Re: CQE repeats the first item?

[Jacob Thompson <jacobT@beta.pyu.ca>]

On Sun, Oct 05, 2025 at 07:09:53PM -0600, Jens Axboe wrote:
> On 10/5/25 3:54 PM, Jacob Thompson wrote:
> > On Sun, Oct 05, 2025 at 02:56:05PM -0600, Jens Axboe wrote:
> >> On 10/5/25 2:21 PM, Jacob Thompson wrote:
> >>> I'm doing something wrong and I wanted to know if anyone knows what I
> >>> did wrong from the description I'm using syscalls to call
> >>> io_uring_setup and io_uring_enter. I managed to submit 1 item without
> >>> an issue but any more gets me the first item over and over again. In
> >>> my test I did a memset -1 on cqes and sqes, I memset 0 the first ten
> >>> sqes with different user_data (0x1234 + i), and I used the opcode
> >>> IORING_OP_NOP. I called "io_uring_enter(fd, 10, 0,
> >>> IORING_ENTER_SQ_WAKEUP, 0)" and looked at the cq. Item 11 has the
> >>> user_data as '18446744073709551615' which is correct, but the first 10
> >>> all has user_data be 0x1234 which is weird AF since only one item has
> >>> that user_data and I submited 10 I considered maybe the debugger was
> >>> giving me incorrect values so I tried printing the user data in a
> >>> loop, I have no idea why the first one repeats 10 times. I only called
> >>> enter once
> >>>
> >>> Id is 4660
> >>> Id is 4660
> >>> Id is 4660
> >>> Id is 4660
> >>> Id is 4660
> >>> Id is 4660
> >>> Id is 4660
> >>> Id is 4660
> >>> Id is 4660
> >>> Id is 4660
> >>> Id is 18446744073709551615
> >>
> >> You're presumably not updating your side of the CQ ring correctly, see
> >> what liburing does when you call io_uring_cqe_seen(). If that's not it,
> >> then you're probably mishandling something else and an example would be
> >> useful as otherwise I'd just be guessing. There's really not much to go
> >> from in this report.
> >>
> >> -- 
> >> Jens Axboe
> > 
> > I tried reproducing it in a smaller file. Assume I did everything wrong but somehow I seem to get results and they're not correct.
> > 
> > The codebase I'd like to use this in has very little activity (could go seconds without a single syscall), then execute a few hundreds-thousand (which I like to be async).
> > SQPOLL sounds like the one best for my usecase. You can see I updated the sq tail before enter and I used IORING_ENTER_SQ_WAKEUP + slept for a second.
> > The sq tail isn't zero which means I have results? and you can see its 10 of the same user_data
> > 
> > cq head is 0 enter result was 10
> > 1234 0
> > 1234 0
> > 1234 0
> > 1234 0
> > 1234 0
> > 1234 0
> > 1234 0
> > 1234 0
> > 1234 0
> > 1234 0
> > FFFFFFFF -1
> 
> I looked at your test code, and you're setting up 10 NOP requests with
> userdata == 0x1234, and hence you get 10 completions with that userdata.
> For some reason you iterate 11 CQEs, which means your last one is the one
> that you already filled with -1.
> 
> In other words, it very much looks like it's working as it should. Any
> reason why you're using the raw interface rather than liburing? All of
> this seems to be not understanding how the ring works, and liburing
> helps isolate you from that. The SQ ring doesn't tell you anything about
> whether you have results (CQEs?), the difference between the SQ head and
> tail just tell you if there's something to submit. The CQ ring head and
> tail would tell you if there are CQEs to reap or not.
> 
> -- 
> Jens Axboe

You must be seeing something that I'm not. I had a +i in the line, should the user_data not increment every item?
The line was 'sqes[i].user_data = 0x1234+i;'. The 11th iteration is intentional to see the value of the memset earlier.

> I'd encourage you to read and understand liburing, even if you aren't going to use it. That will tell you how to reap and iterate CQE events.

I did. I didn't try to compile and step through but I'm in the middle of rereading the manual. I want to finish it before I look at the lib again