io_uring: halt SQO submission on ctx exit

io_sq_thread_finish() is called in io_ring_ctx_free(), so SQPOLL task is
potentially running submitting new requests. It's not a disaster because
of using a "try" variant of percpu_ref_get, but is far from nice.

Remove ctx from the sqd ctx list earlier, before cancellation loop, so
SQPOLL can't find it and so won't submit new requests.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -8564,6 +8564,14 @@ static void io_ring_ctx_wait_and_kill(struct io_ring_ctx *ctx)
 		io_unregister_personality(ctx, index);
 	mutex_unlock(&ctx->uring_lock);
 
+	/* prevent SQPOLL from submitting new requests */
+	if (ctx->sq_data) {
+		io_sq_thread_park(ctx->sq_data);
+		list_del_init(&ctx->sqd_list);
+		io_sqd_update_thread_idle(ctx->sq_data);
+		io_sq_thread_unpark(ctx->sq_data);
+	}
+
 	io_kill_timeouts(ctx, NULL, NULL);
 	io_poll_remove_all(ctx, NULL, NULL);