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

Hao Xu
2021-08-23T18:45:37Z

在 2021/8/23 下午7:02, Pavel Begunkov 写道:
> 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, ...
Sure, I'll send v3 later.
>
> 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);
>> }
>>
>