Commit 6bd97ab640

Andrew Kelley <andrew@ziglang.org>
2023-11-23 00:37:14
std: remove atomic.Stack
This was originally supposed to be a lock-free stack, but I gave up on that and made it be a thread-safe stack which is implemented poorly using spin locks. Nobody should use this data structure. The alternative is a normal stack protected by a mutex.
1 parent 1fa2e91
Changed files (3)
lib/std/atomic/stack.zig
@@ -1,178 +0,0 @@
-const std = @import("../std.zig");
-const builtin = @import("builtin");
-const assert = std.debug.assert;
-const expect = std.testing.expect;
-
-/// Many reader, many writer, non-allocating, thread-safe.
-/// Uses a spinlock to protect `push()` and `pop()`.
-/// When building in single threaded mode, this is a simple linked list.
-pub fn Stack(comptime T: type) type {
-    return struct {
-        root: ?*Node,
-        lock: @TypeOf(lock_init),
-
-        const lock_init = if (builtin.single_threaded) {} else false;
-
-        pub const Self = @This();
-
-        pub const Node = struct {
-            next: ?*Node,
-            data: T,
-        };
-
-        pub fn init() Self {
-            return Self{
-                .root = null,
-                .lock = lock_init,
-            };
-        }
-
-        /// push operation, but only if you are the first item in the stack. if you did not succeed in
-        /// being the first item in the stack, returns the other item that was there.
-        pub fn pushFirst(self: *Self, node: *Node) ?*Node {
-            node.next = null;
-            return @cmpxchgStrong(?*Node, &self.root, null, node, .SeqCst, .SeqCst);
-        }
-
-        pub fn push(self: *Self, node: *Node) void {
-            if (builtin.single_threaded) {
-                node.next = self.root;
-                self.root = node;
-            } else {
-                while (@atomicRmw(bool, &self.lock, .Xchg, true, .SeqCst)) {}
-                defer assert(@atomicRmw(bool, &self.lock, .Xchg, false, .SeqCst));
-
-                node.next = self.root;
-                self.root = node;
-            }
-        }
-
-        pub fn pop(self: *Self) ?*Node {
-            if (builtin.single_threaded) {
-                const root = self.root orelse return null;
-                self.root = root.next;
-                return root;
-            } else {
-                while (@atomicRmw(bool, &self.lock, .Xchg, true, .SeqCst)) {}
-                defer assert(@atomicRmw(bool, &self.lock, .Xchg, false, .SeqCst));
-
-                const root = self.root orelse return null;
-                self.root = root.next;
-                return root;
-            }
-        }
-
-        pub fn isEmpty(self: *Self) bool {
-            return @atomicLoad(?*Node, &self.root, .SeqCst) == null;
-        }
-    };
-}
-
-const Context = struct {
-    allocator: std.mem.Allocator,
-    stack: *Stack(i32),
-    put_sum: isize,
-    get_sum: isize,
-    get_count: usize,
-    puts_done: bool,
-};
-// TODO add lazy evaluated build options and then put puts_per_thread behind
-// some option such as: "AggressiveMultithreadedFuzzTest". In the AppVeyor
-// CI we would use a less aggressive setting since at 1 core, while we still
-// want this test to pass, we need a smaller value since there is so much thrashing
-// we would also use a less aggressive setting when running in valgrind
-const puts_per_thread = 500;
-const put_thread_count = 3;
-
-test "std.atomic.stack" {
-    const plenty_of_memory = try std.heap.page_allocator.alloc(u8, 300 * 1024);
-    defer std.heap.page_allocator.free(plenty_of_memory);
-
-    var fixed_buffer_allocator = std.heap.FixedBufferAllocator.init(plenty_of_memory);
-    const a = fixed_buffer_allocator.threadSafeAllocator();
-
-    var stack = Stack(i32).init();
-    var context = Context{
-        .allocator = a,
-        .stack = &stack,
-        .put_sum = 0,
-        .get_sum = 0,
-        .puts_done = false,
-        .get_count = 0,
-    };
-
-    if (builtin.single_threaded) {
-        {
-            var i: usize = 0;
-            while (i < put_thread_count) : (i += 1) {
-                try expect(startPuts(&context) == 0);
-            }
-        }
-        context.puts_done = true;
-        {
-            var i: usize = 0;
-            while (i < put_thread_count) : (i += 1) {
-                try expect(startGets(&context) == 0);
-            }
-        }
-    } else {
-        var putters: [put_thread_count]std.Thread = undefined;
-        for (&putters) |*t| {
-            t.* = try std.Thread.spawn(.{}, startPuts, .{&context});
-        }
-        var getters: [put_thread_count]std.Thread = undefined;
-        for (&getters) |*t| {
-            t.* = try std.Thread.spawn(.{}, startGets, .{&context});
-        }
-
-        for (putters) |t|
-            t.join();
-        @atomicStore(bool, &context.puts_done, true, .SeqCst);
-        for (getters) |t|
-            t.join();
-    }
-
-    if (context.put_sum != context.get_sum) {
-        std.debug.panic("failure\nput_sum:{} != get_sum:{}", .{ context.put_sum, context.get_sum });
-    }
-
-    if (context.get_count != puts_per_thread * put_thread_count) {
-        std.debug.panic("failure\nget_count:{} != puts_per_thread:{} * put_thread_count:{}", .{
-            context.get_count,
-            @as(u32, puts_per_thread),
-            @as(u32, put_thread_count),
-        });
-    }
-}
-
-fn startPuts(ctx: *Context) u8 {
-    var put_count: usize = puts_per_thread;
-    var prng = std.rand.DefaultPrng.init(0xdeadbeef);
-    const random = prng.random();
-    while (put_count != 0) : (put_count -= 1) {
-        std.time.sleep(1); // let the os scheduler be our fuzz
-        const x = @as(i32, @bitCast(random.int(u32)));
-        const node = ctx.allocator.create(Stack(i32).Node) catch unreachable;
-        node.* = Stack(i32).Node{
-            .next = undefined,
-            .data = x,
-        };
-        ctx.stack.push(node);
-        _ = @atomicRmw(isize, &ctx.put_sum, .Add, x, .SeqCst);
-    }
-    return 0;
-}
-
-fn startGets(ctx: *Context) u8 {
-    while (true) {
-        const last = @atomicLoad(bool, &ctx.puts_done, .SeqCst);
-
-        while (ctx.stack.pop()) |node| {
-            std.time.sleep(1); // let the os scheduler be our fuzz
-            _ = @atomicRmw(isize, &ctx.get_sum, .Add, node.data, .SeqCst);
-            _ = @atomicRmw(usize, &ctx.get_count, .Add, 1, .SeqCst);
-        }
-
-        if (last) return 0;
-    }
-}
lib/std/atomic.zig
@@ -1,16 +1,9 @@
 const std = @import("std.zig");
 const builtin = @import("builtin");
 
