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|
const std = @import("std.zig");
const builtin = @import("builtin");
pub const SpinLock = struct {
state: State,
const State = enum(u8) {
Unlocked,
Locked,
};
pub const Held = struct {
spinlock: *SpinLock,
pub fn release(self: Held) void {
@atomicStore(State, &self.spinlock.state, .Unlocked, .Release);
}
};
pub fn init() SpinLock {
return SpinLock{ .state = .Unlocked };
}
pub fn deinit(self: *SpinLock) void {
self.* = undefined;
}
pub fn tryAcquire(self: *SpinLock) ?Held {
return switch (@atomicRmw(State, &self.state, .Xchg, .Locked, .Acquire)) {
.Unlocked => Held{ .spinlock = self },
.Locked => null,
};
}
pub fn acquire(self: *SpinLock) Held {
while (true) {
return self.tryAcquire() orelse {
yield();
continue;
};
}
}
pub fn yield() void {
// On native windows, SwitchToThread is too expensive,
// and yielding for 380-410 iterations was found to be
// a nice sweet spot. Posix systems on the other hand,
// especially linux, perform better by yielding the thread.
switch (builtin.os.tag) {
.windows => loopHint(400),
else => std.os.sched_yield() catch loopHint(1),
}
}
/// Hint to the cpu that execution is spinning
/// for the given amount of iterations.
pub fn loopHint(iterations: usize) void {
var i = iterations;
while (i != 0) : (i -= 1) {
switch (builtin.arch) {
// these instructions use a memory clobber as they
// flush the pipeline of any speculated reads/writes.
.i386, .x86_64 => asm volatile ("pause"
:
:
: "memory"
),
.arm, .aarch64 => asm volatile ("yield"
:
:
: "memory"
),
else => std.os.sched_yield() catch {},
}
}
}
};
test "spinlock" {
var lock = SpinLock.init();
defer lock.deinit();
const held = lock.acquire();
defer held.release();
}
|
