我在_WebTryThreadLock中偶尔会看到崩溃报告，如下所示：
Exception Type: SIGSEGV
Exception Codes: SEGV_ACCERR at 0xffffffffbbadbeef
Thread 5 Crashed:
0 WebCore 0x00005822 _WebTryThreadLock(bool) + 158
1 WebCore 0x0000576f WebThreadLock + 47
2 UIKit 0x001ae019 -[UIWebView dealloc] + 73
3 CoreFoundation 0x00006f7b -[NSObject(NSObject) release] + 31
4 Foo 0x0000c001 -[FooViewController dealloc] (FooViewController.m:653)
5 CoreFoundation 0x00006f7b -[NSObject(NSObject) release] + 31
6 CoreFoundation 0x0000b3c9 CFRelease + 69
7 CoreFoundation 0x0000a8df _CFAutoreleasePoolPop + 147
8 libSystem.B.dylib 0x000d6c04 _dispatch_worker_thread2 + 228
9 libSystem.B.dylib 0x0007b251 _pthread_wqthread + 265
或
Thread 1 Crashed:
0 WebCore 0x34c263c8 _WebTryThreadLock(bool) + 112
1 WebCore 0x34c281b1 WebThreadLock + 53
2 UIKit 0x35a3765f -[UITextView dealloc] + 31
3 CoreFoundation 0x30329c43 -[NSObject(NSObject) release] + 31
4 UIKit 0x358ad5f5 -[UIView(Hierarchy) removeFromSuperview] + 365
5 UIKit 0x359426cd -[UIScrollView removeFromSuperview] + 49
6 UIKit 0x35a352f1 -[UITextView removeFromSuperview] + 73
7 UIKit 0x358c0de3 -[UIView dealloc] + 155
8 CoreFoundation 0x30329c43 -[NSObject(NSObject) release] + 31
9 UIKit 0x3591900f -[UIViewController dealloc] + 175
10 Foo 0x0000c167 -[FooViewController dealloc] (FooViewController.m:646)
11 CoreFoundation 0x30329c43 -[NSObject(NSObject) release] + 31
12 CoreFoundation 0x3032a1a1 CFRelease + 69
13 CoreFoundation 0x3032cebb _CFAutoreleasePoolPop + 147
14 libdispatch.dylib 0x33d0b6a5 _dispatch_worker_thread2 + 373
15 libsystem_c.dylib 0x33e0a591 _pthread_wqthread + 265
我无法在自己的测试中重现这一点，所以我不确定触发它的原因。它肯定不会经常发生。我在WebThreadLock中看到了在尝试从主线程之外的线程修改UIKit元素时出现崩溃的问题。
我认为这一定是正在发生的事情，因为_dispatch_worker_thread2告诉我这是GCD线程与UIApplicationMain线程之一。我必须在一个GCD线程上运行一些东西，在主线程释放之后保持FooViewController，我想这个非主线程中发生的removeFromSuperview调用是触发问题的原因。
答案 0 :(得分：2)
我想通过Rentzsch的发布/保留调试技巧来解决这个问题：
http://rentzsch.tumblr.com/post/3895140826/retain-release-debugging
问题是我从GCD线程调用了[UINavigationController visibleViewController]，这导致该线程在我的FooViewController上调用retain：
0 Foo 0x00009d98 -[FooViewController retain] + 216
1 UIKit 0x0049e5ad -[UINavigationController topViewController] + 61
2 UIKit 0x0049e4ca -[UINavigationController visibleViewController] + 133
3 Foo 0x0003c0bd -[BarViewController updateStatus:animated:] + 147
4 Foo 0x0003799e -[BarViewController reallyProcessMetadata] + 229
5 Foo 0x0003650e __39-[BarViewController processMetadata]_block_invoke_0828 + 408
6 libdispatch_sim.dylib 0x01a63289 _dispatch_call_block_and_release + 16
7 libdispatch_sim.dylib 0x01a6658a _dispatch_worker_thread2 + 252
8 libSystem.B.dylib 0x98d2bd21 _pthread_wqthread + 390
9 libSystem.B.dylib 0x98d2bb66 start_wqthread + 30
当该线程的自动释放池耗尽时，它是最后一个持有FooViewController的人导致崩溃如果你有幸在调试器中重现这个，你会在控制台中看到这个：
2011-05-06 19:34:40.651 Foo[59365:943b] bool _WebTryThreadLock(bool), 0x11393b30: Tried to obtain the web lock from a thread other than the main thread or the web thread. This may be a result of calling to UIKit from a secondary thread. Crashing now...
Program received signal: “EXC_BAD_ACCESS”.
所以我的情况下的解决方案是不使用此线程中的visibleViewController。我之前只是调用[UINavigationController viewControllers]并检查我的控制器是否是最后一个对象，并且从未崩溃。