1 files changed, 171 insertions, 0 deletions
diff --git a/lib/std/mutex.zig b/lib/std/mutex.zig
new file mode 100644
index 0000000000..5f3b9272d9
--- /dev/null
+++ b/lib/std/mutex.zig
@@ -0,0 +1,171 @@
+const std = @import("std.zig");
+const builtin = @import("builtin");
+const AtomicOrder = builtin.AtomicOrder;
+const AtomicRmwOp = builtin.AtomicRmwOp;
+const testing = std.testing;
+const SpinLock = std.SpinLock;
+const linux = std.os.linux;
+const windows = std.os.windows;
+
+/// Lock may be held only once. If the same thread
+/// tries to acquire the same mutex twice, it deadlocks.
+/// This type must be initialized at runtime, and then deinitialized when no
+/// longer needed, to free resources.
+/// If you need static initialization, use std.StaticallyInitializedMutex.
+/// The Linux implementation is based on mutex3 from
+/// https://www.akkadia.org/drepper/futex.pdf
+/// When an application is built in single threaded release mode, all the functions are
+/// no-ops. In single threaded debug mode, there is deadlock detection.
+pub const Mutex = if (builtin.single_threaded)
+    struct {
+        lock: @typeOf(lock_init),
+
+        const lock_init = if (std.debug.runtime_safety) false else {};
+
+        pub const Held = struct {
+            mutex: *Mutex,
+
+            pub fn release(self: Held) void {
+                if (std.debug.runtime_safety) {
+                    self.mutex.lock = false;
+                }
+            }
+        };
+        pub fn init() Mutex {
+            return Mutex{ .lock = lock_init };
+        }
+        pub fn deinit(self: *Mutex) void {}
+
+        pub fn acquire(self: *Mutex) Held {
+            if (std.debug.runtime_safety and self.lock) {
+                @panic("deadlock detected");
+            }
+            return Held{ .mutex = self };
+        }
+    }
+else switch (builtin.os) {
+    builtin.Os.linux => struct {
+        /// 0: unlocked
+        /// 1: locked, no waiters
+        /// 2: locked, one or more waiters
+        lock: i32,
+
+        pub const Held = struct {
+            mutex: *Mutex,
+
+            pub fn release(self: Held) void {
+                const c = @atomicRmw(i32, &self.mutex.lock, AtomicRmwOp.Sub, 1, AtomicOrder.Release);
+                if (c != 1) {
+                    _ = @atomicRmw(i32, &self.mutex.lock, AtomicRmwOp.Xchg, 0, AtomicOrder.Release);
+                    const rc = linux.futex_wake(&self.mutex.lock, linux.FUTEX_WAKE | linux.FUTEX_PRIVATE_FLAG, 1);
+                    switch (linux.getErrno(rc)) {
+                        0 => {},
+                        linux.EINVAL => unreachable,
+                        else => unreachable,
+                    }
+                }
+            }
+        };
+
+        pub fn init() Mutex {
+            return Mutex{ .lock = 0 };
+        }
+
+        pub fn deinit(self: *Mutex) void {}
+
+        pub fn acquire(self: *Mutex) Held {
+            var c = @cmpxchgWeak(i32, &self.lock, 0, 1, AtomicOrder.Acquire, AtomicOrder.Monotonic) orelse
+                return Held{ .mutex = self };
+            if (c != 2)
+                c = @atomicRmw(i32, &self.lock, AtomicRmwOp.Xchg, 2, AtomicOrder.Acquire);
+            while (c != 0) {
+                const rc = linux.futex_wait(&self.lock, linux.FUTEX_WAIT | linux.FUTEX_PRIVATE_FLAG, 2, null);
+                switch (linux.getErrno(rc)) {
+                    0, linux.EINTR, linux.EAGAIN => {},
+                    linux.EINVAL => unreachable,
+                    else => unreachable,
+                }
+                c = @atomicRmw(i32, &self.lock, AtomicRmwOp.Xchg, 2, AtomicOrder.Acquire);
+            }
+            return Held{ .mutex = self };
+        }
+    },
+    // TODO once https://github.com/ziglang/zig/issues/287 (copy elision) is solved, we can make a
+    // better implementation of this. The problem is we need the init() function to have access to
+    // the address of the CRITICAL_SECTION, and then have it not move.
+    builtin.Os.windows => std.StaticallyInitializedMutex,
+    else => struct {
+        /// TODO better implementation than spin lock.
+        /// When changing this, one must also change the corresponding
+        /// std.StaticallyInitializedMutex code, since it aliases this type,
+        /// under the assumption that it works both statically and at runtime.
+        lock: SpinLock,
+
+        pub const Held = struct {
+            mutex: *Mutex,
+
+            pub fn release(self: Held) void {
+                SpinLock.Held.release(SpinLock.Held{ .spinlock = &self.mutex.lock });
+            }
+        };
+
+        pub fn init() Mutex {
+            return Mutex{ .lock = SpinLock.init() };
+        }
+
+        pub fn deinit(self: *Mutex) void {}
+
+        pub fn acquire(self: *Mutex) Held {
+            _ = self.lock.acquire();
+            return Held{ .mutex = self };
+        }
+    },
+};
+
+const TestContext = struct {
+    mutex: *Mutex,
+    data: i128,
+
+    const incr_count = 10000;
+};
+
+test "std.Mutex" {
+    var plenty_of_memory = try std.heap.direct_allocator.alloc(u8, 300 * 1024);
+    defer std.heap.direct_allocator.free(plenty_of_memory);
+
+    var fixed_buffer_allocator = std.heap.ThreadSafeFixedBufferAllocator.init(plenty_of_memory);
+    var a = &fixed_buffer_allocator.allocator;
+
+    var mutex = Mutex.init();
+    defer mutex.deinit();
+
+    var context = TestContext{
+        .mutex = &mutex,
+        .data = 0,
+    };
+
+    if (builtin.single_threaded) {
+        worker(&context);
+        testing.expect(context.data == TestContext.incr_count);
+    } else {
+        const thread_count = 10;
+        var threads: [thread_count]*std.Thread = undefined;
+        for (threads) |*t| {
+            t.* = try std.Thread.spawn(&context, worker);
+        }
+        for (threads) |t|
+            t.wait();
+
+        testing.expect(context.data == thread_count * TestContext.incr_count);
+    }
+}
+
+fn worker(ctx: *TestContext) void {
+    var i: usize = 0;
+    while (i != TestContext.incr_count) : (i += 1) {
+        const held = ctx.mutex.acquire();
+        defer held.release();
+
+        ctx.data += 1;
+    }
+}