workqueue: wake up a worker when a rescuer is leaving a gcwq

After executing the matching works, a rescuer leaves the gcwq whether
there are more pending works or not. This may decrease the
concurrency level to zero and stall execution until a new work item is
queued on the gcwq.

Make rescuer wake up a regular worker when it leaves a gcwq if there
are more works to execute, so that execution isn't stalled.

Signed-off-by: Tejun Heo <[e-mail]>
Reported-by: Ray Jui <[e-mail]>
Cc: [e-mail]

kernel/workqueue.c

···
 				move_linked_works(work, scheduled, &n);
 
 		process_scheduled_works(rescuer);
+
+		/*
+		 * Leave this gcwq.  If keep_working() is %true, notify a
+		 * regular worker; otherwise, we end up with 0 concurrency
+		 * and stalling the execution.
+		 */
+		if (keep_working(gcwq))
+			wake_up_worker(gcwq);
+
 		spin_unlock_irq(&gcwq->lock);
 	}