[$] Nested bottom-half locking for realtime kernels

Software-interrupt handlers (also called "bottom halves") have a longhistory in the Linux kernel; for much of that history, developers havewished that they could go away. One of their unfortunate characteristicsis that they can add unexpected latency to the execution of unrelatedprocesses; this problem is felt especially acutely in therealtime-preemption community. The solution adopted there has createdproblems of its own, though; in response Sebastian Andrzej Siewior is proposinga new locking mechanism for realtime builds of the kernel that may havebenefits for non-realtime users as well.