[$] A new futex API

The Linux fast user-space mutex ("futex") subsystem debuted with the 2.6.0kernel; it provides a mechanism that can be used to implement user-spacelocking. Since futexes avoid calling into the kernel whenever possible,they can indeed be fast, especially in the uncontended case. The API usedto access futexes has never been seen as one of Linux's strongest points,though, so there has long been a desire to improve it. This patchseries from Peter Zijlstra shows what the future of futexes may looklike.