Question

Why does my code crashes?

    dispatch_async(queue_A, ^{
    @synchronized(self) {
        NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
        [self logInfo:@"queue_A"];
        [pool release];        
    }
});

dispatch_async(queue_B, ^{
    @synchronized(self) {
        NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];
        [self logInfo:@"queue_B"];
        [pool release];        
    }
});

Answer 1

You’re better off avoiding using @syncronized inside dispatch_async, as suggested in the comments. If it’s not safe to call [self logInfo:...] then you might want to either:

1. Use a dispatch group to make sure multiple dispatch_async() calls to a resource don’t crash your app. Mike Ash has a good write-up of this technique. Check out his examples.
2. Use a dispatch semaphore to force separate calls to wait for each other to complete. See the manpage for more.

I’m not sure what logInfo does in this context so you may be able to rewrite that as well.