workqueue: keep track of the flushing task and pool manager

Add wq_barrier->task and worker_pool->manager to keep track of the
flushing task and pool manager respectively. These are purely
informational and will be used to implement sysrq dump of workqueues.

Signed-off-by: Tejun Heo <[e-mail]>

kernel/workqueue.c

···
 
 	/* see manage_workers() for details on the two manager mutexes */
 	struct mutex		manager_arb;	/* manager arbitration */
+	struct worker		*manager;	/* L: purely informational */
 	struct mutex		attach_mutex;	/* attach/detach exclusion */
 	struct list_head	workers;	/* A: attached workers */
 	struct completion	*detach_completion; /* all workers detached */
···
 	 */
 	if (!mutex_trylock(&pool->manager_arb))
 		return false;
+	pool->manager = worker;
 
 	maybe_create_worker(pool);
 
+	pool->manager = NULL;
 	mutex_unlock(&pool->manager_arb);
 	return true;
 }
···
 struct wq_barrier {
 	struct work_struct	work;
 	struct completion	done;
+	struct task_struct	*task;	/* purely informational */
 };
 
 static void wq_barrier_func(struct work_struct *work)
···
 	INIT_WORK_ONSTACK(&barr->work, wq_barrier_func);
 	__set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(&barr->work));
 	init_completion(&barr->done);
+	barr->task = current;
 
 	/*
 	 * If @target is currently being executed, schedule the