Question

我遇到了一个与iOS中的队列死锁相关的非常有趣的问题。有什么方法可以避免这种情况吗？

考虑一下：

创建自定义序列队列。
异步调度此串行队列上的某些任务（＃1）。
此异步任务（＃1）将某个任务（＃2）调度到主队列同步。
主队列将一些任务（＃3）调度到串行队列同步。
结果 - DeadLock

以下是此示例代码。

由于self.opQueue是一个串行队列，任务＃3将在任务＃1完成之前不会启动。
由于任务＃1正在调用主队列同步，因此在主队列完成任务＃2之前它永远不会完成。

由于主队列正在等待opQueue完成任务＃3，并且opQueue正在等待主队列完成任务＃2，因此存在死锁。

#import "ViewController.h"

@interface ViewController ()
@property(nonatomic,strong) dispatch_queue_t opQueue;
@end

@implementation ViewController

- (void)viewDidLoad {
  [super viewDidLoad];

  dispatch_queue_t queue =  dispatch_queue_create("com.tarun.sqqueue",
                                               DISPATCH_QUEUE_SERIAL);
  self.opQueue = queue;

  [self performOperations];
}


/// 1. Dispatch on serial queue async.
/// 2. This async task on serial queue dispatchs some task onto 
///    main queue sync.
/// 3. Main queue dispatched some task onto serial queue sync.
/// 4. Result - DeadLock
- (void)performOperations {

  /// task#1: Dispatch task on the serial Queue Asynchronously.
  /// So this is not blocking.
  dispatch_async(self.opQueue, ^{
    for (int i = 1; i<=100; i++) {
      NSLog(@"%d. Async on Serial Queue from Main Queue.",i);
    }

    /// task#2: Dispatching task on main queue synchronously from serial
    /// queue.So this queue will wait till main queue executes this task.(Blocking)
    dispatch_sync(dispatch_get_main_queue(), ^{
      for (int i = 1; i<=100; i++) {
        NSLog(@"%d. Sync on main queue from Serial Queue.",i);
      }
    });
  });

  /// task#3: Dispatching task on swrial queue synchronously from main
  /// queue.So main queue will wait till serial queue executes this task. (Blocking)
  dispatch_sync(self.opQueue, ^{
    for (int i = 1; i<=100; i++) {
      NSLog(@"%d. Sync on Serial Queue From Main Queue.",i);
    }
  });

  /// Since self.opQueue is a serial queue, task#3 will not start till task#1 completes.
  /// Since task#1 is calling main queue sync, 
  /// so it will never complete till main queue completes task#2.
  /// Since main queue is waiting for opQueue to finish task#3, and opQueue is waiting for main queue to finish task#2 there is a deadlock.

  NSLog(@"Back to main queue");
}


@end

Answer 1

来自Apple

重要提示：您永远不应该调用dispatch_sync或dispatch_sync_f 从您在同一队列中执行的任务中运行计划传递给函数。这对于特别重要串行队列，保证死锁，但也应该是避免并发队列。

保证死锁。避免这种情况的唯一方法是不以这种方式实现它。

从自定义串行队列

1 个答案: