[$] Short waits with umwait

If a user-space process needs to wait for some event to happen, there is awhole range of mechanisms provided by the kernel to make that easy. Butcalling into the kernel tends not to work well for the shortest of waits- those measured in small numbers of microseconds. For delays of thismagnitude, developers often resort to busy loops, which have a muchsmaller potential for turning a small delay into a larger one.Needless to say, busy waiting has its own disadvantages, so Intel has come upwith a set of instructions to support short delays. A patchset from Fenghua Yu to support these instructions is currently workingits way through the review process.