Merge pull request #1198 from ziglang/m-n-threading

M:N threading

1 files changed, 774 insertions, 76 deletions

diff --git a/std/event.zig b/std/event.zig
index c6ac04a9d0..de51f8c87e 100644
--- a/std/event.zig
+++ b/std/event.zig
@@ -4,6 +4,7 @@ const assert = std.debug.assert;
 const event = this;
 const mem = std.mem;
 const posix = std.os.posix;
+const windows = std.os.windows;
 const AtomicRmwOp = builtin.AtomicRmwOp;
 const AtomicOrder = builtin.AtomicOrder;
 
@@ -11,53 +12,69 @@ pub const TcpServer = struct {
     handleRequestFn: async<*mem.Allocator> fn (*TcpServer, *const std.net.Address, *const std.os.File) void,
 
     loop: *Loop,
-    sockfd: i32,
+    sockfd: ?i32,
     accept_coro: ?promise,
     listen_address: std.net.Address,
 
     waiting_for_emfile_node: PromiseNode,
+    listen_resume_node: event.Loop.ResumeNode,
 
     const PromiseNode = std.LinkedList(promise).Node;
 
-    pub fn init(loop: *Loop) !TcpServer {
-        const sockfd = try std.os.posixSocket(posix.AF_INET, posix.SOCK_STREAM | posix.SOCK_CLOEXEC | posix.SOCK_NONBLOCK, posix.PROTO_tcp);
-        errdefer std.os.close(sockfd);
-
+    pub fn init(loop: *Loop) TcpServer {
         // TODO can't initialize handler coroutine here because we need well defined copy elision
         return TcpServer{
             .loop = loop,
-            .sockfd = sockfd,
+            .sockfd = null,
             .accept_coro = null,
             .handleRequestFn = undefined,
             .waiting_for_emfile_node = undefined,
             .listen_address = undefined,
+            .listen_resume_node = event.Loop.ResumeNode{
+                .id = event.Loop.ResumeNode.Id.Basic,
+                .handle = undefined,
+            },
         };
     }
 
-    pub fn listen(self: *TcpServer, address: *const std.net.Address, handleRequestFn: async<*mem.Allocator> fn (*TcpServer, *const std.net.Address, *const std.os.File) void) !void {
+    pub fn listen(
+        self: *TcpServer,
+        address: *const std.net.Address,
+        handleRequestFn: async<*mem.Allocator> fn (*TcpServer, *const std.net.Address, *const std.os.File) void,
+    ) !void {
         self.handleRequestFn = handleRequestFn;
 
-        try std.os.posixBind(self.sockfd, &address.os_addr);
-        try std.os.posixListen(self.sockfd, posix.SOMAXCONN);
-        self.listen_address = std.net.Address.initPosix(try std.os.posixGetSockName(self.sockfd));
+        const sockfd = try std.os.posixSocket(posix.AF_INET, posix.SOCK_STREAM | posix.SOCK_CLOEXEC | posix.SOCK_NONBLOCK, posix.PROTO_tcp);
+        errdefer std.os.close(sockfd);
+        self.sockfd = sockfd;
+
+        try std.os.posixBind(sockfd, &address.os_addr);
+        try std.os.posixListen(sockfd, posix.SOMAXCONN);
+        self.listen_address = std.net.Address.initPosix(try std.os.posixGetSockName(sockfd));
 
         self.accept_coro = try async<self.loop.allocator> TcpServer.handler(self);
         errdefer cancel self.accept_coro.?;
 
-        try self.loop.addFd(self.sockfd, self.accept_coro.?);
-        errdefer self.loop.removeFd(self.sockfd);
+        self.listen_resume_node.handle = self.accept_coro.?;
+        try self.loop.addFd(sockfd, &self.listen_resume_node);
+        errdefer self.loop.removeFd(sockfd);
+    }
+
+    /// Stop listening
+    pub fn close(self: *TcpServer) void {
+        self.loop.removeFd(self.sockfd.?);
+        std.os.close(self.sockfd.?);
     }
 
     pub fn deinit(self: *TcpServer) void {
-        self.loop.removeFd(self.sockfd);
         if (self.accept_coro) |accept_coro| cancel accept_coro;
-        std.os.close(self.sockfd);
+        if (self.sockfd) |sockfd| std.os.close(sockfd);
     }
 
     pub async fn handler(self: *TcpServer) void {
         while (true) {
             var accepted_addr: std.net.Address = undefined;
-            if (std.os.posixAccept(self.sockfd, &accepted_addr.os_addr, posix.SOCK_NONBLOCK | posix.SOCK_CLOEXEC)) |accepted_fd| {
+            if (std.os.posixAccept(self.sockfd.?, &accepted_addr.os_addr, posix.SOCK_NONBLOCK | posix.SOCK_CLOEXEC)) |accepted_fd| {
                 var socket = std.os.File.openHandle(accepted_fd);
                 _ = async<self.loop.allocator> self.handleRequestFn(self, accepted_addr, socket) catch |err| switch (err) {
                     error.OutOfMemory => {
@@ -95,46 +112,276 @@ pub const TcpServer = struct {
 
 pub const Loop = struct {
     allocator: *mem.Allocator,
-    keep_running: bool,
     next_tick_queue: std.atomic.QueueMpsc(promise),
     os_data: OsData,
+    final_resume_node: ResumeNode,
+    dispatch_lock: u8, // TODO make this a bool
+    pending_event_count: usize,
+    extra_threads: []*std.os.Thread,
 
