Re: io_uring_prep_openat_direct() and link/drain

[Miklos Szeredi <miklos@szeredi.hu>]

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;

than it doesn't.  Shouldn't drain have a stronger ordering guarantee than link?

Thanks,
Miklos