workqueue: separate iteration role from worker_idr

worker_idr has the iteration (iterating for attached workers) and
worker ID duties. These two duties don't have to be tied together. We
can separate them and use a list for tracking attached workers and
iteration.

Before this separation, it wasn't possible to add rescuer workers to
worker_idr due to rescuer workers couldn't allocate ID dynamically
because ID-allocation depends on memory-allocation, which rescuer
can't depend on.

After separation, we can easily add the rescuer workers to the list for
iteration without any memory-allocation. It is required when we attach
the rescuer worker to the pool in later patch.

tj: Minor description updates.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue_internal.h

@@ -37,6 +37,8 @@ struct worker {
 	struct task_struct	*task;		/* I: worker task */
 	struct worker_pool	*pool;		/* I: the associated pool */
 						/* L: for rescuers */
+	struct list_head	node;		/* M: anchored at pool->workers */
+						/* M: runs through worker->node */
 
 	unsigned long		last_active;	/* L: last active timestamp */
 	unsigned int		flags;		/* X: flags */