io_uring: cancel work if task_work_add() fails

We currently move it to the io_wqe_manager for execution, but we cannot
safely do so as we may lack some of the state to execute it out of
context. As we cancel work anyway when the ring/task exits, just mark
this request as canceled and io_async_task_func() will do the right
thing.

Fixes: aa96bf8a ("io_uring: use io-wq manager as backup task if task is exiting")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -4135,12 +4135,14 @@ static int __io_async_wake(struct io_kiocb *req, struct io_poll_iocb *poll,
 	req->result = mask;
 	init_task_work(&req->task_work, func);
 	/*
-	 * If this fails, then the task is exiting. Punt to one of the io-wq
-	 * threads to ensure the work gets run, we can't always rely on exit
-	 * cancelation taking care of this.
+	 * If this fails, then the task is exiting. When a task exits, the
+	 * work gets canceled, so just cancel this request as well instead
+	 * of executing it. We can't safely execute it anyway, as we may not
+	 * have the needed state needed for it anyway.
 	 */
 	ret = task_work_add(tsk, &req->task_work, true);
 	if (unlikely(ret)) {
+		WRITE_ONCE(poll->canceled, true);
 		tsk = io_wq_get_task(req->ctx->io_wq);
 		task_work_add(tsk, &req->task_work, true);
 	}