-    const OsData = switch (builtin.os) {
-        builtin.Os.linux => struct {
-            epollfd: i32,
-        },
-        else => struct {},
-    };
+    // pre-allocated eventfds. all permanently active.
+    // this is how we send promises to be resumed on other threads.
+    available_eventfd_resume_nodes: std.atomic.Stack(ResumeNode.EventFd),
+    eventfd_resume_nodes: []std.atomic.Stack(ResumeNode.EventFd).Node,
 
     pub const NextTickNode = std.atomic.QueueMpsc(promise).Node;
 
+    pub const ResumeNode = struct {
+        id: Id,
+        handle: promise,
+
+        pub const Id = enum {
+            Basic,
+            Stop,
+            EventFd,
+        };
+
+        pub const EventFd = switch (builtin.os) {
+            builtin.Os.macosx => MacOsEventFd,
+            builtin.Os.linux => struct {
+                base: ResumeNode,
+                epoll_op: u32,
+                eventfd: i32,
+            },
+            builtin.Os.windows => struct {
+                base: ResumeNode,
+                completion_key: usize,
+            },
+            else => @compileError("unsupported OS"),
+        };
+
+        const MacOsEventFd = struct {
+            base: ResumeNode,
+            kevent: posix.Kevent,
+        };
+    };
+
+    /// After initialization, call run().
+    /// TODO copy elision / named return values so that the threads referencing *Loop
+    /// have the correct pointer value.
+    fn initSingleThreaded(self: *Loop, allocator: *mem.Allocator) !void {
+        return self.initInternal(allocator, 1);
+    }
+
     /// The allocator must be thread-safe because we use it for multiplexing
     /// coroutines onto kernel threads.
-    pub fn init(allocator: *mem.Allocator) !Loop {
-        var self = Loop{
-            .keep_running = true,
+    /// After initialization, call run().
+    /// TODO copy elision / named return values so that the threads referencing *Loop
+    /// have the correct pointer value.
+    fn initMultiThreaded(self: *Loop, allocator: *mem.Allocator) !void {
+        const core_count = try std.os.cpuCount(allocator);
+        return self.initInternal(allocator, core_count);
+    }
+
+    /// Thread count is the total thread count. The thread pool size will be
+    /// max(thread_count - 1, 0)
+    fn initInternal(self: *Loop, allocator: *mem.Allocator, thread_count: usize) !void {
+        self.* = Loop{
+            .pending_event_count = 0,
             .allocator = allocator,
             .os_data = undefined,
             .next_tick_queue = std.atomic.QueueMpsc(promise).init(),
+            .dispatch_lock = 1, // start locked so threads go directly into epoll wait
+            .extra_threads = undefined,
+            .available_eventfd_resume_nodes = std.atomic.Stack(ResumeNode.EventFd).init(),
+            .eventfd_resume_nodes = undefined,
+            .final_resume_node = ResumeNode{
+                .id = ResumeNode.Id.Stop,
+                .handle = undefined,
+            },
         };
-        try self.initOsData();
-        errdefer self.deinitOsData();
+        const extra_thread_count = thread_count - 1;
+        self.eventfd_resume_nodes = try self.allocator.alloc(
+            std.atomic.Stack(ResumeNode.EventFd).Node,
+            extra_thread_count,
+        );
+        errdefer self.allocator.free(self.eventfd_resume_nodes);
+
+        self.extra_threads = try self.allocator.alloc(*std.os.Thread, extra_thread_count);
+        errdefer self.allocator.free(self.extra_threads);
 
-        return self;
+        try self.initOsData(extra_thread_count);
+        errdefer self.deinitOsData();
     }
 
     /// must call stop before deinit
     pub fn deinit(self: *Loop) void {
         self.deinitOsData();
+        self.allocator.free(self.extra_threads);
     }
 
-    const InitOsDataError = std.os.LinuxEpollCreateError;
+    const InitOsDataError = std.os.LinuxEpollCreateError || mem.Allocator.Error || std.os.LinuxEventFdError ||
+        std.os.SpawnThreadError || std.os.LinuxEpollCtlError || std.os.BsdKEventError ||
+        std.os.WindowsCreateIoCompletionPortError;
 
