[$] Relief for insomniac tracepoints

The kernel's tracing infrastructure is designed to be fast and to interfereas little as possible with the normal operation of the system. Oneconsequence of this requirement is that the code that runs when atracepoint is hit cannot sleep; otherwise execution of the tracepoint couldadd an arbitrary delay to the execution of the real work the kernel shouldbe doing. There are times, though, that the ability to sleep within atracepoint would be handy, delays notwithstanding. The sleepabletracepoints patch set from Michael Jeanson sets the stageto make it possible for (some) tracepointhandlers to take a nap while performing their tasks - but stops short ofcompleting the job for now.