Unknow alert void _WebThreadLockFromAnyThread(bool), - iphone

Question

When I am running my code, my code is stuck and gives log as:
void _WebThreadLockFromAnyThread(bool), 0x904db10: Obtaining the web lock from a thread other than the main thread or the web thread. UIKit should not be called from a secondary thread.
What is the reason, and what is the solution for this?
My Code:
-(IBAction)action:(id)sender{
NSLog(#"HI");
[self performSelectorInBackground:#selector(parse) withObject:nil];
}
-(void)parse{
rssOutputData = [[NSMutableArray alloc]init];
//[self performSelectorOnMainThread:#selector() withObject:nil waitUntilDone:NO];
[NSThread detachNewThreadSelector: #selector(startIndicator) toTarget: self withObject: self];
url=[[NSURL alloc]initWithString:[NSString stringWithFormat:#"http://service.technologyorg.com/service1/AuthenticateStudent=%#,%#,1",t1.text,t2.text]];
NSLog(#"A%#",url);
NSURLRequest *lRequest = [NSURLRequest requestWithURL:url
cachePolicy:NSURLRequestUseProtocolCachePolicy
timeoutInterval:30.0];
NSURLResponse *response;
NSError * error;
//[NSURLConnection connectionWithRequest:lRequest delegate:self];
[NSURLConnection sendSynchronousRequest:lRequest returningResponse:&response error:&error];
NSLog(#"AB%#",response);
if (nil == response) {
NSLog(#"%#",error);
[self performSelectorOnMainThread:#selector(updateTable) withObject:nil waitUntilDone:NO];
UIAlertView * alert=[[UIAlertView alloc]initWithTitle:#"Connection failed!" message:nil delegate:nil cancelButtonTitle:#"OK" otherButtonTitles:nil, nil];
[alert show];
}
xmlData=[[NSMutableData alloc]initWithContentsOfURL:url];
NSLog(#"B%#",xmlData);
xmltitle=nil;
xmlParserObject =[[NSXMLParser alloc]initWithData:xmlData];
[xmlParserObject setDelegate:self];
dispatch_async(dispatch_get_global_queue(0, 0), ^{
[xmlParserObject parse];
dispatch_async(dispatch_get_main_queue(), ^{
NSLog(#"XML:%#",xmllink);
if ([xmltitle isEqualToString:#"true"]) {
HomeViewController *lv=[[HomeViewController alloc]init];
lv.s1=[[NSString alloc]initWithFormat:#"%#",xmllink];
lv.s2=[[NSString alloc]initWithFormat:#"%#",xm1];
myStaticPassword=[[NSString alloc]initWithFormat:#"%#",xm2];
[self presentModalViewController:lv animated: YES];
[self performSelectorOnMainThread:#selector(updateTable) withObject:nil waitUntilDone:NO];
}else{
[self performSelectorOnMainThread:#selector(updateTable) withObject:nil waitUntilDone:NO];
UIAlertView * alert=[[UIAlertView alloc]initWithTitle:#"Incorrect Username/Password" message:#"Please check your Username or Password" delegate:nil cancelButtonTitle:#"Cancel" otherButtonTitles:nil, nil];
[alert show];
}
});
});
}