-    fn initOsData(self: *Loop) InitOsDataError!void {
+    const wakeup_bytes = []u8{0x1} ** 8;
+
+    fn initOsData(self: *Loop, extra_thread_count: usize) InitOsDataError!void {
         switch (builtin.os) {
             builtin.Os.linux => {
-                self.os_data.epollfd = try std.os.linuxEpollCreate(std.os.linux.EPOLL_CLOEXEC);
+                errdefer {
+                    while (self.available_eventfd_resume_nodes.pop()) |node| std.os.close(node.data.eventfd);
+                }
+                for (self.eventfd_resume_nodes) |*eventfd_node| {
+                    eventfd_node.* = std.atomic.Stack(ResumeNode.EventFd).Node{
+                        .data = ResumeNode.EventFd{
+                            .base = ResumeNode{
+                                .id = ResumeNode.Id.EventFd,
+                                .handle = undefined,
+                            },
+                            .eventfd = try std.os.linuxEventFd(1, posix.EFD_CLOEXEC | posix.EFD_NONBLOCK),
+                            .epoll_op = posix.EPOLL_CTL_ADD,
+                        },
+                        .next = undefined,
+                    };
+                    self.available_eventfd_resume_nodes.push(eventfd_node);
+                }
+
+                self.os_data.epollfd = try std.os.linuxEpollCreate(posix.EPOLL_CLOEXEC);
                 errdefer std.os.close(self.os_data.epollfd);
+
+                self.os_data.final_eventfd = try std.os.linuxEventFd(0, posix.EFD_CLOEXEC | posix.EFD_NONBLOCK);
+                errdefer std.os.close(self.os_data.final_eventfd);
+
+                self.os_data.final_eventfd_event = posix.epoll_event{
+                    .events = posix.EPOLLIN,
+                    .data = posix.epoll_data{ .ptr = @ptrToInt(&self.final_resume_node) },
+                };
+                try std.os.linuxEpollCtl(
+                    self.os_data.epollfd,
+                    posix.EPOLL_CTL_ADD,
+                    self.os_data.final_eventfd,
+                    &self.os_data.final_eventfd_event,
+                );
+
+                var extra_thread_index: usize = 0;
+                errdefer {
+                    // writing 8 bytes to an eventfd cannot fail
+                    std.os.posixWrite(self.os_data.final_eventfd, wakeup_bytes) catch unreachable;
+                    while (extra_thread_index != 0) {
+                        extra_thread_index -= 1;
+                        self.extra_threads[extra_thread_index].wait();
+                    }
+                }
+                while (extra_thread_index < extra_thread_count) : (extra_thread_index += 1) {
+                    self.extra_threads[extra_thread_index] = try std.os.spawnThread(self, workerRun);
+                }
+            },
+            builtin.Os.macosx => {
+                self.os_data.kqfd = try std.os.bsdKQueue();
+                errdefer std.os.close(self.os_data.kqfd);
+
+                self.os_data.kevents = try self.allocator.alloc(posix.Kevent, extra_thread_count);
+                errdefer self.allocator.free(self.os_data.kevents);
+
+                const eventlist = ([*]posix.Kevent)(undefined)[0..0];
+
+                for (self.eventfd_resume_nodes) |*eventfd_node, i| {
+                    eventfd_node.* = std.atomic.Stack(ResumeNode.EventFd).Node{
+                        .data = ResumeNode.EventFd{
+                            .base = ResumeNode{
+                                .id = ResumeNode.Id.EventFd,
+                                .handle = undefined,
+                            },
+                            // this one is for sending events
+                            .kevent = posix.Kevent{
+                                .ident = i,
+                                .filter = posix.EVFILT_USER,
+                                .flags = posix.EV_CLEAR | posix.EV_ADD | posix.EV_DISABLE,
+                                .fflags = 0,
+                                .data = 0,
+                                .udata = @ptrToInt(&eventfd_node.data.base),
+                            },
+                        },
+                        .next = undefined,
+                    };
+                    self.available_eventfd_resume_nodes.push(eventfd_node);
+                    const kevent_array = (*[1]posix.Kevent)(&eventfd_node.data.kevent);
+                    _ = try std.os.bsdKEvent(self.os_data.kqfd, kevent_array, eventlist, null);
+                    eventfd_node.data.kevent.flags = posix.EV_CLEAR | posix.EV_ENABLE;
+                    eventfd_node.data.kevent.fflags = posix.NOTE_TRIGGER;
+                    // this one is for waiting for events
+                    self.os_data.kevents[i] = posix.Kevent{
+                        .ident = i,
+                        .filter = posix.EVFILT_USER,
+                        .flags = 0,
+                        .fflags = 0,
+                        .data = 0,
+                        .udata = @ptrToInt(&eventfd_node.data.base),
+                    };
+                }
+
+                // Pre-add so that we cannot get error.SystemResources
+                // later when we try to activate it.
+                self.os_data.final_kevent = posix.Kevent{
+                    .ident = extra_thread_count,
+                    .filter = posix.EVFILT_USER,
+                    .flags = posix.EV_ADD | posix.EV_DISABLE,
+                    .fflags = 0,
+                    .data = 0,
+                    .udata = @ptrToInt(&self.final_resume_node),
+                };
+                const kevent_array = (*[1]posix.Kevent)(&self.os_data.final_kevent);
+                _ = try std.os.bsdKEvent(self.os_data.kqfd, kevent_array, eventlist, null);
+                self.os_data.final_kevent.flags = posix.EV_ENABLE;
+                self.os_data.final_kevent.fflags = posix.NOTE_TRIGGER;
+
+                var extra_thread_index: usize = 0;
+                errdefer {
+                    _ = std.os.bsdKEvent(self.os_data.kqfd, kevent_array, eventlist, null) catch unreachable;
+                    while (extra_thread_index != 0) {
+                        extra_thread_index -= 1;
+                        self.extra_threads[extra_thread_index].wait();
+                    }
+                }
+                while (extra_thread_index < extra_thread_count) : (extra_thread_index += 1) {
+                    self.extra_threads[extra_thread_index] = try std.os.spawnThread(self, workerRun);
+                }
+            },
+            builtin.Os.windows => {
+                self.os_data.extra_thread_count = extra_thread_count;
+
+                self.os_data.io_port = try std.os.windowsCreateIoCompletionPort(
+                    windows.INVALID_HANDLE_VALUE,
+                    null,
+                    undefined,
+                    undefined,
+                );
+                errdefer std.os.close(self.os_data.io_port);
+
+                for (self.eventfd_resume_nodes) |*eventfd_node, i| {
+                    eventfd_node.* = std.atomic.Stack(ResumeNode.EventFd).Node{
+                        .data = ResumeNode.EventFd{
+                            .base = ResumeNode{
+                                .id = ResumeNode.Id.EventFd,
+                                .handle = undefined,
+                            },
+                            // this one is for sending events
+                            .completion_key = @ptrToInt(&eventfd_node.data.base),
+                        },
+                        .next = undefined,
+                    };
+                    self.available_eventfd_resume_nodes.push(eventfd_node);
+                }
+
+                var extra_thread_index: usize = 0;
+                errdefer {
+                    var i: usize = 0;
+                    while (i < extra_thread_index) : (i += 1) {
+                        while (true) {
+                            const overlapped = @intToPtr(?*windows.OVERLAPPED, 0x1);
+                            std.os.windowsPostQueuedCompletionStatus(self.os_data.io_port, undefined, @ptrToInt(&self.final_resume_node), overlapped) catch continue;
+                            break;
+                        }
+                    }
+                    while (extra_thread_index != 0) {
+                        extra_thread_index -= 1;
+                        self.extra_threads[extra_thread_index].wait();
+                    }
+                }
+                while (extra_thread_index < extra_thread_count) : (extra_thread_index += 1) {
+                    self.extra_threads[extra_thread_index] = try std.os.spawnThread(self, workerRun);
+                }
             },
             else => {},
         }
@@ -142,65 +389,281 @@ pub const Loop = struct {
 
