io-wq: warn on creating manager while exiting

Add a simple warning making sure that nobody tries to create a new
manager while we're under IO_WQ_BIT_EXIT. That can potentially happen
due to racy work submission after final put.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io-wq.c

@@ -774,6 +774,8 @@ static int io_wq_fork_manager(struct io_wq *wq)
 	if (wq->manager)
 		return 0;
 
+	WARN_ON_ONCE(test_bit(IO_WQ_BIT_EXIT, &wq->state));
+
 	init_completion(&wq->worker_done);
 	atomic_set(&wq->worker_refs, 1);
 	tsk = create_io_thread(io_wq_manager, wq, NUMA_NO_NODE);