True` if the exchange occurred, or `False` if `expected` did not
/// match the current value and so no exchange occurred.
public func compareAndExchange(expected: T, desired: T) -> Bool {
return withExtendedLifetime(desired) {
let expectedPtr = Unmanaged<T>.passUnretained(expected)
let desiredPtr = Unmanaged<T>.passUnretained(desired)
let expectedPtrBits = UInt(bitPattern: expectedPtr.toOpaque())
let desiredPtrBits = UInt(bitPattern: desiredPtr.toOpaque())
while true {
if self.storage.compareAndExchange(expected: expectedPtrBits, desired: desiredPtrBits) {
if desiredPtrBits != expectedPtrBits {
_ = desiredPtr.retain()
expectedPtr.release()
}
return true
} else {
let currentPtrBits = self.storage.load()
if currentPtrBits == 0 || currentPtrBits == expectedPtrBits {
sys_sched_yield()
continue
} else {
return false
}
}
}
}
}
/// Atomically exchanges `value` for the current value of this object.
///
/// This implementation uses a *relaxed* memory ordering. This guarantees nothing
/// more than that this operation is atomic: there is no guarantee that any other
/// event will be ordered before or after this one.
///
/// - warning: The implementation of `exchange` contains a _Compare and Exchange loop_, ie. it may busy wait with
/// 100% CPU load.
///
/// - Parameter value: The new value to set this object to.
/// - Returns: The value previously held by this object.
public func exchange(with value: T) -> T {
let newPtr = Unmanaged<T>.passRetained(value)
let newPtrBits = UInt(bitPattern: newPtr.toOpaque())
// step 1: We need to actually CAS loop here to swap out a non-0 value with the new one.
var oldPtrBits: UInt = 0
while true {
let speculativeVal = self.storage.load()
guard speculativeVal != 0 else {
sys_sched_yield()
continue
}
if self.storage.compareAndExchange(expected: speculativeVal, desired: newPtrBits) {
oldPtrBits = speculativeVal
break
}
}
// step 2: After having gained 'ownership' of the old value, we can release the Unmanaged.
let oldPtr = Unmanaged<T>.fromOpaque(UnsafeRawPointer(bitPattern: oldPtrBits)!)
return oldPtr.takeRetainedValue()
}
/// Atomically loads and returns the value of this object.
///
/// This implementation uses a *relaxed* memory ordering. This guarantees nothing
/// more than that this operation is atomic: there is no guarantee that any other
/// event will be ordered before or after this one.
///
/// - warning: The implementation of `exchange` contains a _Compare and Exchange loop_, ie. it may busy wait with
/// 100% CPU load.
///
/// - Returns: The value of this object
public func load() -> T {
// step 1: We need to gain ownership of the value by successfully swapping 0 (marker value) in.
var ptrBits: UInt = 0
while true {
let speculativeVal = self.storage.load()
guard speculativeVal != 0 else {
sys_sched_yield()
continue
}
if self.storage.compareAndExchange(expected: speculativeVal, desired: 0) {
ptrBits = speculativeVal
break
}
}
// step 2: We now consumed a +1'd version of val, so we have all the time in the world to retain it.
let ptr = Unmanaged<T>.fromOpaque(UnsafeRawPointer(bitPattern: ptrBits)!)
let value = ptr.takeUnretainedValue()
// step 3: Now, let's exchange it back into the store
let casWorked = self.storage.compareAndExchange(expected: 0, desired: ptrBits)
precondition(casWorked) // this _has_ to work because `0` means we own it exclusively.
return value
}
/// Atomically replaces the value of this object with `value`.
///
/// This implementation uses a *relaxed* memory ordering. This guarantees nothing
/// more than that this operation is atomic: there is no guarantee that any other
/// event will be ordered before or after this one.
///
/// - warning: The implementation of `exchange` contains a _Compare and Exchange loop_, ie. it may busy wait with
/// 100% CPU load.
///
/// - Parameter value: The new value to set the object to.
public func store(_ value: T) -> Void {
_ = self.exchange(with: value)
}
}