     fn deinitOsData(self: *Loop) void {
         switch (builtin.os) {
-            builtin.Os.linux => std.os.close(self.os_data.epollfd),
+            builtin.Os.linux => {
+                std.os.close(self.os_data.final_eventfd);
+                while (self.available_eventfd_resume_nodes.pop()) |node| std.os.close(node.data.eventfd);
+                std.os.close(self.os_data.epollfd);
+                self.allocator.free(self.eventfd_resume_nodes);
+            },
+            builtin.Os.macosx => {
+                self.allocator.free(self.os_data.kevents);
+                std.os.close(self.os_data.kqfd);
+            },
+            builtin.Os.windows => {
+                std.os.close(self.os_data.io_port);
+            },
             else => {},
         }
     }
 
-    pub fn addFd(self: *Loop, fd: i32, prom: promise) !void {
+    /// resume_node must live longer than the promise that it holds a reference to.
+    pub fn addFd(self: *Loop, fd: i32, resume_node: *ResumeNode) !void {
+        _ = @atomicRmw(usize, &self.pending_event_count, AtomicRmwOp.Add, 1, AtomicOrder.SeqCst);
+        errdefer {
+            _ = @atomicRmw(usize, &self.pending_event_count, AtomicRmwOp.Sub, 1, AtomicOrder.SeqCst);
+        }
+        try self.modFd(
+            fd,
+            posix.EPOLL_CTL_ADD,
+            std.os.linux.EPOLLIN | std.os.linux.EPOLLOUT | std.os.linux.EPOLLET,
+            resume_node,
+        );
+    }
+
+    pub fn modFd(self: *Loop, fd: i32, op: u32, events: u32, resume_node: *ResumeNode) !void {
         var ev = std.os.linux.epoll_event{
-            .events = std.os.linux.EPOLLIN | std.os.linux.EPOLLOUT | std.os.linux.EPOLLET,
-            .data = std.os.linux.epoll_data{ .ptr = @ptrToInt(prom) },
+            .events = events,
+            .data = std.os.linux.epoll_data{ .ptr = @ptrToInt(resume_node) },
         };
-        try std.os.linuxEpollCtl(self.os_data.epollfd, std.os.linux.EPOLL_CTL_ADD, fd, &ev);
+        try std.os.linuxEpollCtl(self.os_data.epollfd, op, fd, &ev);
     }
 
     pub fn removeFd(self: *Loop, fd: i32) void {
+        self.removeFdNoCounter(fd);
+        _ = @atomicRmw(usize, &self.pending_event_count, AtomicRmwOp.Sub, 1, AtomicOrder.SeqCst);
+    }
+
+    fn removeFdNoCounter(self: *Loop, fd: i32) void {
         std.os.linuxEpollCtl(self.os_data.epollfd, std.os.linux.EPOLL_CTL_DEL, fd, undefined) catch {};
     }
-    async fn waitFd(self: *Loop, fd: i32) !void {
+
+    pub async fn waitFd(self: *Loop, fd: i32) !void {
         defer self.removeFd(fd);
         suspend |p| {
-            try self.addFd(fd, p);
+            // TODO explicitly put this memory in the coroutine frame #1194
+            var resume_node = ResumeNode{
+                .id = ResumeNode.Id.Basic,
+                .handle = p,
+            };
+            try self.addFd(fd, &resume_node);
         }
     }
 
-    pub fn stop(self: *Loop) void {
-        // TODO make atomic
-        self.keep_running = false;
-        // TODO activate an fd in the epoll set which should cancel all the promises
-    }
-
-    /// bring your own linked list node. this means it can't fail.
+    /// Bring your own linked list node. This means it can't fail.
     pub fn onNextTick(self: *Loop, node: *NextTickNode) void {
+        _ = @atomicRmw(usize, &self.pending_event_count, AtomicRmwOp.Add, 1, AtomicOrder.SeqCst);
         self.next_tick_queue.put(node);
     }
 
     pub fn run(self: *Loop) void {
-        while (self.keep_running) {
-            // TODO multiplex the next tick queue and the epoll event results onto a thread pool
-            while (self.next_tick_queue.get()) |node| {
-                resume node.data;
-            }
-            if (!self.keep_running) break;
-
-            self.dispatchOsEvents();
+        _ = @atomicRmw(u8, &self.dispatch_lock, AtomicRmwOp.Xchg, 0, AtomicOrder.SeqCst);
+        self.workerRun();
+        for (self.extra_threads) |extra_thread| {
+            extra_thread.wait();
         }
     }
 
