io-wq: wait for manager exit on wq destroy

The manager waits for the workers, hence the manager is always valid if
workers are running. Now also have wq destroy wait for the manager on
exit, so we now everything is gone.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io-wq.c

@@ -119,6 +119,7 @@ struct io_wq {
 
 	refcount_t refs;
 	struct completion started;
+	struct completion exited;
 
 	atomic_t worker_refs;
 	struct completion worker_done;
@@ -771,6 +772,7 @@ static int io_wq_manager(void *data)
 	if (atomic_read(&wq->worker_refs))
 		wait_for_completion(&wq->worker_done);
 	wq->manager = NULL;
+	complete(&wq->exited);
 	io_wq_put(wq);
 	do_exit(0);
 }
@@ -1066,6 +1068,7 @@ struct io_wq *io_wq_create(unsigned bounded, struct io_wq_data *data)
 
 	wq->task_pid = current->pid;
 	init_completion(&wq->started);
+	init_completion(&wq->exited);
 	refcount_set(&wq->refs, 1);
 
 	init_completion(&wq->worker_done);
@@ -1095,8 +1098,10 @@ static void io_wq_destroy(struct io_wq *wq)
 	cpuhp_state_remove_instance_nocalls(io_wq_online, &wq->cpuhp_node);
 
 	set_bit(IO_WQ_BIT_EXIT, &wq->state);
-	if (wq->manager)
+	if (wq->manager) {
 		wake_up_process(wq->manager);
+		wait_for_completion(&wq->exited);
+	}
 
 	spin_lock_irq(&wq->hash->wait.lock);
 	for_each_node(node) {