Objective-C multi threading - Why does my code produce a deadlock? - ios

Question

I am using the following code to prevent a background task from being started twice. Why does this code block?
- (void)doBackgroundTask {
NSLog(#"Before LOCK: %#", [NSThread currentThread]);
// lockObject is created once in init
#synchronized(lockObject) {
if (taskActive)
taskCanceller.shouldCancel = true;
while (taskActive) {
// NOOP: Busy waiting
//NSLog(#"Busy waiting: %#", [NSThread currentThread]);
}
taskActive = true;
taskCanceller = [[AsyncTaskCanceller alloc] init];
}
NSLog(#"After LOCK: %#", [NSThread currentThread]);
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSLog(#"AsyncStart: %#", [NSThread currentThread]);
AsyncTaskCanceller *canceller = taskCanceller;
[self loadDataAsync:canceller];
NSLog(#"Setting taskActive to FALSE: %#", [NSThread currentThread]);
taskActive = false;
NSLog(#"taskActive == FALSE: %#", [NSThread currentThread]);
if (canceller.shouldCancel == false) {
NSLog(#"Complete: %#", [NSThread currentThread]);
dispatch_async(dispatch_get_main_queue(), ^{
NSLog(#"Completed: %#", [NSThread currentThread]);
...
});
} else {
NSLog(#"Canceled: %#", [NSThread currentThread]);
}
NSLog(#"AsyncComplete: %#", [NSThread currentThread]);
});
NSLog(#"SyncComplete: %#", [NSThread currentThread]);
}
The code produces the following output:
Before LOCK: <NSThread: 0x1706663c0>{number = 5, name = (null)}
After LOCK: <NSThread: 0x1706663c0>{number = 5, name = (null)}
SyncComplete: <NSThread: 0x1706663c0>{number = 5, name = (null)}
AsyncStart: <NSThread: 0x174c75d00>{number = 7, name = (null)}
Before LOCK: <NSThread: 0x1706663c0>{number = 5, name = (null)}
Setting taskActive to FALSE: <NSThread: 0x174c75d00>{number = 7, name = (null)}
taskActive == FALSE: <NSThread: 0x174c75d00>{number = 7, name = (null)}
Canceled: <NSThread: 0x174c75d00>{number = 7, name = (null)}
AsyncComplete: <NSThread: 0x174c75d00>{number = 7, name = (null)}
Which can be translated to:
Thread 5 enters the method
Thread 5 gets the LOCK and check if the Task is running. This is not the case, so it is started in a background thread
Thread 5 exits the method
Task is performed in Thread 7
Thread 5 enters the method again and gets the LOCK. Since Task is running, it switches to busy waiting until taskActive is set to false
Thread 7 sets taskActive = false
Thread 7 finishes
After Thread 7 set taskActiveback to false the while loop should end, shouldn't it? Why does it not?
Even more strange: If I active the NSLog statement within the busy waiting while loop everything works as expected.