-    fn dispatchOsEvents(self: *Loop) void {
-        switch (builtin.os) {
-            builtin.Os.linux => {
-                var events: [16]std.os.linux.epoll_event = undefined;
-                const count = std.os.linuxEpollWait(self.os_data.epollfd, events[0..], -1);
-                for (events[0..count]) |ev| {
-                    const p = @intToPtr(promise, ev.data.ptr);
-                    resume p;
+    fn workerRun(self: *Loop) void {
+        start_over: while (true) {
+            if (@atomicRmw(u8, &self.dispatch_lock, AtomicRmwOp.Xchg, 1, AtomicOrder.SeqCst) == 0) {
+                while (self.next_tick_queue.get()) |next_tick_node| {
+                    const handle = next_tick_node.data;
+                    if (self.next_tick_queue.isEmpty()) {
+                        // last node, just resume it
+                        _ = @atomicRmw(u8, &self.dispatch_lock, AtomicRmwOp.Xchg, 0, AtomicOrder.SeqCst);
+                        resume handle;
+                        _ = @atomicRmw(usize, &self.pending_event_count, AtomicRmwOp.Sub, 1, AtomicOrder.SeqCst);
+                        continue :start_over;
+                    }
+
+                    // non-last node, stick it in the epoll/kqueue set so that
+                    // other threads can get to it
+                    if (self.available_eventfd_resume_nodes.pop()) |resume_stack_node| {
+                        const eventfd_node = &resume_stack_node.data;
+                        eventfd_node.base.handle = handle;
+                        switch (builtin.os) {
+                            builtin.Os.macosx => {
+                                const kevent_array = (*[1]posix.Kevent)(&eventfd_node.kevent);
+                                const eventlist = ([*]posix.Kevent)(undefined)[0..0];
+                                _ = std.os.bsdKEvent(self.os_data.kqfd, kevent_array, eventlist, null) catch {
+                                    // fine, we didn't need it anyway
+                                    _ = @atomicRmw(u8, &self.dispatch_lock, AtomicRmwOp.Xchg, 0, AtomicOrder.SeqCst);
+                                    self.available_eventfd_resume_nodes.push(resume_stack_node);
+                                    resume handle;
+                                    _ = @atomicRmw(usize, &self.pending_event_count, AtomicRmwOp.Sub, 1, AtomicOrder.SeqCst);
+                                    continue :start_over;
+                                };
+                            },
+                            builtin.Os.linux => {
+                                // the pending count is already accounted for
+                                const epoll_events = posix.EPOLLONESHOT | std.os.linux.EPOLLIN | std.os.linux.EPOLLOUT | std.os.linux.EPOLLET;
+                                self.modFd(eventfd_node.eventfd, eventfd_node.epoll_op, epoll_events, &eventfd_node.base) catch {
+                                    // fine, we didn't need it anyway
+                                    _ = @atomicRmw(u8, &self.dispatch_lock, AtomicRmwOp.Xchg, 0, AtomicOrder.SeqCst);
+                                    self.available_eventfd_resume_nodes.push(resume_stack_node);
+                                    resume handle;
+                                    _ = @atomicRmw(usize, &self.pending_event_count, AtomicRmwOp.Sub, 1, AtomicOrder.SeqCst);
+                                    continue :start_over;
+                                };
+                            },
+                            builtin.Os.windows => {
+                                // this value is never dereferenced but we need it to be non-null so that
+                                // the consumer code can decide whether to read the completion key.
+                                // it has to do this for normal I/O, so we match that behavior here.
+                                const overlapped = @intToPtr(?*windows.OVERLAPPED, 0x1);
+                                std.os.windowsPostQueuedCompletionStatus(self.os_data.io_port, undefined, eventfd_node.completion_key, overlapped) catch {
+                                    // fine, we didn't need it anyway
+                                    _ = @atomicRmw(u8, &self.dispatch_lock, AtomicRmwOp.Xchg, 0, AtomicOrder.SeqCst);
+                                    self.available_eventfd_resume_nodes.push(resume_stack_node);
+                                    resume handle;
+                                    _ = @atomicRmw(usize, &self.pending_event_count, AtomicRmwOp.Sub, 1, AtomicOrder.SeqCst);
+                                    continue :start_over;
+                                };
+                            },
+                            else => @compileError("unsupported OS"),
+                        }
+                    } else {
+                        // threads are too busy, can't add another eventfd to wake one up
+                        _ = @atomicRmw(u8, &self.dispatch_lock, AtomicRmwOp.Xchg, 0, AtomicOrder.SeqCst);
+                        resume handle;
+                        _ = @atomicRmw(usize, &self.pending_event_count, AtomicRmwOp.Sub, 1, AtomicOrder.SeqCst);
+                        continue :start_over;
+                    }
                 }
-            },
-            else => {},
+
+                const pending_event_count = @atomicLoad(usize, &self.pending_event_count, AtomicOrder.SeqCst);
+                if (pending_event_count == 0) {
+                    // cause all the threads to stop
+                    switch (builtin.os) {
+                        builtin.Os.linux => {
+                            // writing 8 bytes to an eventfd cannot fail
+                            std.os.posixWrite(self.os_data.final_eventfd, wakeup_bytes) catch unreachable;
+                            return;
+                        },
+                        builtin.Os.macosx => {
+                            const final_kevent = (*[1]posix.Kevent)(&self.os_data.final_kevent);
+                            const eventlist = ([*]posix.Kevent)(undefined)[0..0];
+                            // cannot fail because we already added it and this just enables it
+                            _ = std.os.bsdKEvent(self.os_data.kqfd, final_kevent, eventlist, null) catch unreachable;
+                            return;
+                        },
+                        builtin.Os.windows => {
+                            var i: usize = 0;
+                            while (i < self.os_data.extra_thread_count) : (i += 1) {
+                                while (true) {
+                                    const overlapped = @intToPtr(?*windows.OVERLAPPED, 0x1);
+                                    std.os.windowsPostQueuedCompletionStatus(self.os_data.io_port, undefined, @ptrToInt(&self.final_resume_node), overlapped) catch continue;
+                                    break;
+                                }
+                            }
+                            return;
+                        },
+                        else => @compileError("unsupported OS"),
+                    }
+                }
+
+                _ = @atomicRmw(u8, &self.dispatch_lock, AtomicRmwOp.Xchg, 0, AtomicOrder.SeqCst);
+            }
+
+            switch (builtin.os) {
+                builtin.Os.linux => {
+                    // only process 1 event so we don't steal from other threads
+                    var events: [1]std.os.linux.epoll_event = undefined;
+                    const count = std.os.linuxEpollWait(self.os_data.epollfd, events[0..], -1);
+                    for (events[0..count]) |ev| {
+                        const resume_node = @intToPtr(*ResumeNode, ev.data.ptr);
+                        const handle = resume_node.handle;
+                        const resume_node_id = resume_node.id;
+                        switch (resume_node_id) {
+                            ResumeNode.Id.Basic => {},
+                            ResumeNode.Id.Stop => return,
+                            ResumeNode.Id.EventFd => {
+                                const event_fd_node = @fieldParentPtr(ResumeNode.EventFd, "base", resume_node);
+                                event_fd_node.epoll_op = posix.EPOLL_CTL_MOD;
+                                const stack_node = @fieldParentPtr(std.atomic.Stack(ResumeNode.EventFd).Node, "data", event_fd_node);
+                                self.available_eventfd_resume_nodes.push(stack_node);
+                            },
+                        }
+                        resume handle;
+                        if (resume_node_id == ResumeNode.Id.EventFd) {
+                            _ = @atomicRmw(usize, &self.pending_event_count, AtomicRmwOp.Sub, 1, AtomicOrder.SeqCst);
+                        }
+                    }
+                },
+                builtin.Os.macosx => {
+                    var eventlist: [1]posix.Kevent = undefined;
+                    const count = std.os.bsdKEvent(self.os_data.kqfd, self.os_data.kevents, eventlist[0..], null) catch unreachable;
+                    for (eventlist[0..count]) |ev| {
+                        const resume_node = @intToPtr(*ResumeNode, ev.udata);
+                        const handle = resume_node.handle;
+                        const resume_node_id = resume_node.id;
+                        switch (resume_node_id) {
+                            ResumeNode.Id.Basic => {},
+                            ResumeNode.Id.Stop => return,
+                            ResumeNode.Id.EventFd => {
+                                const event_fd_node = @fieldParentPtr(ResumeNode.EventFd, "base", resume_node);
+                                const stack_node = @fieldParentPtr(std.atomic.Stack(ResumeNode.EventFd).Node, "data", event_fd_node);
+                                self.available_eventfd_resume_nodes.push(stack_node);
+                            },
+                        }
+                        resume handle;
+                        if (resume_node_id == ResumeNode.Id.EventFd) {
+                            _ = @atomicRmw(usize, &self.pending_event_count, AtomicRmwOp.Sub, 1, AtomicOrder.SeqCst);
+                        }
+                    }
+                },
+                builtin.Os.windows => {
+                    var completion_key: usize = undefined;
+                    while (true) {
+                        var nbytes: windows.DWORD = undefined;
+                        var overlapped: ?*windows.OVERLAPPED = undefined;
+                        switch (std.os.windowsGetQueuedCompletionStatus(self.os_data.io_port, &nbytes, &completion_key, &overlapped, windows.INFINITE)) {
+                            std.os.WindowsWaitResult.Aborted => return,
+                            std.os.WindowsWaitResult.Normal => {},
+                        }
+                        if (overlapped != null) break;
+                    }
+                    const resume_node = @intToPtr(*ResumeNode, completion_key);
+                    const handle = resume_node.handle;
+                    const resume_node_id = resume_node.id;
+                    switch (resume_node_id) {
+                        ResumeNode.Id.Basic => {},
+                        ResumeNode.Id.Stop => return,
+                        ResumeNode.Id.EventFd => {
+                            const event_fd_node = @fieldParentPtr(ResumeNode.EventFd, "base", resume_node);
+                            const stack_node = @fieldParentPtr(std.atomic.Stack(ResumeNode.EventFd).Node, "data", event_fd_node);
+                            self.available_eventfd_resume_nodes.push(stack_node);
+                        },
+                    }
+                    resume handle;
+                    if (resume_node_id == ResumeNode.Id.EventFd) {
+                        _ = @atomicRmw(usize, &self.pending_event_count, AtomicRmwOp.Sub, 1, AtomicOrder.SeqCst);
+                    }
+                },
+                else => @compileError("unsupported OS"),
+            }
         }
     }
+
+    const OsData = switch (builtin.os) {
+        builtin.Os.linux => struct {
+            epollfd: i32,
+            final_eventfd: i32,
+            final_eventfd_event: std.os.linux.epoll_event,
+        },
+        builtin.Os.macosx => MacOsData,
+        builtin.Os.windows => struct {
+            io_port: windows.HANDLE,
+            extra_thread_count: usize,
+        },
+        else => struct {},
+    };
+
+    const MacOsData = struct {
+        kqfd: i32,
+        final_kevent: posix.Kevent,
+        kevents: []posix.Kevent,
+    };
 };
 
