Re: io_uring_prep_openat_direct() and link/drain

[Jens Axboe <axboe@kernel.dk>]

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);
> 
> 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.

Can you try and pull:

git://git.kernel.dk/linux-block for-5.18/io_uring

into v5.17 and see if that works for you? It will merge cleanly, no
rejects.

Thanks!

-- 
Jens Axboe