Re: io_uring_prep_openat_direct() and link/drain

[Miklos Szeredi <miklos@szeredi.hu>]

On Tue, 29 Mar 2022 at 20:40, Jens Axboe <axboe@kernel.dk> wrote:
>
> On 3/29/22 12:31 PM, Miklos Szeredi wrote:
> > On Tue, 29 Mar 2022 at 20:26, Jens Axboe <axboe@kernel.dk> wrote:
> >>
> >> On 3/29/22 12:21 PM, Miklos Szeredi wrote:
> >>> On Tue, 29 Mar 2022 at 19:04, Jens Axboe <axboe@kernel.dk> wrote:
> >>>>
> >>>> On 3/29/22 10:08 AM, Jens Axboe wrote:
> >>>>> On 3/29/22 7:20 AM, Miklos Szeredi wrote:
> >>>>>> Hi,
> >>>>>>
> >>>>>> I'm trying to read multiple files with io_uring and getting stuck,
> >>>>>> because the link and drain flags don't seem to do what they are
> >>>>>> documented to do.
> >>>>>>
> >>>>>> Kernel is v5.17 and liburing is compiled from the git tree at
> >>>>>> 7a3a27b6a384 ("add tests for nonblocking accept sockets").
> >>>>>>
> >>>>>> Without those flags the attached example works some of the time, but
> >>>>>> that's probably accidental since ordering is not ensured.
> >>>>>>
> >>>>>> Adding the drain or link flags make it even worse (fail in casese that
> >>>>>> the unordered one didn't).
> >>>>>>
> >>>>>> What am I missing?
> >>>>>
> >>>>> I don't think you're missing anything, it looks like a bug. What you
> >>>>> want here is:
> >>>>>
> >>>>> prep_open_direct(sqe);
> >>>>> sqe->flags |= IOSQE_IO_LINK;
> >>>>> ...
> >>>>> prep_read(sqe);
> >>>
> >>> So with the below merge this works.   But if instead I do
> >>>
> >>> prep_open_direct(sqe);
> >>>  ...
> >>> prep_read(sqe);
> >>> sqe->flags |= IOSQE_IO_DRAIN;

And this doesn't work either:

prep_open_direct(sqe);
sqe->flags |= IOSQE_IO_LINK;
...
prep_read(sqe);
sqe->flags |= IOSQE_IO_LINK;
...
prep_open_direct(sqe);
sqe->flags |= IOSQE_IO_LINK;
...
prep_read(sqe);

Yeah, the link is not needed for the read (unless the fixed file slot
is to be reused), but link/drain should work as general ordering
instructions, not just in special cases.

Thanks,
Miklos