 /// many producer, many consumer, thread-safe, lock-free, runtime configurable buffer size
@@ -304,9 +767,7 @@ pub fn Channel(comptime T: type) type {
             // TODO integrate this function with named return values
             // so we can get rid of this extra result copy
             var result: T = undefined;
-            var debug_handle: usize = undefined;
             suspend |handle| {
-                debug_handle = @ptrToInt(handle);
                 var my_tick_node = Loop.NextTickNode{
                     .next = undefined,
                     .data = handle,
@@ -438,9 +899,8 @@ test "listen on a port, send bytes, receive bytes" {
             const self = @fieldParentPtr(Self, "tcp_server", tcp_server);
             var socket = _socket.*; // TODO https://github.com/ziglang/zig/issues/733
             defer socket.close();
-            const next_handler = async errorableHandler(self, _addr, socket) catch |err| switch (err) {
-                error.OutOfMemory => @panic("unable to handle connection: out of memory"),
-            };
+            // TODO guarantee elision of this allocation
+            const next_handler = async errorableHandler(self, _addr, socket) catch unreachable;
             (await next_handler) catch |err| {
                 std.debug.panic("unable to handle connection: {}\n", err);
             };
@@ -461,17 +921,18 @@ test "listen on a port, send bytes, receive bytes" {
     const ip4addr = std.net.parseIp4("127.0.0.1") catch unreachable;
     const addr = std.net.Address.initIp4(ip4addr, 0);
 
