Re: [PATCH 2/2] io_uring: fix failed linkchain code logic

Pavel Begunkov
2021-08-23T11:02:50Z

On 8/23/21 4:25 AM, Hao Xu wrote:
> Given a linkchain like this:
> req0(link_flag)-->req1(link_flag)-->...-->reqn(no link_flag)
>
> There is a problem:
> - if some intermediate linked req like req1 's submittion fails, reqs
> after it won't be cancelled.
>
> - sqpoll disabled: maybe it's ok since users can get the error info
> of req1 and stop submitting the following sqes.
>
> - sqpoll enabled: definitely a problem, the following sqes will be
> submitted in the next round.
>
> The solution is to refactor the code logic to:
> - if a linked req's submittion fails, just mark it and the head(if it
> exists) as REQ_F_FAIL. Leverage req->result to indicate whether it
> is failed or cancelled.
> - submit or fail the whole chain when we come to the end of it.

This looks good to me, a couple of comments below.

> Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
> ---
> fs/io_uring.c | 61 +++++++++++++++++++++++++++++++++++++--------------
> 1 file changed, 45 insertions(+), 16 deletions(-)
>
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 44b1b2b58e6a..9ae8f2a5c584 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -1776,8 +1776,6 @@ static void io_preinit_req(struct io_kiocb *req, struct io_ring_ctx *ctx)
> req->ctx = ctx;
> req->link = NULL;
> req->async_data = NULL;
> - /* not necessary, but safer to zero */
> - req->result = 0;

Please leave it. I'm afraid of leaking stack to userspace because
->result juggling looks prone to errors. And preinit is pretty cold
anyway.

[...]

>
> @@ -6637,19 +6644,25 @@ static int io_submit_sqe(struct io_ring_ctx *ctx, struct io_kiocb *req,
> ret = io_init_req(ctx, req, sqe);
> if (unlikely(ret)) {
> fail_req:
> + /* fail even hard links since we don't submit */
> if (link->head) {
> - /* fail even hard links since we don't submit */
> - io_req_complete_failed(link->head, -ECANCELED);
> - link->head = NULL;
> + req_set_fail(link->head);

I think it will be more reliable if we set head->result here, ...

if (!(link->head->flags & FAIL))
link->head->result = -ECANCELED;

> - ret = io_req_prep_async(req);
> - if (unlikely(ret))
> - goto fail_req;
> + if (!(req->flags & REQ_F_FAIL)) {
> + ret = io_req_prep_async(req);
> + if (unlikely(ret)) {
> + req->result = ret;
> + req_set_fail(req);
> + req_set_fail(link->head);

... and here (a helper?), ...

> + }
> + }
> trace_io_uring_link(ctx, req, head);
> link->last->link = req;
> link->last = req;
> @@ -6681,6 +6699,17 @@ static int io_submit_sqe(struct io_ring_ctx *ctx, struct io_kiocb *req,
> if (req->flags & (REQ_F_LINK | REQ_F_HARDLINK)) {
> link->head = req;
> link->last = req;
> + /*
> + * we can judge a link req is failed or cancelled by if
> + * REQ_F_FAIL is set, but the head is an exception since
> + * it may be set REQ_F_FAIL because of other req's failure
> + * so let's leverage req->result to distinguish if a head
> + * is set REQ_F_FAIL because of its failure or other req's
> + * failure so that we can set the correct ret code for it.
> + * init result here to avoid affecting the normal path.
> + */
> + if (!(req->flags & REQ_F_FAIL))
> + req->result = 0;

... instead of delaying to this point. Just IMHO, it's easier to look
after the code when it's set on the spot, i.e. may be easy to screw/forget
something while changing bits around.

> } else {
> io_queue_sqe(req);
> }
>

--
Pavel Begunkov