Adaptive modifications of spinning and blocking behavior in spin-then-block mutual exclusion include limiting spinning time to no more than the duration of a context switch. Also, the frequency of spinning versus blocking is limited to a desired amount based on the success rate of recent spin attempts. As an alternative, spinning is bypassed if spinning is unlikely to be successful because the owner is not progressing toward releasing the shared resource, as might occur if the owner is blocked or spinning itself. In another aspect, the duration of spinning is generally limited, but longer spinning is permitted if no other threads are ready to utilize the processor. In another aspect, if the owner of a shared resource is ready to be executed, a thread attempting to acquire ownership performs a "directed yield" of the remainder of its processing quantum to the other thread, and execution of the acquiring thread is suspended.