-    var loop = try Loop.init(std.debug.global_allocator);
-    var server = MyServer{ .tcp_server = try TcpServer.init(&loop) };
+    var loop: Loop = undefined;
+    try loop.initSingleThreaded(std.debug.global_allocator);
+    var server = MyServer{ .tcp_server = TcpServer.init(&loop) };
     defer server.tcp_server.deinit();
     try server.tcp_server.listen(addr, MyServer.handler);
 
-    const p = try async<std.debug.global_allocator> doAsyncTest(&loop, server.tcp_server.listen_address);
+    const p = try async<std.debug.global_allocator> doAsyncTest(&loop, server.tcp_server.listen_address, &server.tcp_server);
     defer cancel p;
     loop.run();
 }
 
-async fn doAsyncTest(loop: *Loop, address: *const std.net.Address) void {
+async fn doAsyncTest(loop: *Loop, address: *const std.net.Address, server: *TcpServer) void {
     errdefer @panic("test failure");
 
     var socket_file = try await try async event.connect(loop, address);
@@ -481,7 +942,7 @@ async fn doAsyncTest(loop: *Loop, address: *const std.net.Address) void {
     const amt_read = try socket_file.read(buf[0..]);
     const msg = buf[0..amt_read];
     assert(mem.eql(u8, msg, "hello from server\n"));
-    loop.stop();
+    server.close();
 }
 
 test "std.event.Channel" {
@@ -490,7 +951,9 @@ test "std.event.Channel" {
 
     const allocator = &da.allocator;
 
-    var loop = try Loop.init(allocator);
+    var loop: Loop = undefined;
+    // TODO make a multi threaded test
+    try loop.initSingleThreaded(allocator);
     defer loop.deinit();
 
     const channel = try Channel(i32).create(&loop, 0);
@@ -515,11 +978,246 @@ async fn testChannelGetter(loop: *Loop, channel: *Channel(i32)) void {
     const value2_promise = try async channel.get();
     const value2 = await value2_promise;
     assert(value2 == 4567);
-
-    loop.stop();
 }
 
