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//===----------------------------------------------------------------------===//
//
// This source file is part of the Swift Atomics open source project
//
// Copyright (c) 2024 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//
// Note: All atomic accesses on WASM are sequentially consistent regardless of
// what ordering we tell LLVM to use.
@_extern(c, "llvm.wasm32.memory.atomic.wait32")
internal func _swift_stdlib_wait(
on: UnsafePointer<UInt32>,
expected: UInt32,
timeout: Int64
) -> UInt32
@_extern(c, "llvm.wasm32.memory.atomic.notify")
internal func _swift_stdlib_wake(on: UnsafePointer<UInt32>, count: UInt32)
extension Atomic where Value == UInt32 {
internal borrowing func _wait(expected: _MutexHandle.State) {
_swift_stdlib_wait(
on: .init(_rawAddress),
expected: expected.rawValue,
// A timeout of < 0 means indefinitely.
timeout: -1
)
}
internal borrowing func _wake() {
// Only wake up 1 thread
_swift_stdlib_wake(on: .init(_rawAddress), count: 1)
}
}
@available(SwiftStdlib 6.0, *)
extension _MutexHandle {
@available(SwiftStdlib 6.0, *)
@frozen
@usableFromInline
internal enum State: UInt32, AtomicRepresentable {
case unlocked
case locked
case contended
}
}
@available(SwiftStdlib 6.0, *)
@frozen
@_staticExclusiveOnly
public struct _MutexHandle: ~Copyable {
@usableFromInline
let storage: Atomic<State>
@available(SwiftStdlib 6.0, *)
@_alwaysEmitIntoClient
@_transparent
public init() {
storage = Atomic(.unlocked)
}
@available(SwiftStdlib 6.0, *)
@usableFromInline
internal borrowing func _lock() {
// Note: We could probably merge this cas into a do/while style loop, but we
// really want to perform the strong variant before attempting to do weak
// ones in the loop.
var (exchanged, state) = storage.compareExchange(
expected: .unlocked,
desired: .locked,
successOrdering: .acquiring,
failureOrdering: .relaxed
)
if _fastPath(exchanged) {
// Locked!
return
}
while !exchanged {
// If we're not already contended, go ahead and transition the mutex state
// into being contended. If when we do this that the value stored there
// was unlocked, then we know we unintentionally acquired the lock. A
// weird quirk that occurs if this happens is that we go directly from
// .unlocked -> .contended when in fact the lock may not be contended.
// We may be able to do another atomic access and change it to .locked if
// acquired it, but it may cause more problems than just potentially
// calling wake with no waiters.
if state != .contended, storage.exchange(
.contended,
ordering: .acquiring
) == .unlocked {
// Locked!
return
}
// Block until unlock has been called. This will return early if the call
// to unlock happened between attempting to acquire and attempting to
// wait while nobody else managed to acquire it yet.
storage._wait(expected: .contended)
(exchanged, state) = storage.weakCompareExchange(
expected: .unlocked,
desired: .locked,
successOrdering: .acquiring,
failureOrdering: .relaxed
)
}
// Locked!
}
@available(SwiftStdlib 6.0, *)
@usableFromInline
internal borrowing func _tryLock() -> Bool {
storage.compareExchange(
expected: .unlocked,
desired: .locked,
successOrdering: .acquiring,
failureOrdering: .relaxed
).exchanged
}
@available(SwiftStdlib 6.0, *)
@usableFromInline
internal borrowing func _unlock() {
// Transition our state from being either .locked or .contended to .unlocked.
// At this point the mutex is freely acquirable. If the value that was
// stored in the mutex was .locked, then no one else was waiting on this
// mutex so we can just skip trying to wake up a thread.
guard storage.exchange(.unlocked, ordering: .releasing) == .contended else {
// Unlocked!
return
}
// Otherwise, wake up our next lucky random thread to acquire the mutex.
// (Assuming no new thread acquires the lock before it does)
storage._wake()
// Unlocked!
}
}
|
