Question for /r/learnprogramming about thread safety

Locks.m

@implementation test
{
  NSData *current;
  NSData *next;
}

//incoming data calls this method on thread A
- (void)gotNewData(NSData *)data
{
/// lock next here?
  next = data;
/// unlock next here?  
}

//thread B sends data through this method
- (void)sendData
{
/// lock next here?
  if( !current && next )
  {
    current = [next copy]
    next = nil;
  }
/// unlock next here?  

  if( current )
  {
    if( [self doSomethingWith:current] )
    {
      current = nil;
    }
  }
}

- (void)doSomethingWith:(NSData *)data
{
  BOOL done;
  
  // process part of data using a counter
  // to move along with each call
  // if end of data is reached in this call 
  // return NO else return YES
  
  return done;
}

@end