workqueue: Minor follow-ups to the rescuer destruction change

* Now that wq->rescuer may be cleared while rescuer is still there,
  switch show_pwq() debug printout to test worker->rescue_wq to
  identify rescuers intead of testing wq->rescuer.

* Update comment on ->rescuer locking.

Signed-off-by: Tejun Heo <tj@kernel.org>
Suggested-by: Lai Jiangshan <jiangshanlai@gmail.com>

Author: htejun

kernel/workqueue.c

@@ -248,7 +248,7 @@ struct workqueue_struct {
 	struct list_head	flusher_overflow; /* WQ: flush overflow list */
 
 	struct list_head	maydays;	/* MD: pwqs requesting rescue */
-	struct worker		*rescuer;	/* I: rescue worker */
+	struct worker		*rescuer;	/* MD: rescue worker */
 
 	int			nr_drainers;	/* WQ: drain in progress */
 	int			saved_max_active; /* WQ: saved pwq max_active */
@@ -4672,7 +4672,7 @@ static void show_pwq(struct pool_workqueue *pwq)
 
 			pr_cont("%s %d%s:%ps", comma ? "," : "",
 				task_pid_nr(worker->task),
-				worker == pwq->wq->rescuer ? "(RESCUER)" : "",
+				worker->rescue_wq ? "(RESCUER)" : "",
 				worker->current_func);
 			list_for_each_entry(work, &worker->scheduled, entry)
 				pr_cont_work(false, work);