 async fn testChannelPutter(channel: *Channel(i32)) void {
     await (async channel.put(1234) catch @panic("out of memory"));
     await (async channel.put(4567) catch @panic("out of memory"));
 }
+
+/// Thread-safe async/await lock.
+/// Does not make any syscalls - coroutines which are waiting for the lock are suspended, and
+/// are resumed when the lock is released, in order.
+pub const Lock = struct {
+    loop: *Loop,
+    shared_bit: u8, // TODO make this a bool
+    queue: Queue,
+    queue_empty_bit: u8, // TODO make this a bool
+
+    const Queue = std.atomic.QueueMpsc(promise);
+
+    pub const Held = struct {
+        lock: *Lock,
+
+        pub fn release(self: Held) void {
+            // Resume the next item from the queue.
+            if (self.lock.queue.get()) |node| {
+                self.lock.loop.onNextTick(node);
+                return;
+            }
+
+            // We need to release the lock.
+            _ = @atomicRmw(u8, &self.lock.queue_empty_bit, AtomicRmwOp.Xchg, 1, AtomicOrder.SeqCst);
+            _ = @atomicRmw(u8, &self.lock.shared_bit, AtomicRmwOp.Xchg, 0, AtomicOrder.SeqCst);
+
+            // There might be a queue item. If we know the queue is empty, we can be done,
+            // because the other actor will try to obtain the lock.
+            // But if there's a queue item, we are the actor which must loop and attempt
+            // to grab the lock again.
+            if (@atomicLoad(u8, &self.lock.queue_empty_bit, AtomicOrder.SeqCst) == 1) {
+                return;
+            }
+
+            while (true) {
+                const old_bit = @atomicRmw(u8, &self.lock.shared_bit, AtomicRmwOp.Xchg, 1, AtomicOrder.SeqCst);
+                if (old_bit != 0) {
+                    // We did not obtain the lock. Great, the queue is someone else's problem.
+                    return;
+                }
+
+                // Resume the next item from the queue.
+                if (self.lock.queue.get()) |node| {
+                    self.lock.loop.onNextTick(node);
+                    return;
+                }
+
+                // Release the lock again.
+                _ = @atomicRmw(u8, &self.lock.queue_empty_bit, AtomicRmwOp.Xchg, 1, AtomicOrder.SeqCst);
+                _ = @atomicRmw(u8, &self.lock.shared_bit, AtomicRmwOp.Xchg, 0, AtomicOrder.SeqCst);
+
+                // Find out if we can be done.
+                if (@atomicLoad(u8, &self.lock.queue_empty_bit, AtomicOrder.SeqCst) == 1) {
+                    return;
+                }
+            }
+        }
+    };
+
+    pub fn init(loop: *Loop) Lock {
+        return Lock{
+            .loop = loop,
+            .shared_bit = 0,
+            .queue = Queue.init(),
+            .queue_empty_bit = 1,
+        };
+    }
+
+    /// Must be called when not locked. Not thread safe.
+    /// All calls to acquire() and release() must complete before calling deinit().
+    pub fn deinit(self: *Lock) void {
+        assert(self.shared_bit == 0);
+        while (self.queue.get()) |node| cancel node.data;
+    }
+
+    pub async fn acquire(self: *Lock) Held {
+        s: suspend |handle| {
+            // TODO explicitly put this memory in the coroutine frame #1194
+            var my_tick_node = Loop.NextTickNode{
+                .data = handle,
+                .next = undefined,
+            };
+
+            self.queue.put(&my_tick_node);
+
+            // At this point, we are in the queue, so we might have already been resumed and this coroutine
+            // frame might be destroyed. For the rest of the suspend block we cannot access the coroutine frame.
+
+            // We set this bit so that later we can rely on the fact, that if queue_empty_bit is 1, some actor
+            // will attempt to grab the lock.
+            _ = @atomicRmw(u8, &self.queue_empty_bit, AtomicRmwOp.Xchg, 0, AtomicOrder.SeqCst);
+
+            while (true) {
+                const old_bit = @atomicRmw(u8, &self.shared_bit, AtomicRmwOp.Xchg, 1, AtomicOrder.SeqCst);
+                if (old_bit != 0) {
+                    // We did not obtain the lock. Trust that our queue entry will resume us, and allow
+                    // suspend to complete.
+                    break;
+                }
+                // We got the lock. However we might have already been resumed from the queue.
+                if (self.queue.get()) |node| {
+                    // Whether this node is us or someone else, we tail resume it.
+                    resume node.data;
+                    break;
+                } else {
+                    // We already got resumed, and there are none left in the queue, which means that
+                    // we aren't even supposed to hold the lock right now.
+                    _ = @atomicRmw(u8, &self.queue_empty_bit, AtomicRmwOp.Xchg, 1, AtomicOrder.SeqCst);
+                    _ = @atomicRmw(u8, &self.shared_bit, AtomicRmwOp.Xchg, 0, AtomicOrder.SeqCst);
+
+                    // There might be a queue item. If we know the queue is empty, we can be done,
+                    // because the other actor will try to obtain the lock.
+                    // But if there's a queue item, we are the actor which must loop and attempt
+                    // to grab the lock again.
+                    if (@atomicLoad(u8, &self.queue_empty_bit, AtomicOrder.SeqCst) == 1) {
+                        break;
+                    } else {
+                        continue;
+                    }
+                }
+                unreachable;
+            }
+        }
+
+        return Held{ .lock = self };
+    }
+};
+
+/// Thread-safe async/await lock that protects one piece of data.
+/// Does not make any syscalls - coroutines which are waiting for the lock are suspended, and
+/// are resumed when the lock is released, in order.
+pub fn Locked(comptime T: type) type {
+    return struct {
+        lock: Lock,
+        private_data: T,
+
+        const Self = this;
+
+        pub const HeldLock = struct {
+            value: *T,
+            held: Lock.Held,
+
+            pub fn release(self: HeldLock) void {
+                self.held.release();
+            }
+        };
+
+        pub fn init(loop: *Loop, data: T) Self {
+            return Self{
+                .lock = Lock.init(loop),
+                .private_data = data,
+            };
+        }
+
+        pub fn deinit(self: *Self) void {
+            self.lock.deinit();
+        }
+
+        pub async fn acquire(self: *Self) HeldLock {
+            return HeldLock{
+            // TODO guaranteed allocation elision
+                .held = await (async self.lock.acquire() catch unreachable),
+                .value = &self.private_data,
+            };
+        }
+    };
+}
+
+test "std.event.Lock" {
+    var da = std.heap.DirectAllocator.init();
+    defer da.deinit();
+
+    const allocator = &da.allocator;
+
+    var loop: Loop = undefined;
+    try loop.initMultiThreaded(allocator);
+    defer loop.deinit();
+
+    var lock = Lock.init(&loop);
+    defer lock.deinit();
+
+    const handle = try async<allocator> testLock(&loop, &lock);
+    defer cancel handle;
+    loop.run();
+
+    assert(mem.eql(i32, shared_test_data, [1]i32{3 * @intCast(i32, shared_test_data.len)} ** shared_test_data.len));
+}
+
+async fn testLock(loop: *Loop, lock: *Lock) void {
+    // TODO explicitly put next tick node memory in the coroutine frame #1194
+    suspend |p| {
+        resume p;
+    }
+    const handle1 = async lockRunner(lock) catch @panic("out of memory");
+    var tick_node1 = Loop.NextTickNode{
+        .next = undefined,
+        .data = handle1,
+    };
+    loop.onNextTick(&tick_node1);
+
+    const handle2 = async lockRunner(lock) catch @panic("out of memory");
+    var tick_node2 = Loop.NextTickNode{
+        .next = undefined,
+        .data = handle2,
+    };
+    loop.onNextTick(&tick_node2);
+
+    const handle3 = async lockRunner(lock) catch @panic("out of memory");
+    var tick_node3 = Loop.NextTickNode{
+        .next = undefined,
+        .data = handle3,
+    };
+    loop.onNextTick(&tick_node3);
+
+    await handle1;
+    await handle2;
+    await handle3;
+}
+
+var shared_test_data = [1]i32{0} ** 10;
+var shared_test_index: usize = 0;
+
+async fn lockRunner(lock: *Lock) void {
+    suspend; // resumed by onNextTick
+
+    var i: usize = 0;
+    while (i < shared_test_data.len) : (i += 1) {
+        const lock_promise = async lock.acquire() catch @panic("out of memory");
+        const handle = await lock_promise;
+        defer handle.release();
+
+        shared_test_index = 0;
+        while (shared_test_index < shared_test_data.len) : (shared_test_index += 1) {
+            shared_test_data[shared_test_index] = shared_test_data[shared_test_index] + 1;
+        }
+    }
+}