A system, method, and product are disclosed for testing multiple threads simultaneously. The threads share a real memory space. A first portion of the real memory space is designated as exclusive memory such that the first portion appears to be reserved for use by only one of the threads. The threads are simultaneously executed. The threads access the first portion during execution. Apparent exclusive use of the first portion of the real memory space is permitted by a first one of the threads. Simultaneously with permitting apparent exclusive use of the first portion by the first one of the threads, apparent exclusive use of the first portion of the real memory space is also permitted by a second one of the threads. The threads simultaneously appear to have exclusive use of the first portion and may simultaneously access the first portion.