91 lines
2.5 KiB
Zig
91 lines
2.5 KiB
Zig
// SPDX-License-Identifier: MIT
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// Copyright (c) 2015-2020 Zig Contributors
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// This file is part of [zig](https://ziglang.org/), which is MIT licensed.
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// The MIT license requires this copyright notice to be included in all copies
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// and substantial portions of the software.
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const std = @import("std.zig");
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const builtin = @import("builtin");
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pub const SpinLock = struct {
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state: State,
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const State = enum(u8) {
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Unlocked,
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Locked,
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};
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pub const Held = struct {
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spinlock: *SpinLock,
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pub fn release(self: Held) void {
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@atomicStore(State, &self.spinlock.state, .Unlocked, .Release);
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}
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};
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pub fn init() SpinLock {
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return SpinLock{ .state = .Unlocked };
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}
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pub fn deinit(self: *SpinLock) void {
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self.* = undefined;
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}
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pub fn tryAcquire(self: *SpinLock) ?Held {
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return switch (@atomicRmw(State, &self.state, .Xchg, .Locked, .Acquire)) {
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.Unlocked => Held{ .spinlock = self },
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.Locked => null,
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};
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}
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pub fn acquire(self: *SpinLock) Held {
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while (true) {
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return self.tryAcquire() orelse {
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yield();
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continue;
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};
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}
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}
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pub fn yield() void {
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// On native windows, SwitchToThread is too expensive,
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// and yielding for 380-410 iterations was found to be
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// a nice sweet spot. Posix systems on the other hand,
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// especially linux, perform better by yielding the thread.
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switch (builtin.os.tag) {
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.windows => loopHint(400),
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else => std.os.sched_yield() catch loopHint(1),
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}
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}
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/// Hint to the cpu that execution is spinning
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/// for the given amount of iterations.
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pub fn loopHint(iterations: usize) void {
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var i = iterations;
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while (i != 0) : (i -= 1) {
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switch (builtin.arch) {
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// these instructions use a memory clobber as they
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// flush the pipeline of any speculated reads/writes.
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.i386, .x86_64 => asm volatile ("pause"
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:
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:
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: "memory"
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),
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.arm, .aarch64 => asm volatile ("yield"
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:
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:
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: "memory"
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),
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else => std.os.sched_yield() catch {},
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}
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}
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}
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};
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test "spinlock" {
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var lock = SpinLock.init();
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defer lock.deinit();
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const held = lock.acquire();
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defer held.release();
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}
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