GCD and OSSpinLockLock

Question

I probably ask my question the wrong way and risk being blocked by stackoverflow completely. I have Asperger and no social skills, so I am very sorry for asking my last (?) question (because systems like these are only made for people without handicaps).

I am using GCD to load images and video from Instagram. I do this in an app that is very 'busy' with its user interface, and I want to keep that running smoothly, so I load the Instagram-media in the background.

The code below (which I probably formatted the wrong way so I apologize up front) works fine and does exactly want I want it to do. It loads images in the background (I left video's out to keep things simple) and my main UI is response while the images load. I display them between loads, and there works fine too.

However.

After 10 minutes, sometimes 20 minutes, sometimes 30 minutes and even sometimes after two hours my app gets OSSpinLockLock freezes. If I remove the code below I get no media, but the app never freezes.

I have searched for days on the web about alternative ways to do the job and to find an explanation for the OSSpinLockLock. No luck. Only thing I found was that using GCD could not result in an OSSpinLockLock. I have used all my knowledge of Instruments (which I must admit is more limited than I thought), but I cannot find a fault.

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0ul);
    dispatch_async(queue, ^(void) {
        [[InstagramEngine sharedEngine] getMediaAtLocation:location count:kInstagramLocationBufferSize maxId:nil withSuccess:^(NSArray* media, InstagramPaginationInfo* paginationInfo) {
            if (media && media.count>0) {
                for (InstagramMedia* mediaObject in media) {
                    NSData* data = [[NSData alloc] initWithContentsOfURL:mediaObject.standardResolutionImageURL];
                    if (data) {
                        UIImage* img = [UIImage imageWithData:data];
                        if (img)
                            [self.locationBuffer addObject:img];
                        data = nil;
                    }
                }
            }
         } failure:^(NSError *error) {
             ;
         }];
    });

If you look at this code, do you see anything that might cause that lock? Because I most certainly don't.

self.locationBuffer is declared in the .h as

@property (nonatomic,strong) NSMutableArray* locationBuffer;

and is properly allocated and initialized (otherwise it would be rather clear what the problem was). I have also tried not to put the UIImage but the NSData in the array but that made no difference whatsoever.

On my iPad mini retina for instance the CPU-load goes to 195% and stays around that number for a very long time. Eventually, sometimes after several hours, the app crashes.

Any suggestions would be very welcome.

Edit: As I see now on the ips-file on the iPad itself (which for some mysterious reason I cannot paste into this webpage (is stackoverflow still in an experimental stage?)) I see that the iPad did spent 16.000+ seconds on NSURLConnection...

Answer 1

my tought is that some timeout or failure blocks your gcd queues for too much time. try rewriting that code with NSOperationQueue, that way you can stop the queue on errors or view/controller going away.

https://developer.apple.com/library/ios/documentation/Cocoa/Reference/NSOperationQueue_class/index.html

Update 1:

Here is my trial at your code, i added queue and logs, check them and also check the timeout value. This way if a request does not finish (most likely) you can trace it. All requests are serial so if one of them stops you should immediately notice it. You can create more than one queue and access them sequentially (round robin) to have more requests simultaneously. I would not go with more than 4 queues which is also the default for most desktop internet browsers.

// keep the same queue for all request (class member?), don't put it in the same block
NSOperationQueue* queue= [[NSOperationQueue alloc] init];

// keep this in another code block from queue
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0ul), ^(void) {
 [[InstagramEngine sharedEngine] getMediaAtLocation:location count:kInstagramLocationBufferSize maxId:nil withSuccess:^(NSArray* media, InstagramPaginationInfo* paginationInfo) {
     if (media && media.count>0) {
         for (InstagramMedia* mediaObject in media) {
             NSURL* url= mediaObject.standardResolutionImageURL;
             NSLog(@"Start loading from %@", url);
             NSURLRequest* req= [NSURLRequest requestWithURL:mediaObject.standardResolutionImageURL
                                                                                     cachePolicy:NSURLRequestUseProtocolCachePolicy
                                                                             timeoutInterval:30]; // 30 seconds timeout
             [NSURLConnection sendAsynchronousRequest:req queue:queue completionHandler:^(NSURLResponse *response, NSData *data, NSError *connectionError) {
                 NSLog(@"Stop loading from %@ %@ %@", url, response, connectionError);
                 if (data != nil && connectionError == nil) {
                     UIImage* img = [UIImage imageWithData:data];
                     if (img) {
                         // if you are triggering some ui update run on main thread
                         // [self.locationBuffer addObject:img];
                         [self.locationBuffer performSelectorOnMainThread:@selector(addObject:)
                                                                                                     withObject:img
                                                                                                waitUntilDone:NO];
                     }
                 }
             }];
         }
     }
 } failure:^(NSError *error) {
     NSLog(@"Error getting media list: %@", error);
 }];
});

Answer 2

OSSpinLocks has bug on iOS8.3+: report
,which leads to such freezes.
I think you should replace OSSpinLocks with something else to fix this in your app (maybe create your own realization of spin lock - search SO for OSSpinLock).