Merge pull request #6441 from kprotty/lock
New std.event.Lock implementation
This commit is contained in:
Andrew Kelley
GitHub
commit
a0c0f9ead5
@ -16,107 +16,107 @@ const Loop = std.event.Loop;
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/// Allows only one actor to hold the lock.
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/// TODO: make this API also work in blocking I/O mode.
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pub const Lock = struct {
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shared: bool,
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queue: Queue,
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queue_empty: bool,
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mutex: std.Mutex = std.Mutex{},
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head: usize = UNLOCKED,
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const Queue = std.atomic.Queue(anyframe);
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const UNLOCKED = 0;
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const LOCKED = 1;
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const global_event_loop = Loop.instance orelse
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@compileError("std.event.Lock currently only works with event-based I/O");
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const Waiter = struct {
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// forced Waiter alignment to ensure it doesn't clash with LOCKED
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next: ?*Waiter align(2),
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tail: *Waiter,
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node: Loop.NextTickNode,
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};
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pub fn acquire(self: *Lock) Held {
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const held = self.mutex.acquire();
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// self.head transitions from multiple stages depending on the value:
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// UNLOCKED -> LOCKED:
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// acquire Lock ownership when theres no waiters
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// LOCKED -> <Waiter head ptr>:
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// Lock is already owned, enqueue first Waiter
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// <head ptr> -> <head ptr>:
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// Lock is owned with pending waiters. Push our waiter to the queue.
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if (self.head == UNLOCKED) {
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self.head = LOCKED;
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held.release();
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return Held{ .lock = self };
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}
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var waiter: Waiter = undefined;
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waiter.next = null;
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waiter.tail = &waiter;
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const head = switch (self.head) {
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UNLOCKED => unreachable,
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LOCKED => null,
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else => @intToPtr(*Waiter, self.head),
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};
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if (head) |h| {
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h.tail.next = &waiter;
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h.tail = &waiter;
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} else {
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self.head = @ptrToInt(&waiter);
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}
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suspend {
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waiter.node = Loop.NextTickNode{
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.prev = undefined,
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.next = undefined,
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.data = @frame(),
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};
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held.release();
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}
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return Held{ .lock = self };
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}
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pub const Held = struct {
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lock: *Lock,
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pub fn release(self: Held) void {
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// Resume the next item from the queue.
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if (self.lock.queue.get()) |node| {
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global_event_loop.onNextTick(node);
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return;
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}
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const waiter = blk: {
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const held = self.lock.mutex.acquire();
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defer held.release();
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// We need to release the lock.
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@atomicStore(bool, &self.lock.queue_empty, true, .SeqCst);
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@atomicStore(bool, &self.lock.shared, false, .SeqCst);
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// self.head goes through the reverse transition from acquire():
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// <head ptr> -> <new head ptr>:
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// pop a waiter from the queue to give Lock ownership when theres still others pending
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// <head ptr> -> LOCKED:
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// pop the laster waiter from the queue, while also giving it lock ownership when awaken
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// LOCKED -> UNLOCKED:
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// last lock owner releases lock while no one else is waiting for it
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// There might be a queue item. If we know the queue is empty, we can be done,
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// because the other actor will try to obtain the lock.
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// But if there's a queue item, we are the actor which must loop and attempt
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// to grab the lock again.
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if (@atomicLoad(bool, &self.lock.queue_empty, .SeqCst)) {
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return;
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}
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while (true) {
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if (@atomicRmw(bool, &self.lock.shared, .Xchg, true, .SeqCst)) {
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// We did not obtain the lock. Great, the queue is someone else's problem.
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return;
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switch (self.lock.head) {
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UNLOCKED => {
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unreachable; // Lock unlocked while unlocking
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},
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LOCKED => {
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self.lock.head = UNLOCKED;
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break :blk null;
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},
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else => {
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const waiter = @intToPtr(*Waiter, self.lock.head);
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self.lock.head = if (waiter.next == null) LOCKED else @ptrToInt(waiter.next);
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if (waiter.next) |next|
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next.tail = waiter.tail;
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break :blk waiter;
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},
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}
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};
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// Resume the next item from the queue.
