1. A computer readable storage medium storing program instructions executable by a processor of a multithreaded system to implement a mutual exclusion mechanism comprising: a biasable lock that is concurrently accessible to a plurality of threads on the multithreaded system and is biased to at most one of the plurality of threads at a given time, wherein the biasable lock includes a quick-lock field; acquisition and release sequences that, when executed by a bias-holding thread on the processor, wherein the biasable lock is biased to the bias-holding thread, are free of atomic read-modify-write operations and wherein the acquisition sequence comprises: storing a value in the quick-lock field, the value indicating that the biasable lock is held by the bias-holding thread; and subsequent to said storing, determining whether the biasable lock is still biased to the bias-holding thread, wherein the acquisition sequence completes successfully and free of atomic read-modify-write operations if the biasable lock is determined to still be biased to the bias-holding thread.