From: Rusty Russell <rusty@rustcorp.com.au>

Workqueues are a great primitive for running things from user context from
a completely clean environment.  Unfortunately, they currently insist on
creating one thread per CPU, which is overkill for many situations, so the
more generic keventd workqueue is used for these.  Recently deadlocks using
keventd were demonstrated, showing that it is not suitable for all uses.

1) Clean up CPU iterators.  Always a nice touch.

2) Add __create_workqueue() and create_singlethread_workqueue(),
   keeping source compatibility.

3) Put workqueues in workqueue list even if !CONFIG_HOTPLUG_CPU (means
   we need a lock to protect that list).  Now we can tell if a wq is
   single-threaded using list_empty(&wq->list).

4) For single-threaded workqueues, override CPU in queue_work,
   delayed_work_timer_fn and flush_workqueue to be 0.  flush_workqueue
   now does redundant passes for single-threaded workqueues, but the
   code remains simple.

5) Make create_workqueue_thread return the thread, so we can easily
   kthread_bind for multi-threaded workqueues.


akpm fixes:

- Fix up is_single_threaded() handling

- single-threaded wq thread does not have "/0" appended.