偶现问题复现 ,for 循环10万次增加偶现概率

- (void)viewDidLoad {
    [super viewDidLoad];
    dispatch_queue_t concurrentQueue = dispatch_queue_create("tianyao", DISPATCH_QUEUE_CONCURRENT);
    
    
    dispatch_async(concurrentQueue, ^{
        for(int i = 0;i < 100000;i++){
            [self writeTest];
            NSLog(@"%d 线程:%@", i, [NSThread currentThread]);
        }
    });
    
//    dispatch_queue_t concurrentQueue2 = dispatch_queue_create("tianyao2", DISPATCH_QUEUE_CONCURRENT);

    dispatch_async(concurrentQueue, ^{
        for(int i = 0;i < 100000;i++){
            [self readTest];
            NSLog(@"%d 线程:%@", i, [NSThread currentThread]);
            
        }
    });
      
}



- (void)writeTest{
    DYTestManager *manager = [DYTestManager shareInstant];
    NSArray *arr = @[@"1",@"2",@"3",@"4",@"5"];
    NSMutableArray *arrM  = [NSMutableArray arrayWithArray:arr];
    [arrM addObject:@(random())];
    manager.dayRankList = arrM;
    NSLog(@"写入的数组%@",arrM);
}


- (void)readTest{
    DYTestManager *manager = [DYTestManager shareInstant];
    NSArray *arr =  manager.dayRankList;
    NSLog(@"读出的数组");
}

尝试5次, 每次分别给予10万次读和写 , 出现4次crash

问题复现

dayrankList变量存在同时写入的情况, 这个情况是极低的概率出现, 所以会出现偶现的crash

问题修复

//
//  ViewController.m
//  writeread
//
//  Created by wanggang on 2021/3/9.
//

#import "ViewController.h"
#import "DYTestManager.h"

@interface ViewController ()

@end

@implementation ViewController


- (void)viewDidLoad {
    [super viewDidLoad];
    dispatch_queue_t concurrentQueue = dispatch_queue_create("tianyao", DISPATCH_QUEUE_CONCURRENT);
    
    
    dispatch_async(concurrentQueue, ^{
        for(int i = 0;i < 100000;i++){
            [self writeTest];
            NSLog(@"%d 线程:%@", i, [NSThread currentThread]);
        }
    });
    
//    dispatch_queue_t concurrentQueue2 = dispatch_queue_create("tianyao2", DISPATCH_QUEUE_CONCURRENT);

    dispatch_async(concurrentQueue, ^{
        for(int i = 0;i < 100000;i++){
            [self readTest];
            NSLog(@"%d 线程:%@", i, [NSThread currentThread]);
            
        }
    });
      
}



- (void)writeTest{
    DYTestManager *manager = [DYTestManager shareInstant];
    NSArray *arr = @[@"1",@"2",@"3",@"4",@"5"];
    NSMutableArray *arrM  = [NSMutableArray arrayWithArray:arr];
    [arrM addObject:@(random())];
    
    // 写操作
    dispatch_queue_t queue = manager.readWriteQueue;
    dispatch_barrier_async(queue, ^{
        // Write action.
        manager.dayRankList = arrM;
        NSLog(@"内部写 线程:%@",  [NSThread currentThread]);

    });
    NSLog(@"写入的数组%@",arrM);
}


- (void)readTest{
    DYTestManager *manager = [DYTestManager shareInstant];
    // 继续执行读操作
    dispatch_queue_t queue = manager.readWriteQueue;
    dispatch_sync(queue, ^{
        // Read action3.
        NSArray *arr =  manager.dayRankList;
        NSLog(@"内部读 线程:%@",  [NSThread currentThread]);
    });
    

}



@end

读写不同线程不会crash

image.png

问题总结

• 同时读写都在一个线程 同步不会crash
• 读写在不同的线程, 偶现的crash , 非必现 ,demo验证10万分之1左右
• 读写在不同的线程通过增加锁验证可以解决
  增加锁的主要代码如下:

// 创建一个并行队列
dispatch_queue_t queue = dispatch_queue_create("LLDebugtool.com", DISPATCH_QUEUE_CONCURRENT);

// 执行一些读操作
dispatch_sync(queue, ^{
    // Read action1.
});

dispatch_sync(queue, ^{
    // Read action2.
});

...
// 写操作
dispatch_barrier_async(queue, ^{
    // Write action.
});

// 继续执行读操作
dispatch_sync(queue, ^{
    // Read action3.
});

dispatch_sync(queue, ^{
    // Read action4.
});

线程加读写锁之后的特点

• 同一时间，只能有一个线程进行写的操作。
• 同一时间，允许有多个线程进行读的操作。
• 同一时间，不允许既有写的操作，又有读的操作。

参考文献

https://juejin.cn/post/6844903966099128328