Re: io_uring_prep_openat_direct() and link/drain

[Jens Axboe <axboe@kernel.dk>]

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);

submit();

You don't want link on the read, it just depends on that previous open.
And you don't need drain.

But there's an issue with file assignment, it's done with the read is
prepped, not before it is run. Hence it will fail with EBADF currently,
which is the issue at hand.

Let me write up a fix for this, would be great if you could test.

-- 
Jens Axboe