-pub const Stack = @import("atomic/stack.zig").Stack;
 pub const Queue = @import("atomic/queue.zig").Queue;
 pub const Atomic = @import("atomic/Atomic.zig").Atomic;
 
-test {
-    _ = @import("atomic/stack.zig");
-    _ = @import("atomic/queue.zig");
-    _ = @import("atomic/Atomic.zig");
-}
-
 /// Signals to the processor that the caller is inside a busy-wait spin-loop.
 pub inline fn spinLoopHint() void {
     switch (builtin.target.cpu.arch) {
@@ -90,3 +83,8 @@ pub const cache_line = switch (builtin.cpu.arch) {
     // - https://github.com/golang/go/blob/3dd58676054223962cd915bb0934d1f9f489d4d2/src/internal/cpu/cpu_wasm.go#L7
     else => 64,
 };
+
+test {
+    _ = Queue;
+    _ = Atomic;
+}
CMakeLists.txt
@@ -211,7 +211,6 @@ set(ZIG_STAGE2_SOURCES
     "${CMAKE_SOURCE_DIR}/lib/std/atomic.zig"
     "${CMAKE_SOURCE_DIR}/lib/std/atomic/Atomic.zig"
     "${CMAKE_SOURCE_DIR}/lib/std/atomic/queue.zig"
-    "${CMAKE_SOURCE_DIR}/lib/std/atomic/stack.zig"
     "${CMAKE_SOURCE_DIR}/lib/std/base64.zig"
     "${CMAKE_SOURCE_DIR}/lib/std/BitStack.zig"
     "${CMAKE_SOURCE_DIR}/lib/std/buf_map.zig"