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if (self.lock.queue.get()) |node| {
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global_event_loop.onNextTick(node);
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return;
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}
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// Release the lock again.
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@atomicStore(bool, &self.lock.queue_empty, true, .SeqCst);
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@atomicStore(bool, &self.lock.shared, false, .SeqCst);
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// Find out if we can be done.
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if (@atomicLoad(bool, &self.lock.queue_empty, .SeqCst)) {
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return;
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}
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if (waiter) |w| {
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global_event_loop.onNextTick(&w.node);
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}
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}
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};
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pub fn init() Lock {
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return Lock{
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.shared = false,
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.queue = Queue.init(),
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.queue_empty = true,
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};
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}
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pub fn initLocked() Lock {
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return Lock{
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.shared = true,
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.queue = Queue.init(),
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.queue_empty = true,
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};
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}
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/// Must be called when not locked. Not thread safe.
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/// All calls to acquire() and release() must complete before calling deinit().
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pub fn deinit(self: *Lock) void {
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assert(!self.shared);
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while (self.queue.get()) |node| resume node.data;
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}
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pub fn acquire(self: *Lock) callconv(.Async) Held {
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var my_tick_node = Loop.NextTickNode.init(@frame());
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errdefer _ = self.queue.remove(&my_tick_node); // TODO test canceling an acquire
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suspend {
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self.queue.put(&my_tick_node);
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// At this point, we are in the queue, so we might have already been resumed.
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// We set this bit so that later we can rely on the fact, that if queue_empty == true, some actor
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// will attempt to grab the lock.
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@atomicStore(bool, &self.queue_empty, false, .SeqCst);
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if (!@atomicRmw(bool, &self.shared, .Xchg, true, .SeqCst)) {
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if (self.queue.get()) |node| {
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// Whether this node is us or someone else, we tail resume it.
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resume node.data;
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}
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}
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}
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return Held{ .lock = self };
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}
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};
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test "std.event.Lock" {
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@ -128,41 +128,16 @@ test "std.event.Lock" {
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// TODO https://github.com/ziglang/zig/issues/3251
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if (builtin.os.tag == .freebsd) return error.SkipZigTest;
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// TODO this file has bit-rotted. repair it
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if (true) return error.SkipZigTest;
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var lock = Lock.init();
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defer lock.deinit();
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_ = async testLock(&lock);
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var lock = Lock{};
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testLock(&lock);
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const expected_result = [1]i32{3 * @intCast(i32, shared_test_data.len)} ** shared_test_data.len;
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testing.expectEqualSlices(i32, &expected_result, &shared_test_data);
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}
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fn testLock(lock: *Lock) callconv(.Async) void {
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fn testLock(lock: *Lock) void {
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var handle1 = async lockRunner(lock);
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var tick_node1 = Loop.NextTickNode{
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.prev = undefined,
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.next = undefined,
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.data = &handle1,
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};
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Loop.instance.?.onNextTick(&tick_node1);
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var handle2 = async lockRunner(lock);
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var tick_node2 = Loop.NextTickNode{
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.prev = undefined,
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.next = undefined,
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.data = &handle2,
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};
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Loop.instance.?.onNextTick(&tick_node2);
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var handle3 = async lockRunner(lock);
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var tick_node3 = Loop.NextTickNode{
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.prev = undefined,
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.next = undefined,
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.data = &handle3,
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};
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Loop.instance.?.onNextTick(&tick_node3);
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await handle1;
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await handle2;
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@ -171,13 +146,13 @@ fn testLock(lock: *Lock) callconv(.Async) void {
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var shared_test_data = [1]i32{0} ** 10;
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var shared_test_index: usize = 0;
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fn lockRunner(lock: *Lock) callconv(.Async) void {
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suspend; // resumed by onNextTick
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fn lockRunner(lock: *Lock) void {
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Lock.global_event_loop.yield();
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var i: usize = 0;
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while (i < shared_test_data.len) : (i += 1) {
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var lock_frame = async lock.acquire();
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const handle = await lock_frame;
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const handle = lock.acquire();
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defer handle.release();
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shared_test_index = 0;
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