threading/mutex objc problem

gistfile1.txt

- (void)updateData:(NSTimer*)timer {

    // if already getting data do nothing
    if(!self.isFetchDataComplete) {
        NSLog(@"Do not get data - flag is OFF...");
        return;
    }
    
    // reset the flag
    self.isFetchDataComplete = FALSE;
    
    // get new data
    [self.pulseDataStore dataFrom:self.startDataDate
                             till:self.endDataDate
                       completion:^(NSArray *array) {
                          dispatch_sync(dispatch_get_main_queue(), ^{
                            self.isFetchDataComplete = TRUE;
                          });
                       }
                       }];
  }