Swift 与 os_unfair_lock_lock 的访问竞赛

Question

我制作了一个自定义属性包装器，它提供了一种使用 os_unfair_lock 在互斥上下文中访问数据的方法。在启用 TSAN 测试我的包装器后，在使用 os_unfair_lock_lock（如下图所示）

获取锁时报告了访问竞争错误

不知何故，TSAN 报告了一个本应是线程安全的锁定结构，但实际上并非如此。这是怎么回事？

Answer 1

根据 WWDC 2016 演讲“Swift 3 中的 GCD 并发编程”，演讲者在 18:07 左右指出

Traditionally you would use a lock. And in Swift, since you have the entire Darwin module at your disposition, you will actually see the struct based traditional C locks. However [emphasis added], Swift assumes that anything that's a struct can be moved, and that doesn't work with a mutex or with a lock.

解决方案是桥接到 Objective-C 并创建一个 class 将 os_unfair_lock 包装为 ivar:

And if you want something that's smaller and that looks like the locks that you have in C, then you have to call into Objective-C and introduce a base class in Objective-C that has your lock as an ivar

在这种情况下，类似于

UnfairLock.h

#ifndef UnfairLock_h
#define UnfairLock_h

@import Foundation;
@import os;

@interface UnfairLock : NSObject

-(void)unfairlyAcquire;
-(void)unlock;

@end


#endif /* UnfairLock_h */

UnfairLock.m

#import <Foundation/Foundation.h>
#import "UnfairLock.h"

@implementation UnfairLock {
    os_unfair_lock _lock;
}

-(instancetype)init {
    self = [super init];
    
    if (self) {
        _lock = OS_UNFAIR_LOCK_INIT;
    }
    
    return self;
}


-(void)unfairlyAcquire {
    os_unfair_lock_lock(&_lock);
}

-(void)unlock {
    os_unfair_lock_unlock(&_lock);
}

@end

Answer 2

您的的另一种（可能更直接）方法是直接在 Swift 中堆分配锁，而不是桥接到 Objective-C 来执行此操作. Objective-C 方法通过从具有不同语义的不同语言调用锁定函数来避免此问题 — C 和 Objective-C 不移动或传递逻辑删除值类型通过 inout 引用进入函数；但是你也可以通过完全不使用 inout 引用来避免纯 Swift 中的问题：

let lock = UnsafeMutablePointer<os_unfair_lock>.allocate(capacity: 1)
lock.initialize(to: .init())

// later:

os_unfair_lock_lock(lock)
defer { os_unfair_lock_unlock(lock) }

堆分配允许您将指针直接传递给函数，指针在 Swift 中是 reference 类型 — 而 Swift 可以移动指针值本身，它引用的内存将保持不变（并且有效）。

如果你走这条路，当你想拆除锁时不要忘记取消初始化和释放内存：

lock.deinitialize(count: 1)
lock.deallocate()

如果您愿意，可以在 Swift 中创建类似的 UnfairLock 界面，包括您自己的功能 mutexExecute:

typealias UnfairLock = UnsafeMutablePointer<os_unfair_lock>

extension UnfairLock {
    static func createLock() -> UnfairLock {
        let l = UnfairLock.allocate(capacity: 1)
        l.initialize(to: .init())
        return l
    }

    static func destructLock(_ lock: UnfairLock) {
        lock.deinitialize(count: 1)
        lock.deallocate()
    }

    func whileLocked<T>(_ action: () throws -> T) rethrows -> T {
        os_unfair_lock_lock(self)
        defer { os_unfair_lock_unlock(self) }
        return try action()
    }
}

用法：

init() {
    lock = UnfairLock.createLock()
}

deinit {
    UnfairLock.destructLock(lock)
}

func performThing() -> Foo {
    return lock.whileLocked {
        // some operation that returns a Foo
    }
}

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