workqueue: implement current_is_workqueue_rescuer()

Implement a function which queries whether it currently is running off
a workqueue rescuer.  This will be used to convert writeback to
workqueue.
Signed-off-by: Tejun Heo <tj@kernel.org>

include/linux/workqueue.h

@@ -451,6 +451,7 @@ extern bool cancel_delayed_work_sync(struct delayed_work *dwork);
 
 extern void workqueue_set_max_active(struct workqueue_struct *wq,
 				     int max_active);
+extern bool current_is_workqueue_rescuer(void);
 extern bool workqueue_congested(int cpu, struct workqueue_struct *wq);
 extern unsigned int work_busy(struct work_struct *work);
 

kernel/workqueue.c

@@ -4071,6 +4071,19 @@ void workqueue_set_max_active(struct workqueue_struct *wq, int max_active)
 }
 EXPORT_SYMBOL_GPL(workqueue_set_max_active);
 
+/**
+ * current_is_workqueue_rescuer - is %current workqueue rescuer?
+ *
+ * Determine whether %current is a workqueue rescuer.  Can be used from
+ * work functions to determine whether it's being run off the rescuer task.
+ */
+bool current_is_workqueue_rescuer(void)
+{
+	struct worker *worker = current_wq_worker();
+
+	return worker && worker == worker->current_pwq->wq->rescuer;
+}
+
 /**
  * workqueue_congested - test whether a workqueue is congested
  * @cpu: CPU in question